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issue23591: add docs; code cleanup; more tests
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Ethan Furman
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3 changed files with 213 additions and 78 deletions
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@ -23,9 +23,9 @@ by identity, and the enumeration itself can be iterated over.
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Module Contents
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---------------
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This module defines two enumeration classes that can be used to define unique
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sets of names and values: :class:`Enum` and :class:`IntEnum`. It also defines
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one decorator, :func:`unique`.
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This module defines four enumeration classes that can be used to define unique
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sets of names and values: :class:`Enum`, :class:`IntEnum`, and
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:class:`IntFlags`. It also defines one decorator, :func:`unique`.
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.. class:: Enum
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@ -37,10 +37,23 @@ one decorator, :func:`unique`.
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Base class for creating enumerated constants that are also
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subclasses of :class:`int`.
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.. class:: IntFlag
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Base class for creating enumerated constants that can be combined using
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the bitwise operators without losing their :class:`IntFlag` membership.
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:class:`IntFlag` members are also subclasses of :class:`int`.
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.. class:: Flag
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Base class for creating enumerated constants that can be combined using
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the bitwise operations without losing their :class:`Flag` membership.
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.. function:: unique
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Enum class decorator that ensures only one name is bound to any one value.
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.. versionadded:: 3.6 ``Flag``, ``IntFlag``
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Creating an Enum
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----------------
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@ -478,7 +491,7 @@ Derived Enumerations
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IntEnum
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^^^^^^^
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A variation of :class:`Enum` is provided which is also a subclass of
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The first variation of :class:`Enum` that is provided is also a subclass of
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:class:`int`. Members of an :class:`IntEnum` can be compared to integers;
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by extension, integer enumerations of different types can also be compared
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to each other::
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@ -521,13 +534,54 @@ However, they still can't be compared to standard :class:`Enum` enumerations::
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>>> [i for i in range(Shape.square)]
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[0, 1]
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For the vast majority of code, :class:`Enum` is strongly recommended,
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since :class:`IntEnum` breaks some semantic promises of an enumeration (by
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being comparable to integers, and thus by transitivity to other
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unrelated enumerations). It should be used only in special cases where
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there's no other choice; for example, when integer constants are
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replaced with enumerations and backwards compatibility is required with code
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that still expects integers.
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IntFlag
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^^^^^^^
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The next variation of :class:`Enum` provided, :class:`IntFlag`, is also based
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on :class:`int`. The difference being :class:`IntFlag` members can be combined
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using the bitwise operators (&, \|, ^, ~) and the result is still an
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:class:`IntFlag` member. However, as the name implies, :class:`IntFlag`
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members also subclass :class:`int` and can be used wherever an :class:`int` is.
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Any operation on an :class:`IntFlag` member besides the bit-wise operations
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will lose the :class:`IntFlag` membership.
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>>> from enum import IntFlag
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>>> class Perm(IntFlag):
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... R = 4
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... W = 2
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... X = 1
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...
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>>> Perm.R | Perm.W
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<Perm.R|W: 6>
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>>> Perm.R + Perm.W
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6
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>>> RW = Perm.R | Perm.W
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>>> Perm.R in RW
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True
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.. versionadded:: 3.6
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Flag
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^^^^
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The last variation is :class:`Flag`. Like :class:`IntFlag`, :class:`Flag`
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members can be combined using the bitwise operators (^, \|, ^, ~). Unlike
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:class:`IntFlag`, they cannot be combined with, nor compared against, any
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other :class:`Flag` enumeration nor :class:`int`.
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.. versionadded:: 3.6
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.. note::
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For the majority of new code, :class:`Enum` and :class:`Flag` are strongly
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recommended, since :class:`IntEnum` and :class:`IntFlag` break some
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semantic promises of an enumeration (by being comparable to integers, and
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thus by transitivity to other unrelated enumerations). :class:`IntEnum`
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and :class:`IntFlag` should be used only in cases where :class:`Enum` and
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:class:`Flag` will not do; for example, when integer constants are replaced
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with enumerations, or for interoperability with other systems.
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Others
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@ -567,10 +621,10 @@ Some rules:
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Interesting examples
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--------------------
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While :class:`Enum` and :class:`IntEnum` are expected to cover the majority of
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use-cases, they cannot cover them all. Here are recipes for some different
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types of enumerations that can be used directly, or as examples for creating
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one's own.
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While :class:`Enum`, :class:`IntEnum`, :class:`IntFlag`, and :class:`Flag` are
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expected to cover the majority of use-cases, they cannot cover them all. Here
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are recipes for some different types of enumerations that can be used directly,
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or as examples for creating one's own.
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AutoNumber
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@ -731,55 +785,33 @@ member instances.
