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.. _datamodel:
**********
Data model
**********
.. _objects:
Objects, values and types
=========================
.. index::
single: object
single: data
:dfn:`Objects` are Python's abstraction for data. All data in a Python program
is represented by objects or by relations between objects. (In a sense, and in
conformance to Von Neumann's model of a "stored program computer," code is also
represented by objects.)
.. index::
builtin: id
builtin: type
single: identity of an object
single: value of an object
single: type of an object
single: mutable object
single: immutable object
.. XXX it *is* now possible in some cases to change an object's
type, under certain controlled conditions
Every object has an identity, a type and a value. An object's *identity* never
changes once it has been created; you may think of it as the object's address in
memory. The ':keyword:`is`' operator compares the identity of two objects; the
:func:`id` function returns an integer representing its identity.
.. impl-detail::
For CPython, ``id(x)`` is the memory address where ``x`` is stored.
An object's type determines the operations that the object supports (e.g., "does
it have a length?") and also defines the possible values for objects of that
type. The :func:`type` function returns an object's type (which is an object
itself). Like its identity, an object's :dfn:`type` is also unchangeable.
[#]_
The *value* of some objects can change. Objects whose value can
change are said to be *mutable*; objects whose value is unchangeable once they
are created are called *immutable*. (The value of an immutable container object
that contains a reference to a mutable object can change when the latter's value
is changed; however the container is still considered immutable, because the
collection of objects it contains cannot be changed. So, immutability is not
strictly the same as having an unchangeable value, it is more subtle.) An
object's mutability is determined by its type; for instance, numbers, strings
and tuples are immutable, while dictionaries and lists are mutable.
.. index::
single: garbage collection
single: reference counting
single: unreachable object
Objects are never explicitly destroyed; however, when they become unreachable
they may be garbage-collected. An implementation is allowed to postpone garbage
collection or omit it altogether --- it is a matter of implementation quality
how garbage collection is implemented, as long as no objects are collected that
are still reachable.
.. impl-detail::
CPython currently uses a reference-counting scheme with (optional) delayed
detection of cyclically linked garbage, which collects most objects as soon
as they become unreachable, but is not guaranteed to collect garbage
containing circular references. See the documentation of the :mod:`gc`
module for information on controlling the collection of cyclic garbage.
Other implementations act differently and CPython may change.
Do not depend on immediate finalization of objects when they become
unreachable (ex: always close files).
Note that the use of the implementation's tracing or debugging facilities may
keep objects alive that would normally be collectable. Also note that catching
an exception with a ':keyword:`try`...\ :keyword:`except`' statement may keep
objects alive.
Some objects contain references to "external" resources such as open files or
windows. It is understood that these resources are freed when the object is
garbage-collected, but since garbage collection is not guaranteed to happen,
such objects also provide an explicit way to release the external resource,
usually a :meth:`close` method. Programs are strongly recommended to explicitly
close such objects. The ':keyword:`try`...\ :keyword:`finally`' statement
and the ':keyword:`with`' statement provide convenient ways to do this.
.. index:: single: container
Some objects contain references to other objects; these are called *containers*.
Examples of containers are tuples, lists and dictionaries. The references are
part of a container's value. In most cases, when we talk about the value of a
container, we imply the values, not the identities of the contained objects;
however, when we talk about the mutability of a container, only the identities
of the immediately contained objects are implied. So, if an immutable container
(like a tuple) contains a reference to a mutable object, its value changes if
that mutable object is changed.
Types affect almost all aspects of object behavior. Even the importance of
object identity is affected in some sense: for immutable types, operations that
compute new values may actually return a reference to any existing object with
the same type and value, while for mutable objects this is not allowed. E.g.,
after ``a = 1; b = 1``, ``a`` and ``b`` may or may not refer to the same object
with the value one, depending on the implementation, but after ``c = []; d =
[]``, ``c`` and ``d`` are guaranteed to refer to two different, unique, newly
created empty lists. (Note that ``c = d = []`` assigns the same object to both
``c`` and ``d``.)
.. _types:
The standard type hierarchy
===========================
.. index::
single: type
pair: data; type
pair: type; hierarchy
pair: extension; module
pair: C; language
Below is a list of the types that are built into Python. Extension modules
(written in C, Java, or other languages, depending on the implementation) can
define additional types. Future versions of Python may add types to the type
hierarchy (e.g., rational numbers, efficiently stored arrays of integers, etc.),
although such additions will often be provided via the standard library instead.
.. index::
single: attribute
pair: special; attribute
triple: generic; special; attribute
Some of the type descriptions below contain a paragraph listing 'special
attributes.' These are attributes that provide access to the implementation and
are not intended for general use. Their definition may change in the future.
None
.. index:: object: None
This type has a single value. There is a single object with this value. This
object is accessed through the built-in name ``None``. It is used to signify the
absence of a value in many situations, e.g., it is returned from functions that
don't explicitly return anything. Its truth value is false.
NotImplemented
.. index:: object: NotImplemented
This type has a single value. There is a single object with this value. This
object is accessed through the built-in name ``NotImplemented``. Numeric methods
and rich comparison methods may return this value if they do not implement the
operation for the operands provided. (The interpreter will then try the
reflected operation, or some other fallback, depending on the operator.) Its
truth value is true.
Ellipsis
.. index:: object: Ellipsis
This type has a single value. There is a single object with this value. This
object is accessed through the literal ``...`` or the built-in name
``Ellipsis``. Its truth value is true.
:class:`numbers.Number`
.. index:: object: numeric
These are created by numeric literals and returned as results by arithmetic
operators and arithmetic built-in functions. Numeric objects are immutable;
once created their value never changes. Python numbers are of course strongly
related to mathematical numbers, but subject to the limitations of numerical
representation in computers.
Python distinguishes between integers, floating point numbers, and complex
numbers:
:class:`numbers.Integral`
.. index:: object: integer
These represent elements from the mathematical set of integers (positive and
negative).
There are two types of integers:
Integers (:class:`int`)
These represent numbers in an unlimited range, subject to available (virtual)
memory only. For the purpose of shift and mask operations, a binary
representation is assumed, and negative numbers are represented in a variant of
2's complement which gives the illusion of an infinite string of sign bits
extending to the left.
Booleans (:class:`bool`)
.. index::
object: Boolean
single: False
single: True
These represent the truth values False and True. The two objects representing
the values False and True are the only Boolean objects. The Boolean type is a
subtype of the integer type, and Boolean values behave like the values 0 and 1,
respectively, in almost all contexts, the exception being that when converted to
a string, the strings ``"False"`` or ``"True"`` are returned, respectively.
.. index:: pair: integer; representation
The rules for integer representation are intended to give the most meaningful
interpretation of shift and mask operations involving negative integers.
:class:`numbers.Real` (:class:`float`)
.. index::
object: floating point
pair: floating point; number
pair: C; language
pair: Java; language
These represent machine-level double precision floating point numbers. You are
at the mercy of the underlying machine architecture (and C or Java
implementation) for the accepted range and handling of overflow. Python does not
support single-precision floating point numbers; the savings in processor and
memory usage that are usually the reason for using these is dwarfed by the
overhead of using objects in Python, so there is no reason to complicate the
language with two kinds of floating point numbers.
:class:`numbers.Complex` (:class:`complex`)
.. index::
object: complex
pair: complex; number
These represent complex numbers as a pair of machine-level double precision
floating point numbers. The same caveats apply as for floating point numbers.
The real and imaginary parts of a complex number ``z`` can be retrieved through
the read-only attributes ``z.real`` and ``z.imag``.
Sequences
.. index::
builtin: len
object: sequence
single: index operation
single: item selection
single: subscription
These represent finite ordered sets indexed by non-negative numbers. The
built-in function :func:`len` returns the number of items of a sequence. When
the length of a sequence is *n*, the index set contains the numbers 0, 1,
..., *n*-1. Item *i* of sequence *a* is selected by ``a[i]``.
.. index:: single: slicing
Sequences also support slicing: ``a[i:j]`` selects all items with index *k* such
that *i* ``<=`` *k* ``<`` *j*. When used as an expression, a slice is a
sequence of the same type. This implies that the index set is renumbered so
that it starts at 0.
Some sequences also support "extended slicing" with a third "step" parameter:
``a[i:j:k]`` selects all items of *a* with index *x* where ``x = i + n*k``, *n*
``>=`` ``0`` and *i* ``<=`` *x* ``<`` *j*.
Sequences are distinguished according to their mutability:
Immutable sequences
.. index::
object: immutable sequence
object: immutable
An object of an immutable sequence type cannot change once it is created. (If
the object contains references to other objects, these other objects may be
mutable and may be changed; however, the collection of objects directly
referenced by an immutable object cannot change.)
The following types are immutable sequences:
.. index::
single: string; immutable sequences
Strings
.. index::
builtin: chr
builtin: ord
single: character
single: integer
single: Unicode
A string is a sequence of values that represent Unicode codepoints.
All the codepoints in range ``U+0000 - U+10FFFF`` can be represented
in a string. Python doesn't have a :c:type:`chr` type, and
every character in the string is represented as a string object
with length ``1``. The built-in function :func:`ord` converts a
character to its codepoint (as an integer); :func:`chr` converts
an integer in range ``0 - 10FFFF`` to the corresponding character.
:meth:`str.encode` can be used to convert a :class:`str` to
:class:`bytes` using the given encoding, and :meth:`bytes.decode` can
be used to achieve the opposite.
Tuples
.. index::
object: tuple
pair: singleton; tuple
pair: empty; tuple
The items of a tuple are arbitrary Python objects. Tuples of two or
more items are formed by comma-separated lists of expressions. A tuple
of one item (a 'singleton') can be formed by affixing a comma to an
expression (an expression by itself does not create a tuple, since
parentheses must be usable for grouping of expressions). An empty
tuple can be formed by an empty pair of parentheses.
Bytes
.. index:: bytes, byte
A bytes object is an immutable array. The items are 8-bit bytes,
represented by integers in the range 0 <= x < 256. Bytes literals
(like ``b'abc'``) and the built-in function :func:`bytes` can be used to
construct bytes objects. Also, bytes objects can be decoded to strings
via the :meth:`decode` method.
Mutable sequences
.. index::
object: mutable sequence
object: mutable
pair: assignment; statement
single: delete
statement: del
single: subscription
single: slicing
Mutable sequences can be changed after they are created. The subscription and
slicing notations can be used as the target of assignment and :keyword:`del`
(delete) statements.
There are currently two intrinsic mutable sequence types:
Lists
.. index:: object: list
The items of a list are arbitrary Python objects. Lists are formed by
placing a comma-separated list of expressions in square brackets. (Note
that there are no special cases needed to form lists of length 0 or 1.)
Byte Arrays
.. index:: bytearray
A bytearray object is a mutable array. They are created by the built-in
:func:`bytearray` constructor. Aside from being mutable (and hence
unhashable), byte arrays otherwise provide the same interface and
functionality as immutable bytes objects.
.. index:: module: array
The extension module :mod:`array` provides an additional example of a
mutable sequence type, as does the :mod:`collections` module.
Set types
.. index::
builtin: len
object: set type
These represent unordered, finite sets of unique, immutable objects. As such,
they cannot be indexed by any subscript. However, they can be iterated over, and
the built-in function :func:`len` returns the number of items in a set. Common
uses for sets are fast membership testing, removing duplicates from a sequence,
and computing mathematical operations such as intersection, union, difference,
and symmetric difference.
For set elements, the same immutability rules apply as for dictionary keys. Note
that numeric types obey the normal rules for numeric comparison: if two numbers
compare equal (e.g., ``1`` and ``1.0``), only one of them can be contained in a
set.
There are currently two intrinsic set types:
Sets
.. index:: object: set
These represent a mutable set. They are created by the built-in :func:`set`
constructor and can be modified afterwards by several methods, such as
:meth:`add`.
Frozen sets
.. index:: object: frozenset
These represent an immutable set. They are created by the built-in
:func:`frozenset` constructor. As a frozenset is immutable and
:term:`hashable`, it can be used again as an element of another set, or as
a dictionary key.
Mappings
.. index::
builtin: len
single: subscription
object: mapping
These represent finite sets of objects indexed by arbitrary index sets. The
subscript notation ``a[k]`` selects the item indexed by ``k`` from the mapping
``a``; this can be used in expressions and as the target of assignments or
:keyword:`del` statements. The built-in function :func:`len` returns the number
of items in a mapping.
There is currently a single intrinsic mapping type:
Dictionaries
.. index:: object: dictionary
These represent finite sets of objects indexed by nearly arbitrary values. The
only types of values not acceptable as keys are values containing lists or
dictionaries or other mutable types that are compared by value rather than by
object identity, the reason being that the efficient implementation of
dictionaries requires a key's hash value to remain constant. Numeric types used
for keys obey the normal rules for numeric comparison: if two numbers compare
equal (e.g., ``1`` and ``1.0``) then they can be used interchangeably to index
the same dictionary entry.
Dictionaries are mutable; they can be created by the ``{...}`` notation (see
section :ref:`dict`).
.. index::
module: dbm.ndbm
module: dbm.gnu
The extension modules :mod:`dbm.ndbm` and :mod:`dbm.gnu` provide
additional examples of mapping types, as does the :mod:`collections`
module.
Callable types
.. index::
object: callable
pair: function; call
single: invocation
pair: function; argument
These are the types to which the function call operation (see section
:ref:`calls`) can be applied:
User-defined functions
.. index::
pair: user-defined; function
object: function
object: user-defined function
A user-defined function object is created by a function definition (see
section :ref:`function`). It should be called with an argument list
containing the same number of items as the function's formal parameter
list.
Special attributes:
.. tabularcolumns:: |l|L|l|
+-------------------------+-------------------------------+-----------+
| Attribute | Meaning | |
+=========================+===============================+===========+
| :attr:`__doc__` | The function's documentation | Writable |
| | string, or ``None`` if | |
| | unavailable | |
+-------------------------+-------------------------------+-----------+
| :attr:`__name__` | The function's name | Writable |
+-------------------------+-------------------------------+-----------+
| :attr:`__qualname__` | The function's | Writable |
| | :term:`qualified name` | |
| | | |
| | .. versionadded:: 3.3 | |
+-------------------------+-------------------------------+-----------+
| :attr:`__module__` | The name of the module the | Writable |
| | function was defined in, or | |
| | ``None`` if unavailable. | |
+-------------------------+-------------------------------+-----------+
| :attr:`__defaults__` | A tuple containing default | Writable |
| | argument values for those | |
| | arguments that have defaults, | |
| | or ``None`` if no arguments | |
| | have a default value | |
+-------------------------+-------------------------------+-----------+
| :attr:`__code__` | The code object representing | Writable |
| | the compiled function body. | |
+-------------------------+-------------------------------+-----------+
| :attr:`__globals__` | A reference to the dictionary | Read-only |
| | that holds the function's | |
| | global variables --- the | |
| | global namespace of the | |
| | module in which the function | |
| | was defined. | |
+-------------------------+-------------------------------+-----------+
| :attr:`__dict__` | The namespace supporting | Writable |
| | arbitrary function | |
| | attributes. | |
+-------------------------+-------------------------------+-----------+
| :attr:`__closure__` | ``None`` or a tuple of cells | Read-only |
| | that contain bindings for the | |
| | function's free variables. | |
+-------------------------+-------------------------------+-----------+
| :attr:`__annotations__` | A dict containing annotations | Writable |
| | of parameters. The keys of | |
| | the dict are the parameter | |
| | names, or ``'return'`` for | |
| | the return annotation, if | |
| | provided. | |
+-------------------------+-------------------------------+-----------+
| :attr:`__kwdefaults__` | A dict containing defaults | Writable |
| | for keyword-only parameters. | |
+-------------------------+-------------------------------+-----------+
Most of the attributes labelled "Writable" check the type of the assigned value.
Function objects also support getting and setting arbitrary attributes, which
can be used, for example, to attach metadata to functions. Regular attribute
dot-notation is used to get and set such attributes. *Note that the current
implementation only supports function attributes on user-defined functions.
Function attributes on built-in functions may be supported in the future.*
Additional information about a function's definition can be retrieved from its
code object; see the description of internal types below.
.. index::
single: __doc__ (function attribute)
single: __name__ (function attribute)
single: __module__ (function attribute)
single: __dict__ (function attribute)
single: __defaults__ (function attribute)
single: __closure__ (function attribute)
single: __code__ (function attribute)
single: __globals__ (function attribute)
single: __annotations__ (function attribute)
single: __kwdefaults__ (function attribute)
pair: global; namespace
Instance methods
.. index::
object: method
object: user-defined method
pair: user-defined; method
An instance method object combines a class, a class instance and any
callable object (normally a user-defined function).
.. index::
single: __func__ (method attribute)
single: __self__ (method attribute)
single: __doc__ (method attribute)
single: __name__ (method attribute)
single: __module__ (method attribute)
Special read-only attributes: :attr:`__self__` is the class instance object,
:attr:`__func__` is the function object; :attr:`__doc__` is the method's
documentation (same as ``__func__.__doc__``); :attr:`__name__` is the
method name (same as ``__func__.__name__``); :attr:`__module__` is the
name of the module the method was defined in, or ``None`` if unavailable.
Methods also support accessing (but not setting) the arbitrary function
attributes on the underlying function object.
User-defined method objects may be created when getting an attribute of a
class (perhaps via an instance of that class), if that attribute is a
user-defined function object or a class method object.
When an instance method object is created by retrieving a user-defined
function object from a class via one of its instances, its
:attr:`__self__` attribute is the instance, and the method object is said
to be bound. The new method's :attr:`__func__` attribute is the original
function object.
When a user-defined method object is created by retrieving another method
object from a class or instance, the behaviour is the same as for a
function object, except that the :attr:`__func__` attribute of the new
instance is not the original method object but its :attr:`__func__`
attribute.
When an instance method object is created by retrieving a class method
object from a class or instance, its :attr:`__self__` attribute is the
class itself, and its :attr:`__func__` attribute is the function object
underlying the class method.
When an instance method object is called, the underlying function
(:attr:`__func__`) is called, inserting the class instance
(:attr:`__self__`) in front of the argument list. For instance, when
:class:`C` is a class which contains a definition for a function
:meth:`f`, and ``x`` is an instance of :class:`C`, calling ``x.f(1)`` is
equivalent to calling ``C.f(x, 1)``.
When an instance method object is derived from a class method object, the
"class instance" stored in :attr:`__self__` will actually be the class
itself, so that calling either ``x.f(1)`` or ``C.f(1)`` is equivalent to
calling ``f(C,1)`` where ``f`` is the underlying function.