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Finer Points
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^^^^^^^^^^^^
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:class:`Enum` members are instances of an :class:`Enum` class, and even
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though they are accessible as `EnumClass.member`, they should not be accessed
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directly from the member as that lookup may fail or, worse, return something
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besides the :class:`Enum` member you looking for::
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Supported ``__dunder__`` names
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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>>> class FieldTypes(Enum):
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... name = 0
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... value = 1
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... size = 2
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...
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>>> FieldTypes.value.size
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<FieldTypes.size: 2>
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>>> FieldTypes.size.value
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2
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:attr:`__members__` is an :class:`OrderedDict` of ``member_name``:``member``
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items. It is only available on the class.
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.. versionchanged:: 3.5
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:meth:`__new__`, if specified, must create and return the enum members; it is
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also a very good idea to set the member's :attr:`_value_` appropriately. Once
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all the members are created it is no longer used.
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Boolean evaluation: Enum classes that are mixed with non-Enum types (such as
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:class:`int`, :class:`str`, etc.) are evaluated according to the mixed-in
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type's rules; otherwise, all members evaluate as ``True``. To make your own
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Enum's boolean evaluation depend on the member's value add the following to
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your class::
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def __bool__(self):
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return bool(self.value)
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Supported ``_sunder_`` names
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The :attr:`__members__` attribute is only available on the class.
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- ``_name_`` -- name of the member
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- ``_value_`` -- value of the member; can be set / modified in ``__new__``
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If you give your :class:`Enum` subclass extra methods, like the `Planet`_
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class above, those methods will show up in a :func:`dir` of the member,
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but not of the class::
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- ``_missing_`` -- a lookup function used when a value is not found; may be
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overridden
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- ``_order_`` -- used in Python 2/3 code to ensure member order is consistent
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(class attribute, removed during class creation)
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>>> dir(Planet)
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['EARTH', 'JUPITER', 'MARS', 'MERCURY', 'NEPTUNE', 'SATURN', 'URANUS', 'VENUS', '__class__', '__doc__', '__members__', '__module__']
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>>> dir(Planet.EARTH)
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['__class__', '__doc__', '__module__', 'name', 'surface_gravity', 'value']
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.. versionadded:: 3.6 ``_missing_``, ``_order_``
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The :meth:`__new__` method will only be used for the creation of the
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:class:`Enum` members -- after that it is replaced. Any custom :meth:`__new__`
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method must create the object and set the :attr:`_value_` attribute
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appropriately.
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If you wish to change how :class:`Enum` members are looked up you should either
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write a helper function or a :func:`classmethod` for the :class:`Enum`
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subclass.
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To help keep Python 2 / Python 3 code in sync a user-specified :attr:`_order_`,
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if provided, will be checked to ensure the actual order of the enumeration
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matches::
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To help keep Python 2 / Python 3 code in sync an :attr:`_order_` attribute can
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be provided. It will be checked against the actual order of the enumeration
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and raise an error if the two do not match::
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>>> class Color(Enum):
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... _order_ = 'red green blue'
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@ -794,4 +826,53 @@ matches::
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.. note::
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In Python 2 code the :attr:`_order_` attribute is necessary as definition
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order is lost during class creation.
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order is lost before it can be recorded.
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``Enum`` member type
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~~~~~~~~~~~~~~~~~~~~
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:class:`Enum` members are instances of an :class:`Enum` class, and even
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though they are accessible as `EnumClass.member`, they should not be accessed
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directly from the member as that lookup may fail or, worse, return something
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besides the ``Enum`` member you looking for::
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>>> class FieldTypes(Enum):
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... name = 0
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... value = 1
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... size = 2
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...
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>>> FieldTypes.value.size
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<FieldTypes.size: 2>
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>>> FieldTypes.size.value
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2
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.. versionchanged:: 3.5
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Boolean value of ``Enum`` classes and members
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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``Enum`` members that are mixed with non-Enum types (such as
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:class:`int`, :class:`str`, etc.) are evaluated according to the mixed-in
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type's rules; otherwise, all members evaluate as :data:`True`. To make your own
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Enum's boolean evaluation depend on the member's value add the following to
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your class::
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def __bool__(self):
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return bool(self.value)
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``Enum`` classes always evaluate as :data:`True`.
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``Enum`` classes with methods
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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If you give your :class:`Enum` subclass extra methods, like the `Planet`_
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class above, those methods will show up in a :func:`dir` of the member,
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but not of the class::
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>>> dir(Planet)
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['EARTH', 'JUPITER', 'MARS', 'MERCURY', 'NEPTUNE', 'SATURN', 'URANUS', 'VENUS', '__class__', '__doc__', '__members__', '__module__']
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>>> dir(Planet.EARTH)
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['__class__', '__doc__', '__module__', 'name', 'surface_gravity', 'value']
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