Note that the transformation from function object to instance method
object happens each time the attribute is retrieved from the instance. In
some cases, a fruitful optimization is to assign the attribute to a local
variable and call that local variable. Also notice that this
transformation only happens for user-defined functions; other callable
objects (and all non-callable objects) are retrieved without
transformation. It is also important to note that user-defined functions
which are attributes of a class instance are not converted to bound
methods; this *only* happens when the function is an attribute of the
class.
Generator functions
.. index::
single: generator; function
single: generator; iterator
A function or method which uses the :keyword:`yield` statement (see section
:ref:`yield`) is called a :dfn:`generator function`. Such a function, when
called, always returns an iterator object which can be used to execute the
body of the function: calling the iterator's :meth:`iterator__next__`
method will cause the function to execute until it provides a value
using the :keyword:`yield` statement. When the function executes a
:keyword:`return` statement or falls off the end, a :exc:`StopIteration`
exception is raised and the iterator will have reached the end of the set of
values to be returned.
Built-in functions
.. index::
object: built-in function
object: function
pair: C; language
A built-in function object is a wrapper around a C function. Examples of
built-in functions are :func:`len` and :func:`math.sin` (:mod:`math` is a
standard built-in module). The number and type of the arguments are
determined by the C function. Special read-only attributes:
:attr:`__doc__` is the function's documentation string, or ``None`` if
unavailable; :attr:`__name__` is the function's name; :attr:`__self__` is
set to ``None`` (but see the next item); :attr:`__module__` is the name of
the module the function was defined in or ``None`` if unavailable.
Built-in methods
.. index::
object: built-in method
object: method
pair: built-in; method
This is really a different disguise of a built-in function, this time containing
an object passed to the C function as an implicit extra argument. An example of
a built-in method is ``alist.append()``, assuming *alist* is a list object. In
this case, the special read-only attribute :attr:`__self__` is set to the object
denoted by *alist*.
Classes
Classes are callable. These objects normally act as factories for new
instances of themselves, but variations are possible for class types that
override :meth:`__new__`. The arguments of the call are passed to
:meth:`__new__` and, in the typical case, to :meth:`__init__` to
initialize the new instance.
Class Instances
Instances of arbitrary classes can be made callable by defining a
:meth:`__call__` method in their class.
Modules
.. index::
statement: import
object: module
Modules are a basic organizational unit of Python code, and are created by
the :ref:`import system <importsystem>` as invoked either by the
:keyword:`import` statement (see :keyword:`import`), or by calling
functions such as :func:`importlib.import_module` and built-in
:func:`__import__`. A module object has a namespace implemented by a
dictionary object (this is the dictionary referenced by the ``__globals__``
attribute of functions defined in the module). Attribute references are
translated to lookups in this dictionary, e.g., ``m.x`` is equivalent to
``m.__dict__["x"]``. A module object does not contain the code object used
to initialize the module (since it isn't needed once the initialization is
done).
Attribute assignment updates the module's namespace dictionary, e.g.,
``m.x = 1`` is equivalent to ``m.__dict__["x"] = 1``.
.. index:: single: __dict__ (module attribute)
Special read-only attribute: :attr:`__dict__` is the module's namespace as a
dictionary object.
.. impl-detail::
Because of the way CPython clears module dictionaries, the module
dictionary will be cleared when the module falls out of scope even if the
dictionary still has live references. To avoid this, copy the dictionary
or keep the module around while using its dictionary directly.
.. index::
single: __name__ (module attribute)
single: __doc__ (module attribute)
single: __file__ (module attribute)
pair: module; namespace
Predefined (writable) attributes: :attr:`__name__` is the module's name;
:attr:`__doc__` is the module's documentation string, or ``None`` if
unavailable; :attr:`__file__` is the pathname of the file from which the
module was loaded, if it was loaded from a file. The :attr:`__file__`
attribute may be missing for certain types of modules, such as C modules
that are statically linked into the interpreter; for extension modules
loaded dynamically from a shared library, it is the pathname of the shared
library file.
Custom classes
Custom class types are typically created by class definitions (see section
:ref:`class`). A class has a namespace implemented by a dictionary object.
Class attribute references are translated to lookups in this dictionary, e.g.,
``C.x`` is translated to ``C.__dict__["x"]`` (although there are a number of
hooks which allow for other means of locating attributes). When the attribute
name is not found there, the attribute search continues in the base classes.
This search of the base classes uses the C3 method resolution order which
behaves correctly even in the presence of 'diamond' inheritance structures
where there are multiple inheritance paths leading back to a common ancestor.
Additional details on the C3 MRO used by Python can be found in the
documentation accompanying the 2.3 release at
http://www.python.org/download/releases/2.3/mro/.
.. XXX: Could we add that MRO doc as an appendix to the language ref?
.. index::
object: class
object: class instance
object: instance
pair: class object; call
single: container
object: dictionary
pair: class; attribute
When a class attribute reference (for class :class:`C`, say) would yield a
class method object, it is transformed into an instance method object whose
:attr:`__self__` attributes is :class:`C`. When it would yield a static
method object, it is transformed into the object wrapped by the static method
object. See section :ref:`descriptors` for another way in which attributes
retrieved from a class may differ from those actually contained in its
:attr:`__dict__`.
.. index:: triple: class; attribute; assignment
Class attribute assignments update the class's dictionary, never the dictionary
of a base class.
.. index:: pair: class object; call
A class object can be called (see above) to yield a class instance (see below).
.. index::
single: __name__ (class attribute)
single: __module__ (class attribute)
single: __dict__ (class attribute)
single: __bases__ (class attribute)
single: __doc__ (class attribute)
Special attributes: :attr:`__name__` is the class name; :attr:`__module__` is
the module name in which the class was defined; :attr:`__dict__` is the
dictionary containing the class's namespace; :attr:`__bases__` is a tuple
(possibly empty or a singleton) containing the base classes, in the order of
their occurrence in the base class list; :attr:`__doc__` is the class's
documentation string, or None if undefined.
Class instances
.. index::
object: class instance
object: instance
pair: class; instance
pair: class instance; attribute
A class instance is created by calling a class object (see above). A class
instance has a namespace implemented as a dictionary which is the first place
in which attribute references are searched. When an attribute is not found
there, and the instance's class has an attribute by that name, the search
continues with the class attributes. If a class attribute is found that is a
user-defined function object, it is transformed into an instance method
object whose :attr:`__self__` attribute is the instance. Static method and
class method objects are also transformed; see above under "Classes". See
section :ref:`descriptors` for another way in which attributes of a class
retrieved via its instances may differ from the objects actually stored in
the class's :attr:`__dict__`. If no class attribute is found, and the
object's class has a :meth:`__getattr__` method, that is called to satisfy
the lookup.
.. index:: triple: class instance; attribute; assignment
Attribute assignments and deletions update the instance's dictionary, never a
class's dictionary. If the class has a :meth:`__setattr__` or
:meth:`__delattr__` method, this is called instead of updating the instance
dictionary directly.
.. index::
object: numeric
object: sequence
object: mapping
Class instances can pretend to be numbers, sequences, or mappings if they have
methods with certain special names. See section :ref:`specialnames`.
.. index::
single: __dict__ (instance attribute)
single: __class__ (instance attribute)
Special attributes: :attr:`__dict__` is the attribute dictionary;
:attr:`__class__` is the instance's class.
I/O objects (also known as file objects)
.. index::
builtin: open
module: io
single: popen() (in module os)
single: makefile() (socket method)
single: sys.stdin
single: sys.stdout
single: sys.stderr
single: stdio
single: stdin (in module sys)
single: stdout (in module sys)
single: stderr (in module sys)
A :term:`file object` represents an open file. Various shortcuts are
available to create file objects: the :func:`open` built-in function, and
also :func:`os.popen`, :func:`os.fdopen`, and the :meth:`makefile` method
of socket objects (and perhaps by other functions or methods provided
by extension modules).
The objects ``sys.stdin``, ``sys.stdout`` and ``sys.stderr`` are
initialized to file objects corresponding to the interpreter's standard
input, output and error streams; they are all open in text mode and
therefore follow the interface defined by the :class:`io.TextIOBase`
abstract class.
Internal types
.. index::
single: internal type
single: types, internal
A few types used internally by the interpreter are exposed to the user. Their
definitions may change with future versions of the interpreter, but they are
mentioned here for completeness.
Code objects
.. index::
single: bytecode
object: code
Code objects represent *byte-compiled* executable Python code, or :term:`bytecode`.
The difference between a code object and a function object is that the function
object contains an explicit reference to the function's globals (the module in
which it was defined), while a code object contains no context; also the default
argument values are stored in the function object, not in the code object
(because they represent values calculated at run-time). Unlike function
objects, code objects are immutable and contain no references (directly or
indirectly) to mutable objects.
.. index::
single: co_argcount (code object attribute)
single: co_code (code object attribute)
single: co_consts (code object attribute)
single: co_filename (code object attribute)
single: co_firstlineno (code object attribute)
single: co_flags (code object attribute)
single: co_lnotab (code object attribute)
single: co_name (code object attribute)
single: co_names (code object attribute)
single: co_nlocals (code object attribute)
single: co_stacksize (code object attribute)
single: co_varnames (code object attribute)
single: co_cellvars (code object attribute)
single: co_freevars (code object attribute)
Special read-only attributes: :attr:`co_name` gives the function name;
:attr:`co_argcount` is the number of positional arguments (including arguments
with default values); :attr:`co_nlocals` is the number of local variables used
by the function (including arguments); :attr:`co_varnames` is a tuple containing
the names of the local variables (starting with the argument names);
:attr:`co_cellvars` is a tuple containing the names of local variables that are
referenced by nested functions; :attr:`co_freevars` is a tuple containing the
names of free variables; :attr:`co_code` is a string representing the sequence
of bytecode instructions; :attr:`co_consts` is a tuple containing the literals
used by the bytecode; :attr:`co_names` is a tuple containing the names used by
the bytecode; :attr:`co_filename` is the filename from which the code was
compiled; :attr:`co_firstlineno` is the first line number of the function;
:attr:`co_lnotab` is a string encoding the mapping from bytecode offsets to
line numbers (for details see the source code of the interpreter);
:attr:`co_stacksize` is the required stack size (including local variables);
:attr:`co_flags` is an integer encoding a number of flags for the interpreter.
.. index:: object: generator
The following flag bits are defined for :attr:`co_flags`: bit ``0x04`` is set if
the function uses the ``*arguments`` syntax to accept an arbitrary number of
positional arguments; bit ``0x08`` is set if the function uses the
``**keywords`` syntax to accept arbitrary keyword arguments; bit ``0x20`` is set
if the function is a generator.
Future feature declarations (``from __future__ import division``) also use bits
in :attr:`co_flags` to indicate whether a code object was compiled with a
particular feature enabled: bit ``0x2000`` is set if the function was compiled
with future division enabled; bits ``0x10`` and ``0x1000`` were used in earlier
versions of Python.
Other bits in :attr:`co_flags` are reserved for internal use.
.. index:: single: documentation string
If a code object represents a function, the first item in :attr:`co_consts` is
the documentation string of the function, or ``None`` if undefined.
.. _frame-objects:
Frame objects
.. index:: object: frame
Frame objects represent execution frames. They may occur in traceback objects
(see below).
.. index::
single: f_back (frame attribute)
single: f_code (frame attribute)
single: f_globals (frame attribute)
single: f_locals (frame attribute)
single: f_lasti (frame attribute)
single: f_builtins (frame attribute)
Special read-only attributes: :attr:`f_back` is to the previous stack frame
(towards the caller), or ``None`` if this is the bottom stack frame;
:attr:`f_code` is the code object being executed in this frame; :attr:`f_locals`
is the dictionary used to look up local variables; :attr:`f_globals` is used for
global variables; :attr:`f_builtins` is used for built-in (intrinsic) names;
:attr:`f_lasti` gives the precise instruction (this is an index into the
bytecode string of the code object).
.. index::
single: f_trace (frame attribute)
single: f_lineno (frame attribute)
Special writable attributes: :attr:`f_trace`, if not ``None``, is a function
called at the start of each source code line (this is used by the debugger);
:attr:`f_lineno` is the current line number of the frame --- writing to this
from within a trace function jumps to the given line (only for the bottom-most
frame). A debugger can implement a Jump command (aka Set Next Statement)
by writing to f_lineno.
Frame objects support one method:
.. method:: frame.clear()
This method clears all references to local variables held by the
frame. Also, if the frame belonged to a generator, the generator
is finalized. This helps break reference cycles involving frame
objects (for example when catching an exception and storing its
traceback for later use).
:exc:`RuntimeError` is raised if the frame is currently executing.
.. versionadded:: 3.4
Traceback objects
.. index::
object: traceback
pair: stack; trace
pair: exception; handler
pair: execution; stack
single: exc_info (in module sys)
single: last_traceback (in module sys)
single: sys.exc_info
single: sys.last_traceback
Traceback objects represent a stack trace of an exception. A traceback object
is created when an exception occurs. When the search for an exception handler
unwinds the execution stack, at each unwound level a traceback object is
inserted in front of the current traceback. When an exception handler is
entered, the stack trace is made available to the program. (See section
:ref:`try`.) It is accessible as the third item of the
tuple returned by ``sys.exc_info()``. When the program contains no suitable
handler, the stack trace is written (nicely formatted) to the standard error
stream; if the interpreter is interactive, it is also made available to the user
as ``sys.last_traceback``.
.. index::
single: tb_next (traceback attribute)
single: tb_frame (traceback attribute)
single: tb_lineno (traceback attribute)
single: tb_lasti (traceback attribute)
statement: try
Special read-only attributes: :attr:`tb_next` is the next level in the stack
trace (towards the frame where the exception occurred), or ``None`` if there is
no next level; :attr:`tb_frame` points to the execution frame of the current
level; :attr:`tb_lineno` gives the line number where the exception occurred;
:attr:`tb_lasti` indicates the precise instruction. The line number and last
instruction in the traceback may differ from the line number of its frame object
if the exception occurred in a :keyword:`try` statement with no matching except
clause or with a finally clause.
Slice objects
.. index:: builtin: slice
Slice objects are used to represent slices for :meth:`__getitem__`
methods. They are also created by the built-in :func:`slice` function.
.. index::
single: start (slice object attribute)
single: stop (slice object attribute)
single: step (slice object attribute)
Special read-only attributes: :attr:`start` is the lower bound; :attr:`stop` is
the upper bound; :attr:`step` is the step value; each is ``None`` if omitted.
These attributes can have any type.
Slice objects support one method:
.. method:: slice.indices(self, length)
This method takes a single integer argument *length* and computes
information about the slice that the slice object would describe if
applied to a sequence of *length* items. It returns a tuple of three
integers; respectively these are the *start* and *stop* indices and the
*step* or stride length of the slice. Missing or out-of-bounds indices
are handled in a manner consistent with regular slices.
Static method objects
Static method objects provide a way of defeating the transformation of function
objects to method objects described above. A static method object is a wrapper
around any other object, usually a user-defined method object. When a static
method object is retrieved from a class or a class instance, the object actually
returned is the wrapped object, which is not subject to any further
transformation. Static method objects are not themselves callable, although the
objects they wrap usually are. Static method objects are created by the built-in
:func:`staticmethod` constructor.
Class method objects
A class method object, like a static method object, is a wrapper around another
object that alters the way in which that object is retrieved from classes and
class instances. The behaviour of class method objects upon such retrieval is
described above, under "User-defined methods". Class method objects are created
by the built-in :func:`classmethod` constructor.
.. _specialnames:
Special method names
====================
.. index::
pair: operator; overloading
single: __getitem__() (mapping object method)
A class can implement certain operations that are invoked by special syntax
(such as arithmetic operations or subscripting and slicing) by defining methods
with special names. This is Python's approach to :dfn:`operator overloading`,
allowing classes to define their own behavior with respect to language
operators. For instance, if a class defines a method named :meth:`__getitem__`,
and ``x`` is an instance of this class, then ``x[i]`` is roughly equivalent
to ``type(x).__getitem__(x, i)``. Except where mentioned, attempts to execute an
operation raise an exception when no appropriate method is defined (typically
:exc:`AttributeError` or :exc:`TypeError`).
When implementing a class that emulates any built-in type, it is important that
the emulation only be implemented to the degree that it makes sense for the
object being modelled. For example, some sequences may work well with retrieval
of individual elements, but extracting a slice may not make sense. (One example
of this is the :class:`NodeList` interface in the W3C's Document Object Model.)
.. _customization:
Basic customization
-------------------
.. method:: object.__new__(cls[, ...])
.. index:: pair: subclassing; immutable types
Called to create a new instance of class *cls*. :meth:`__new__` is a static
method (special-cased so you need not declare it as such) that takes the class
of which an instance was requested as its first argument. The remaining
arguments are those passed to the object constructor expression (the call to the
class). The return value of :meth:`__new__` should be the new object instance
(usually an instance of *cls*).
Typical implementations create a new instance of the class by invoking the
superclass's :meth:`__new__` method using ``super(currentclass,
cls).__new__(cls[, ...])`` with appropriate arguments and then modifying the
newly-created instance as necessary before returning it.
If :meth:`__new__` returns an instance of *cls*, then the new instance's
:meth:`__init__` method will be invoked like ``__init__(self[, ...])``, where
*self* is the new instance and the remaining arguments are the same as were
passed to :meth:`__new__`.
If :meth:`__new__` does not return an instance of *cls*, then the new instance's
:meth:`__init__` method will not be invoked.
:meth:`__new__` is intended mainly to allow subclasses of immutable types (like
int, str, or tuple) to customize instance creation. It is also commonly
overridden in custom metaclasses in order to customize class creation.
.. method:: object.__init__(self[, ...])
.. index:: pair: class; constructor
Called when the instance is created. The arguments are those passed to the
class constructor expression. If a base class has an :meth:`__init__` method,
the derived class's :meth:`__init__` method, if any, must explicitly call it to
ensure proper initialization of the base class part of the instance; for
example: ``BaseClass.__init__(self, [args...])``. As a special constraint on
constructors, no value may be returned; doing so will cause a :exc:`TypeError`
to be raised at runtime.
.. method:: object.__del__(self)
.. index::
single: destructor
statement: del
Called when the instance is about to be destroyed. This is also called a
destructor. If a base class has a :meth:`__del__` method, the derived class's
:meth:`__del__` method, if any, must explicitly call it to ensure proper
deletion of the base class part of the instance. Note that it is possible
(though not recommended!) for the :meth:`__del__` method to postpone destruction
of the instance by creating a new reference to it. It may then be called at a
later time when this new reference is deleted. It is not guaranteed that
:meth:`__del__` methods are called for objects that still exist when the
interpreter exits.
.. note::
``del x`` doesn't directly call ``x.__del__()`` --- the former decrements
the reference count for ``x`` by one, and the latter is only called when
``x``'s reference count reaches zero. Some common situations that may
prevent the reference count of an object from going to zero include:
circular references between objects (e.g., a doubly-linked list or a tree
data structure with parent and child pointers); a reference to the object
on the stack frame of a function that caught an exception (the traceback
stored in ``sys.exc_info()[2]`` keeps the stack frame alive); or a
reference to the object on the stack frame that raised an unhandled
exception in interactive mode (the traceback stored in
``sys.last_traceback`` keeps the stack frame alive). The first situation
can only be remedied by explicitly breaking the cycles; the latter two
situations can be resolved by storing ``None`` in ``sys.last_traceback``.
Circular references which are garbage are detected and cleaned up when
the cyclic garbage collector is enabled (it's on by default). Refer to the
documentation for the :mod:`gc` module for more information about this
topic.
.. warning::
Due to the precarious circumstances under which :meth:`__del__` methods are
invoked, exceptions that occur during their execution are ignored, and a warning
is printed to ``sys.stderr`` instead. Also, when :meth:`__del__` is invoked in
response to a module being deleted (e.g., when execution of the program is
done), other globals referenced by the :meth:`__del__` method may already have
been deleted or in the process of being torn down (e.g. the import
machinery shutting down). For this reason, :meth:`__del__` methods
should do the absolute
minimum needed to maintain external invariants. Starting with version 1.5,
Python guarantees that globals whose name begins with a single underscore are
deleted from their module before other globals are deleted; if no other
references to such globals exist, this may help in assuring that imported
modules are still available at the time when the :meth:`__del__` method is
called.
.. index::
single: repr() (built-in function); __repr__() (object method)
.. method:: object.__repr__(self)
Called by the :func:`repr` built-in function to compute the "official" string
representation of an object. If at all possible, this should look like a
valid Python expression that could be used to recreate an object with the
same value (given an appropriate environment). If this is not possible, a
string of the form ``<...some useful description...>`` should be returned.
The return value must be a string object. If a class defines :meth:`__repr__`
but not :meth:`__str__`, then :meth:`__repr__` is also used when an
"informal" string representation of instances of that class is required.
This is typically used for debugging, so it is important that the representation
is information-rich and unambiguous.
.. index::
single: string; __str__() (object method)
single: format() (built-in function); __str__() (object method)
single: print() (built-in function); __str__() (object method)
.. method:: object.__str__(self)
Called by :func:`str(object) <str>` and the built-in functions
:func:`format` and :func:`print` to compute the "informal" or nicely
printable string representation of an object. The return value must be a
:ref:`string <textseq>` object.
This method differs from :meth:`object.__repr__` in that there is no
expectation that :meth:`__str__` return a valid Python expression: a more
convenient or concise representation can be used.
The default implementation defined by the built-in type :class:`object`
calls :meth:`object.__repr__`.
.. XXX what about subclasses of string?
.. method:: object.__bytes__(self)
.. index:: builtin: bytes
Called by :func:`bytes` to compute a byte-string representation of an
object. This should return a ``bytes`` object.
.. index::
single: string; __format__() (object method)
pair: string; conversion
builtin: print
.. method:: object.__format__(self, format_spec)
Called by the :func:`format` built-in function (and by extension, the
:meth:`str.format` method of class :class:`str`) to produce a "formatted"
string representation of an object. The ``format_spec`` argument is
a string that contains a description of the formatting options desired.
The interpretation of the ``format_spec`` argument is up to the type
implementing :meth:`__format__`, however most classes will either
delegate formatting to one of the built-in types, or use a similar
formatting option syntax.
See :ref:`formatspec` for a description of the standard formatting syntax.
The return value must be a string object.
.. _richcmpfuncs:
.. method:: object.__lt__(self, other)
object.__le__(self, other)
object.__eq__(self, other)
object.__ne__(self, other)
object.__gt__(self, other)
object.__ge__(self, other)
.. index::
single: comparisons
These are the so-called "rich comparison" methods. The correspondence between
operator symbols and method names is as follows: ``x<y`` calls ``x.__lt__(y)``,
``x<=y`` calls ``x.__le__(y)``, ``x==y`` calls ``x.__eq__(y)``, ``x!=y`` calls
``x.__ne__(y)``, ``x>y`` calls ``x.__gt__(y)``, and ``x>=y`` calls
``x.__ge__(y)``.
A rich comparison method may return the singleton ``NotImplemented`` if it does
not implement the operation for a given pair of arguments. By convention,
``False`` and ``True`` are returned for a successful comparison. However, these
methods can return any value, so if the comparison operator is used in a Boolean
context (e.g., in the condition of an ``if`` statement), Python will call
:func:`bool` on the value to determine if the result is true or false.
There are no implied relationships among the comparison operators. The truth
of ``x==y`` does not imply that ``x!=y`` is false. Accordingly, when
defining :meth:`__eq__`, one should also define :meth:`__ne__` so that the
operators will behave as expected. See the paragraph on :meth:`__hash__` for
some important notes on creating :term:`hashable` objects which support
custom comparison operations and are usable as dictionary keys.
There are no swapped-argument versions of these methods (to be used when the
left argument does not support the operation but the right argument does);
rather, :meth:`__lt__` and :meth:`__gt__` are each other's reflection,
:meth:`__le__` and :meth:`__ge__` are each other's reflection, and
:meth:`__eq__` and :meth:`__ne__` are their own reflection.
Arguments to rich comparison methods are never coerced.
To automatically generate ordering operations from a single root operation,
see :func:`functools.total_ordering`.
.. method:: object.__hash__(self)
.. index::
object: dictionary
builtin: hash
Called by built-in function :func:`hash` and for operations on members of
hashed collections including :class:`set`, :class:`frozenset`, and
:class:`dict`. :meth:`__hash__` should return an integer. The only
required property is that objects which compare equal have the same hash
value; it is advised to somehow mix together (e.g. using exclusive or) the
hash values for the components of the object that also play a part in
comparison of objects.
.. note::
:func:`hash` truncates the value returned from an object's custom
:meth:`__hash__` method to the size of a :c:type:`Py_ssize_t`. This is
typically 8 bytes on 64-bit builds and 4 bytes on 32-bit builds. If an
object's :meth:`__hash__` must interoperate on builds of different bit
sizes, be sure to check the width on all supported builds. An easy way
to do this is with
``python -c "import sys; print(sys.hash_info.width)"``
If a class does not define an :meth:`__eq__` method it should not define a
:meth:`__hash__` operation either; if it defines :meth:`__eq__` but not
:meth:`__hash__`, its instances will not be usable as items in hashable
collections. If a class defines mutable objects and implements an
:meth:`__eq__` method, it should not implement :meth:`__hash__`, since the
implementation of hashable collections requires that a key's hash value is
immutable (if the object's hash value changes, it will be in the wrong hash
bucket).
User-defined classes have :meth:`__eq__` and :meth:`__hash__` methods
by default; with them, all objects compare unequal (except with themselves)
and ``x.__hash__()`` returns an appropriate value such that ``x == y``
implies both that ``x is y`` and ``hash(x) == hash(y)``.
A class that overrides :meth:`__eq__` and does not define :meth:`__hash__`
will have its :meth:`__hash__` implicitly set to ``None``. When the
:meth:`__hash__` method of a class is ``None``, instances of the class will
raise an appropriate :exc:`TypeError` when a program attempts to retrieve
their hash value, and will also be correctly identified as unhashable when
checking ``isinstance(obj, collections.Hashable``).
If a class that overrides :meth:`__eq__` needs to retain the implementation
of :meth:`__hash__` from a parent class, the interpreter must be told this
explicitly by setting ``__hash__ = <ParentClass>.__hash__``.
If a class that does not override :meth:`__eq__` wishes to suppress hash
support, it should include ``__hash__ = None`` in the class definition.
A class which defines its own :meth:`__hash__` that explicitly raises
a :exc:`TypeError` would be incorrectly identified as hashable by
an ``isinstance(obj, collections.Hashable)`` call.
.. note::
By default, the :meth:`__hash__` values of str, bytes and datetime
objects are "salted" with an unpredictable random value. Although they
remain constant within an individual Python process, they are not
predictable between repeated invocations of Python.
This is intended to provide protection against a denial-of-service caused
by carefully-chosen inputs that exploit the worst case performance of a
dict insertion, O(n^2) complexity. See
http://www.ocert.org/advisories/ocert-2011-003.html for details.
Changing hash values affects the iteration order of dicts, sets and
other mappings. Python has never made guarantees about this ordering
(and it typically varies between 32-bit and 64-bit builds).
See also :envvar:`PYTHONHASHSEED`.
.. versionchanged:: 3.3
Hash randomization is enabled by default.
.. method:: object.__bool__(self)
.. index:: single: __len__() (mapping object method)
Called to implement truth value testing and the built-in operation
``bool()``; should return ``False`` or ``True``. When this method is not
defined, :meth:`__len__` is called, if it is defined, and the object is
considered true if its result is nonzero. If a class defines neither
:meth:`__len__` nor :meth:`__bool__`, all its instances are considered
true.
.. _attribute-access:
Customizing attribute access
----------------------------
The following methods can be defined to customize the meaning of attribute
access (use of, assignment to, or deletion of ``x.name``) for class instances.
.. XXX explain how descriptors interfere here!
.. method:: object.__getattr__(self, name)
Called when an attribute lookup has not found the attribute in the usual places
(i.e. it is not an instance attribute nor is it found in the class tree for
``self``). ``name`` is the attribute name. This method should return the
(computed) attribute value or raise an :exc:`AttributeError` exception.
Note that if the attribute is found through the normal mechanism,
:meth:`__getattr__` is not called. (This is an intentional asymmetry between
:meth:`__getattr__` and :meth:`__setattr__`.) This is done both for efficiency
reasons and because otherwise :meth:`__getattr__` would have no way to access
other attributes of the instance. Note that at least for instance variables,
you can fake total control by not inserting any values in the instance attribute
dictionary (but instead inserting them in another object). See the
:meth:`__getattribute__` method below for a way to actually get total control
over attribute access.
.. method:: object.__getattribute__(self, name)
Called unconditionally to implement attribute accesses for instances of the
class. If the class also defines :meth:`__getattr__`, the latter will not be
called unless :meth:`__getattribute__` either calls it explicitly or raises an
:exc:`AttributeError`. This method should return the (computed) attribute value
or raise an :exc:`AttributeError` exception. In order to avoid infinite
recursion in this method, its implementation should always call the base class
method with the same name to access any attributes it needs, for example,
``object.__getattribute__(self, name)``.
.. note::
This method may still be bypassed when looking up special methods as the
result of implicit invocation via language syntax or built-in functions.
See :ref:`special-lookup`.
.. method:: object.__setattr__(self, name, value)
Called when an attribute assignment is attempted. This is called instead of
the normal mechanism (i.e. store the value in the instance dictionary).
*name* is the attribute name, *value* is the value to be assigned to it.
If :meth:`__setattr__` wants to assign to an instance attribute, it should
call the base class method with the same name, for example,
``object.__setattr__(self, name, value)``.
.. method:: object.__delattr__(self, name)
Like :meth:`__setattr__` but for attribute deletion instead of assignment. This
should only be implemented if ``del obj.name`` is meaningful for the object.
.. method:: object.__dir__(self)
Called when :func:`dir` is called on the object. A sequence must be
returned. :func:`dir` converts the returned sequence to a list and sorts it.
.. _descriptors:
Implementing Descriptors
^^^^^^^^^^^^^^^^^^^^^^^^
The following methods only apply when an instance of the class containing the
method (a so-called *descriptor* class) appears in an *owner* class (the
descriptor must be in either the owner's class dictionary or in the class
dictionary for one of its parents). In the examples below, "the attribute"
refers to the attribute whose name is the key of the property in the owner
class' :attr:`__dict__`.
.. method:: object.__get__(self, instance, owner)
Called to get the attribute of the owner class (class attribute access) or of an
instance of that class (instance attribute access). *owner* is always the owner
class, while *instance* is the instance that the attribute was accessed through,
or ``None`` when the attribute is accessed through the *owner*. This method
should return the (computed) attribute value or raise an :exc:`AttributeError`
exception.
.. method:: object.__set__(self, instance, value)
Called to set the attribute on an instance *instance* of the owner class to a
new value, *value*.
.. method:: object.__delete__(self, instance)
Called to delete the attribute on an instance *instance* of the owner class.
.. _descriptor-invocation:
Invoking Descriptors
^^^^^^^^^^^^^^^^^^^^
In general, a descriptor is an object attribute with "binding behavior", one
whose attribute access has been overridden by methods in the descriptor
protocol: :meth:`__get__`, :meth:`__set__`, and :meth:`__delete__`. If any of
those methods are defined for an object, it is said to be a descriptor.
The default behavior for attribute access is to get, set, or delete the
attribute from an object's dictionary. For instance, ``a.x`` has a lookup chain
starting with ``a.__dict__['x']``, then ``type(a).__dict__['x']``, and
continuing through the base classes of ``type(a)`` excluding metaclasses.
However, if the looked-up value is an object defining one of the descriptor
methods, then Python may override the default behavior and invoke the descriptor
method instead. Where this occurs in the precedence chain depends on which
descriptor methods were defined and how they were called.
The starting point for descriptor invocation is a binding, ``a.x``. How the
arguments are assembled depends on ``a``:
Direct Call
The simplest and least common call is when user code directly invokes a
descriptor method: ``x.__get__(a)``.
Instance Binding
If binding to an object instance, ``a.x`` is transformed into the call:
``type(a).__dict__['x'].__get__(a, type(a))``.
Class Binding
If binding to a class, ``A.x`` is transformed into the call:
``A.__dict__['x'].__get__(None, A)``.
Super Binding
If ``a`` is an instance of :class:`super`, then the binding ``super(B,
obj).m()`` searches ``obj.__class__.__mro__`` for the base class ``A``
immediately preceding ``B`` and then invokes the descriptor with the call:
``A.__dict__['m'].__get__(obj, obj.__class__)``.
For instance bindings, the precedence of descriptor invocation depends on the
which descriptor methods are defined. A descriptor can define any combination
of :meth:`__get__`, :meth:`__set__` and :meth:`__delete__`. If it does not
define :meth:`__get__`, then accessing the attribute will return the descriptor
object itself unless there is a value in the object's instance dictionary. If
the descriptor defines :meth:`__set__` and/or :meth:`__delete__`, it is a data
descriptor; if it defines neither, it is a non-data descriptor. Normally, data
descriptors define both :meth:`__get__` and :meth:`__set__`, while non-data
descriptors have just the :meth:`__get__` method. Data descriptors with
:meth:`__set__` and :meth:`__get__` defined always override a redefinition in an
instance dictionary. In contrast, non-data descriptors can be overridden by
instances.
Python methods (including :func:`staticmethod` and :func:`classmethod`) are
implemented as non-data descriptors. Accordingly, instances can redefine and
override methods. This allows individual instances to acquire behaviors that
differ from other instances of the same class.
The :func:`property` function is implemented as a data descriptor. Accordingly,
instances cannot override the behavior of a property.
.. _slots:
__slots__
^^^^^^^^^
By default, instances of classes have a dictionary for attribute storage. This
wastes space for objects having very few instance variables. The space
consumption can become acute when creating large numbers of instances.
The default can be overridden by defining *__slots__* in a class definition.
The *__slots__* declaration takes a sequence of instance variables and reserves
just enough space in each instance to hold a value for each variable. Space is
saved because *__dict__* is not created for each instance.
.. data:: object.__slots__
This class variable can be assigned a string, iterable, or sequence of
strings with variable names used by instances. If defined in a
class, *__slots__* reserves space for the declared variables and prevents the
automatic creation of *__dict__* and *__weakref__* for each instance.
Notes on using *__slots__*
""""""""""""""""""""""""""
* When inheriting from a class without *__slots__*, the *__dict__* attribute of
that class will always be accessible, so a *__slots__* definition in the
subclass is meaningless.
* Without a *__dict__* variable, instances cannot be assigned new variables not
listed in the *__slots__* definition. Attempts to assign to an unlisted
variable name raises :exc:`AttributeError`. If dynamic assignment of new
variables is desired, then add ``'__dict__'`` to the sequence of strings in
the *__slots__* declaration.
* Without a *__weakref__* variable for each instance, classes defining
*__slots__* do not support weak references to its instances. If weak reference
support is needed, then add ``'__weakref__'`` to the sequence of strings in the
*__slots__* declaration.
* *__slots__* are implemented at the class level by creating descriptors
(:ref:`descriptors`) for each variable name. As a result, class attributes
cannot be used to set default values for instance variables defined by
*__slots__*; otherwise, the class attribute would overwrite the descriptor
assignment.
* The action of a *__slots__* declaration is limited to the class where it is
defined. As a result, subclasses will have a *__dict__* unless they also define
*__slots__* (which must only contain names of any *additional* slots).
* If a class defines a slot also defined in a base class, the instance variable
defined by the base class slot is inaccessible (except by retrieving its
descriptor directly from the base class). This renders the meaning of the
program undefined. In the future, a check may be added to prevent this.
* Nonempty *__slots__* does not work for classes derived from "variable-length"
built-in types such as :class:`int`, :class:`str` and :class:`tuple`.
* Any non-string iterable may be assigned to *__slots__*. Mappings may also be
used; however, in the future, special meaning may be assigned to the values
corresponding to each key.
* *__class__* assignment works only if both classes have the same *__slots__*.
.. _metaclasses:
Customizing class creation
--------------------------
By default, classes are constructed using :func:`type`. The class body is
executed in a new namespace and the class name is bound locally to the
result of ``type(name, bases, namespace)``.
The class creation process can be customised by passing the ``metaclass``
keyword argument in the class definition line, or by inheriting from an
existing class that included such an argument. In the following example,
both ``MyClass`` and ``MySubclass`` are instances of ``Meta``::
class Meta(type):
pass
class MyClass(metaclass=Meta):
pass
class MySubclass(MyClass):
pass
Any other keyword arguments that are specified in the class definition are
passed through to all metaclass operations described below.
When a class definition is executed, the following steps occur:
* the appropriate metaclass is determined
* the class namespace is prepared
* the class body is executed
* the class object is created
Determining the appropriate metaclass
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The appropriate metaclass for a class definition is determined as follows:
* if no bases and no explicit metaclass are given, then :func:`type` is used
* if an explicit metaclass is given and it is *not* an instance of
:func:`type`, then it is used directly as the metaclass
* if an instance of :func:`type` is given as the explicit metaclass, or
bases are defined, then the most derived metaclass is used
The most derived metaclass is selected from the explicitly specified
metaclass (if any) and the metaclasses (i.e. ``type(cls)``) of all specified
base classes. The most derived metaclass is one which is a subtype of *all*
of these candidate metaclasses. If none of the candidate metaclasses meets
that criterion, then the class definition will fail with ``TypeError``.
Preparing the class namespace
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Once the appropriate metaclass has been identified, then the class namespace
is prepared. If the metaclass has a ``__prepare__`` attribute, it is called
as ``namespace = metaclass.__prepare__(name, bases, **kwds)`` (where the
additional keyword arguments, if any, come from the class definition).
If the metaclass has no ``__prepare__`` attribute, then the class namespace
is initialised as an empty :func:`dict` instance.
.. seealso::
:pep:`3115` - Metaclasses in Python 3000
Introduced the ``__prepare__`` namespace hook
Executing the class body
^^^^^^^^^^^^^^^^^^^^^^^^
The class body is executed (approximately) as
``exec(body, globals(), namespace)``. The key difference from a normal
call to :func:`exec` is that lexical scoping allows the class body (including
any methods) to reference names from the current and outer scopes when the
class definition occurs inside a function.
However, even when the class definition occurs inside the function, methods
defined inside the class still cannot see names defined at the class scope.
Class variables must be accessed through the first parameter of instance or
class methods, and cannot be accessed at all from static methods.
Creating the class object
^^^^^^^^^^^^^^^^^^^^^^^^^
Once the class namespace has been populated by executing the class body,
the class object is created by calling
``metaclass(name, bases, namespace, **kwds)`` (the additional keywords
passed here are the same as those passed to ``__prepare__``).
This class object is the one that will be referenced by the zero-argument
form of :func:`super`. ``__class__`` is an implicit closure reference
created by the compiler if any methods in a class body refer to either
``__class__`` or ``super``. This allows the zero argument form of
:func:`super` to correctly identify the class being defined based on
lexical scoping, while the class or instance that was used to make the
current call is identified based on the first argument passed to the method.
After the class object is created, it is passed to the class decorators
included in the class definition (if any) and the resulting object is bound
in the local namespace as the defined class.
.. seealso::
:pep:`3135` - New super
Describes the implicit ``__class__`` closure reference
Metaclass example
^^^^^^^^^^^^^^^^^
The potential uses for metaclasses are boundless. Some ideas that have been
explored include logging, interface checking, automatic delegation, automatic
property creation, proxies, frameworks, and automatic resource
locking/synchronization.
Here is an example of a metaclass that uses an :class:`collections.OrderedDict`
to remember the order that class members were defined::
class OrderedClass(type):
@classmethod
def __prepare__(metacls, name, bases, **kwds):
return collections.OrderedDict()
def __new__(cls, name, bases, namespace, **kwds):
result = type.__new__(cls, name, bases, dict(namespace))
result.members = tuple(namespace)
return result
class A(metaclass=OrderedClass):
def one(self): pass
def two(self): pass
def three(self): pass
def four(self): pass
>>> A.members
('__module__', 'one', 'two', 'three', 'four')
When the class definition for *A* gets executed, the process begins with
calling the metaclass's :meth:`__prepare__` method which returns an empty
:class:`collections.OrderedDict`. That mapping records the methods and
attributes of *A* as they are defined within the body of the class statement.
Once those definitions are executed, the ordered dictionary is fully populated
and the metaclass's :meth:`__new__` method gets invoked. That method builds
the new type and it saves the ordered dictionary keys in an attribute
called ``members``.
Customizing instance and subclass checks
----------------------------------------
The following methods are used to override the default behavior of the
:func:`isinstance` and :func:`issubclass` built-in functions.
In particular, the metaclass :class:`abc.ABCMeta` implements these methods in
order to allow the addition of Abstract Base Classes (ABCs) as "virtual base
classes" to any class or type (including built-in types), including other
ABCs.
.. method:: class.__instancecheck__(self, instance)
Return true if *instance* should be considered a (direct or indirect)
instance of *class*. If defined, called to implement ``isinstance(instance,
class)``.
.. method:: class.__subclasscheck__(self, subclass)
Return true if *subclass* should be considered a (direct or indirect)
subclass of *class*. If defined, called to implement ``issubclass(subclass,
class)``.
Note that these methods are looked up on the type (metaclass) of a class. They
cannot be defined as class methods in the actual class. This is consistent with
the lookup of special methods that are called on instances, only in this
case the instance is itself a class.
.. seealso::
:pep:`3119` - Introducing Abstract Base Classes
Includes the specification for customizing :func:`isinstance` and
:func:`issubclass` behavior through :meth:`__instancecheck__` and
:meth:`__subclasscheck__`, with motivation for this functionality in the
context of adding Abstract Base Classes (see the :mod:`abc` module) to the
language.
.. _callable-types:
Emulating callable objects
--------------------------
.. method:: object.__call__(self[, args...])
.. index:: pair: call; instance
Called when the instance is "called" as a function; if this method is defined,
``x(arg1, arg2, ...)`` is a shorthand for ``x.__call__(arg1, arg2, ...)``.
.. _sequence-types:
Emulating container types
-------------------------
The following methods can be defined to implement container objects. Containers
usually are sequences (such as lists or tuples) or mappings (like dictionaries),
but can represent other containers as well. The first set of methods is used
either to emulate a sequence or to emulate a mapping; the difference is that for
a sequence, the allowable keys should be the integers *k* for which ``0 <= k <
N`` where *N* is the length of the sequence, or slice objects, which define a
range of items. It is also recommended that mappings provide the methods
:meth:`keys`, :meth:`values`, :meth:`items`, :meth:`get`, :meth:`clear`,
:meth:`setdefault`, :meth:`pop`, :meth:`popitem`, :meth:`copy`, and
:meth:`update` behaving similar to those for Python's standard dictionary
objects. The :mod:`collections` module provides a :class:`MutableMapping`
abstract base class to help create those methods from a base set of
:meth:`__getitem__`, :meth:`__setitem__`, :meth:`__delitem__`, and :meth:`keys`.
Mutable sequences should provide methods :meth:`append`, :meth:`count`,
:meth:`index`, :meth:`extend`, :meth:`insert`, :meth:`pop`, :meth:`remove`,
:meth:`reverse` and :meth:`sort`, like Python standard list objects. Finally,
sequence types should implement addition (meaning concatenation) and
multiplication (meaning repetition) by defining the methods :meth:`__add__`,
:meth:`__radd__`, :meth:`__iadd__`, :meth:`__mul__`, :meth:`__rmul__` and
:meth:`__imul__` described below; they should not define other numerical
operators. It is recommended that both mappings and sequences implement the
:meth:`__contains__` method to allow efficient use of the ``in`` operator; for
mappings, ``in`` should search the mapping's keys; for sequences, it should
search through the values. It is further recommended that both mappings and
sequences implement the :meth:`__iter__` method to allow efficient iteration
through the container; for mappings, :meth:`__iter__` should be the same as
:meth:`keys`; for sequences, it should iterate through the values.
.. method:: object.__len__(self)
.. index::
builtin: len
single: __bool__() (object method)
Called to implement the built-in function :func:`len`. Should return the length
of the object, an integer ``>=`` 0. Also, an object that doesn't define a
:meth:`__bool__` method and whose :meth:`__len__` method returns zero is
considered to be false in a Boolean context.
.. method:: object.__length_hint__(self)
Called to implement :func:`operator.length_hint`. Should return an estimated
length for the object (which may be greater or less than the actual length).
The length must be an integer ``>=`` 0. This method is purely an
optimization and is never required for correctness.
.. versionadded:: 3.4
.. note::
Slicing is done exclusively with the following three methods. A call like ::
a[1:2] = b
is translated to ::
a[slice(1, 2, None)] = b
and so forth. Missing slice items are always filled in with ``None``.
.. method:: object.__getitem__(self, key)
.. index:: object: slice
Called to implement evaluation of ``self[key]``. For sequence types, the
accepted keys should be integers and slice objects. Note that the special
interpretation of negative indexes (if the class wishes to emulate a sequence
type) is up to the :meth:`__getitem__` method. If *key* is of an inappropriate
type, :exc:`TypeError` may be raised; if of a value outside the set of indexes
for the sequence (after any special interpretation of negative values),
:exc:`IndexError` should be raised. For mapping types, if *key* is missing (not
in the container), :exc:`KeyError` should be raised.
.. note::
:keyword:`for` loops expect that an :exc:`IndexError` will be raised for illegal
indexes to allow proper detection of the end of the sequence.
.. method:: object.__setitem__(self, key, value)
Called to implement assignment to ``self[key]``. Same note as for
:meth:`__getitem__`. This should only be implemented for mappings if the
objects support changes to the values for keys, or if new keys can be added, or
for sequences if elements can be replaced. The same exceptions should be raised
for improper *key* values as for the :meth:`__getitem__` method.
.. method:: object.__delitem__(self, key)
Called to implement deletion of ``self[key]``. Same note as for
:meth:`__getitem__`. This should only be implemented for mappings if the
objects support removal of keys, or for sequences if elements can be removed
from the sequence. The same exceptions should be raised for improper *key*
values as for the :meth:`__getitem__` method.
.. method:: object.__iter__(self)
This method is called when an iterator is required for a container. This method
should return a new iterator object that can iterate over all the objects in the
container. For mappings, it should iterate over the keys of the container, and
should also be made available as the method :meth:`keys`.
Iterator objects also need to implement this method; they are required to return
themselves. For more information on iterator objects, see :ref:`typeiter`.
.. method:: object.__reversed__(self)
Called (if present) by the :func:`reversed` built-in to implement
reverse iteration. It should return a new iterator object that iterates
over all the objects in the container in reverse order.
If the :meth:`__reversed__` method is not provided, the :func:`reversed`
built-in will fall back to using the sequence protocol (:meth:`__len__` and
:meth:`__getitem__`). Objects that support the sequence protocol should
only provide :meth:`__reversed__` if they can provide an implementation
that is more efficient than the one provided by :func:`reversed`.
The membership test operators (:keyword:`in` and :keyword:`not in`) are normally
implemented as an iteration through a sequence. However, container objects can
supply the following special method with a more efficient implementation, which
also does not require the object be a sequence.
.. method:: object.__contains__(self, item)
Called to implement membership test operators. Should return true if *item*
is in *self*, false otherwise. For mapping objects, this should consider the
keys of the mapping rather than the values or the key-item pairs.
For objects that don't define :meth:`__contains__`, the membership test first
tries iteration via :meth:`__iter__`, then the old sequence iteration
protocol via :meth:`__getitem__`, see :ref:`this section in the language
reference <membership-test-details>`.
.. _numeric-types:
Emulating numeric types
-----------------------
The following methods can be defined to emulate numeric objects. Methods
corresponding to operations that are not supported by the particular kind of
number implemented (e.g., bitwise operations for non-integral numbers) should be
left undefined.
.. method:: object.__add__(self, other)
object.__sub__(self, other)
object.__mul__(self, other)
object.__truediv__(self, other)
object.__floordiv__(self, other)
object.__mod__(self, other)
object.__divmod__(self, other)
object.__pow__(self, other[, modulo])
object.__lshift__(self, other)
object.__rshift__(self, other)
object.__and__(self, other)
object.__xor__(self, other)
object.__or__(self, other)
.. index::
builtin: divmod
builtin: pow
builtin: pow
These methods are called to implement the binary arithmetic operations (``+``,
``-``, ``*``, ``/``, ``//``, ``%``, :func:`divmod`, :func:`pow`, ``**``, ``<<``,
``>>``, ``&``, ``^``, ``|``). For instance, to evaluate the expression
``x + y``, where *x* is an instance of a class that has an :meth:`__add__`
method, ``x.__add__(y)`` is called. The :meth:`__divmod__` method should be the
equivalent to using :meth:`__floordiv__` and :meth:`__mod__`; it should not be
related to :meth:`__truediv__`. Note that :meth:`__pow__` should be defined
to accept an optional third argument if the ternary version of the built-in
:func:`pow` function is to be supported.
If one of those methods does not support the operation with the supplied
arguments, it should return ``NotImplemented``.
.. method:: object.__radd__(self, other)
object.__rsub__(self, other)
object.__rmul__(self, other)
object.__rtruediv__(self, other)
object.__rfloordiv__(self, other)
object.__rmod__(self, other)
object.__rdivmod__(self, other)
object.__rpow__(self, other)
object.__rlshift__(self, other)
object.__rrshift__(self, other)
object.__rand__(self, other)
object.__rxor__(self, other)
object.__ror__(self, other)
.. index::
builtin: divmod
builtin: pow
These methods are called to implement the binary arithmetic operations (``+``,
``-``, ``*``, ``/``, ``//``, ``%``, :func:`divmod`, :func:`pow`, ``**``,
``<<``, ``>>``, ``&``, ``^``, ``|``) with reflected (swapped) operands.
These functions are only called if the left operand does not support the
corresponding operation and the operands are of different types. [#]_ For
instance, to evaluate the expression ``x - y``, where *y* is an instance of
a class that has an :meth:`__rsub__` method, ``y.__rsub__(x)`` is called if
``x.__sub__(y)`` returns *NotImplemented*.
.. index:: builtin: pow
Note that ternary :func:`pow` will not try calling :meth:`__rpow__` (the
coercion rules would become too complicated).
.. note::
If the right operand's type is a subclass of the left operand's type and that
subclass provides the reflected method for the operation, this method will be
called before the left operand's non-reflected method. This behavior allows
subclasses to override their ancestors' operations.
.. method:: object.__iadd__(self, other)
object.__isub__(self, other)
object.__imul__(self, other)
object.__itruediv__(self, other)
object.__ifloordiv__(self, other)
object.__imod__(self, other)
object.__ipow__(self, other[, modulo])
object.__ilshift__(self, other)
object.__irshift__(self, other)
object.__iand__(self, other)
object.__ixor__(self, other)
object.__ior__(self, other)
These methods are called to implement the augmented arithmetic assignments
(``+=``, ``-=``, ``*=``, ``/=``, ``//=``, ``%=``, ``**=``, ``<<=``, ``>>=``,
``&=``, ``^=``, ``|=``). These methods should attempt to do the operation
in-place (modifying *self*) and return the result (which could be, but does
not have to be, *self*). If a specific method is not defined, the augmented
assignment falls back to the normal methods. For instance, to execute the
statement ``x += y``, where *x* is an instance of a class that has an
:meth:`__iadd__` method, ``x.__iadd__(y)`` is called. If *x* is an instance
of a class that does not define a :meth:`__iadd__` method, ``x.__add__(y)``
and ``y.__radd__(x)`` are considered, as with the evaluation of ``x + y``.
.. method:: object.__neg__(self)
object.__pos__(self)
object.__abs__(self)
object.__invert__(self)
.. index:: builtin: abs
Called to implement the unary arithmetic operations (``-``, ``+``, :func:`abs`
and ``~``).
.. method:: object.__complex__(self)
object.__int__(self)
object.__float__(self)
object.__round__(self, [,n])
.. index::
builtin: complex
builtin: int
builtin: float
builtin: round
Called to implement the built-in functions :func:`complex`,
:func:`int`, :func:`float` and :func:`round`. Should return a value
of the appropriate type.
.. method:: object.__index__(self)
Called to implement :func:`operator.index`. Also called whenever Python needs
an integer object (such as in slicing, or in the built-in :func:`bin`,
:func:`hex` and :func:`oct` functions). Must return an integer.
.. _context-managers:
With Statement Context Managers
-------------------------------
A :dfn:`context manager` is an object that defines the runtime context to be
established when executing a :keyword:`with` statement. The context manager
handles the entry into, and the exit from, the desired runtime context for the
execution of the block of code. Context managers are normally invoked using the
:keyword:`with` statement (described in section :ref:`with`), but can also be
used by directly invoking their methods.
.. index::
statement: with
single: context manager
Typical uses of context managers include saving and restoring various kinds of
global state, locking and unlocking resources, closing opened files, etc.
For more information on context managers, see :ref:`typecontextmanager`.
.. method:: object.__enter__(self)
Enter the runtime context related to this object. The :keyword:`with` statement
will bind this method's return value to the target(s) specified in the
:keyword:`as` clause of the statement, if any.
.. method:: object.__exit__(self, exc_type, exc_value, traceback)
Exit the runtime context related to this object. The parameters describe the
exception that caused the context to be exited. If the context was exited
without an exception, all three arguments will be :const:`None`.
If an exception is supplied, and the method wishes to suppress the exception
(i.e., prevent it from being propagated), it should return a true value.
Otherwise, the exception will be processed normally upon exit from this method.
Note that :meth:`__exit__` methods should not reraise the passed-in exception;
this is the caller's responsibility.
.. seealso::
:pep:`0343` - The "with" statement
The specification, background, and examples for the Python :keyword:`with`
statement.
.. _special-lookup:
Special method lookup
---------------------
For custom classes, implicit invocations of special methods are only guaranteed
to work correctly if defined on an object's type, not in the object's instance
dictionary. That behaviour is the reason why the following code raises an
exception::
>>> class C:
... pass
...
>>> c = C()
>>> c.__len__ = lambda: 5
>>> len(c)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: object of type 'C' has no len()
The rationale behind this behaviour lies with a number of special methods such
as :meth:`__hash__` and :meth:`__repr__` that are implemented by all objects,
including type objects. If the implicit lookup of these methods used the
conventional lookup process, they would fail when invoked on the type object
itself::
>>> 1 .__hash__() == hash(1)
True
>>> int.__hash__() == hash(int)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: descriptor '__hash__' of 'int' object needs an argument
Incorrectly attempting to invoke an unbound method of a class in this way is
sometimes referred to as 'metaclass confusion', and is avoided by bypassing
the instance when looking up special methods::
>>> type(1).__hash__(1) == hash(1)
True
>>> type(int).__hash__(int) == hash(int)
True
In addition to bypassing any instance attributes in the interest of
correctness, implicit special method lookup generally also bypasses the
:meth:`__getattribute__` method even of the object's metaclass::
>>> class Meta(type):
... def __getattribute__(*args):
... print("Metaclass getattribute invoked")
... return type.__getattribute__(*args)
...
>>> class C(object, metaclass=Meta):
... def __len__(self):
... return 10
... def __getattribute__(*args):
... print("Class getattribute invoked")
... return object.__getattribute__(*args)
...
>>> c = C()
>>> c.__len__() # Explicit lookup via instance
Class getattribute invoked
10
>>> type(c).__len__(c) # Explicit lookup via type
Metaclass getattribute invoked
10
>>> len(c) # Implicit lookup
10
Bypassing the :meth:`__getattribute__` machinery in this fashion
provides significant scope for speed optimisations within the
interpreter, at the cost of some flexibility in the handling of
special methods (the special method *must* be set on the class
object itself in order to be consistently invoked by the interpreter).
.. rubric:: Footnotes
.. [#] It *is* possible in some cases to change an object's type, under certain
controlled conditions. It generally isn't a good idea though, since it can
lead to some very strange behaviour if it is handled incorrectly.
.. [#] For operands of the same type, it is assumed that if the non-reflected method
(such as :meth:`__add__`) fails the operation is not supported, which is why the
reflected method is not called.
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