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Migrate to Sphinx 1.0 C language constructs.

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Georg Brandl 2010-10-06 10:11:56 +00:00
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Doc/c-api/intro.rst
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@ -88,15 +88,15 @@ Objects, Types and Reference Counts
.. index:: object: type
Most Python/C API functions have one or more arguments as well as a return value
of type :ctype:`PyObject\*`. This type is a pointer to an opaque data type
of type :c:type:`PyObject\*`. This type is a pointer to an opaque data type
representing an arbitrary Python object. Since all Python object types are
treated the same way by the Python language in most situations (e.g.,
assignments, scope rules, and argument passing), it is only fitting that they
should be represented by a single C type. Almost all Python objects live on the
heap: you never declare an automatic or static variable of type
:ctype:`PyObject`, only pointer variables of type :ctype:`PyObject\*` can be
:c:type:`PyObject`, only pointer variables of type :c:type:`PyObject\*` can be
declared. The sole exception are the type objects; since these must never be
deallocated, they are typically static :ctype:`PyTypeObject` objects.
deallocated, they are typically static :c:type:`PyTypeObject` objects.
All Python objects (even Python integers) have a :dfn:`type` and a
:dfn:`reference count`. An object's type determines what kind of object it is
@ -127,8 +127,8 @@ that.")
single: Py_DECREF()
Reference counts are always manipulated explicitly. The normal way is to use
the macro :cfunc:`Py_INCREF` to increment an object's reference count by one,
and :cfunc:`Py_DECREF` to decrement it by one. The :cfunc:`Py_DECREF` macro
the macro :c:func:`Py_INCREF` to increment an object's reference count by one,
and :c:func:`Py_DECREF` to decrement it by one. The :c:func:`Py_DECREF` macro
is considerably more complex than the incref one, since it must check whether
the reference count becomes zero and then cause the object's deallocator to be
called. The deallocator is a function pointer contained in the object's type
@ -159,13 +159,13 @@ for a while without incrementing its reference count. Some other operation might
conceivably remove the object from the list, decrementing its reference count
and possible deallocating it. The real danger is that innocent-looking
operations may invoke arbitrary Python code which could do this; there is a code
path which allows control to flow back to the user from a :cfunc:`Py_DECREF`, so
path which allows control to flow back to the user from a :c:func:`Py_DECREF`, so
almost any operation is potentially dangerous.
A safe approach is to always use the generic operations (functions whose name
begins with ``PyObject_``, ``PyNumber_``, ``PySequence_`` or ``PyMapping_``).
These operations always increment the reference count of the object they return.
This leaves the caller with the responsibility to call :cfunc:`Py_DECREF` when
This leaves the caller with the responsibility to call :c:func:`Py_DECREF` when
they are done with the result; this soon becomes second nature.
@ -180,7 +180,7 @@ to objects (objects are not owned: they are always shared). "Owning a
reference" means being responsible for calling Py_DECREF on it when the
reference is no longer needed. Ownership can also be transferred, meaning that
the code that receives ownership of the reference then becomes responsible for
eventually decref'ing it by calling :cfunc:`Py_DECREF` or :cfunc:`Py_XDECREF`
eventually decref'ing it by calling :c:func:`Py_DECREF` or :c:func:`Py_XDECREF`
when it's no longer needed---or passing on this responsibility (usually to its
caller). When a function passes ownership of a reference on to its caller, the
caller is said to receive a *new* reference. When no ownership is transferred,
@ -198,7 +198,7 @@ responsible for it any longer.
single: PyTuple_SetItem()
Few functions steal references; the two notable exceptions are
:cfunc:`PyList_SetItem` and :cfunc:`PyTuple_SetItem`, which steal a reference
:c:func:`PyList_SetItem` and :c:func:`PyTuple_SetItem`, which steal a reference
to the item (but not to the tuple or list into which the item is put!). These
functions were designed to steal a reference because of a common idiom for
populating a tuple or list with newly created objects; for example, the code to
@ -212,21 +212,21 @@ error handling for the moment; a better way to code this is shown below)::
PyTuple_SetItem(t, 1, PyLong_FromLong(2L));
PyTuple_SetItem(t, 2, PyString_FromString("three"));
Here, :cfunc:`PyLong_FromLong` returns a new reference which is immediately
stolen by :cfunc:`PyTuple_SetItem`. When you want to keep using an object
although the reference to it will be stolen, use :cfunc:`Py_INCREF` to grab
Here, :c:func:`PyLong_FromLong` returns a new reference which is immediately
stolen by :c:func:`PyTuple_SetItem`. When you want to keep using an object
although the reference to it will be stolen, use :c:func:`Py_INCREF` to grab
another reference before calling the reference-stealing function.
Incidentally, :cfunc:`PyTuple_SetItem` is the *only* way to set tuple items;
:cfunc:`PySequence_SetItem` and :cfunc:`PyObject_SetItem` refuse to do this
Incidentally, :c:func:`PyTuple_SetItem` is the *only* way to set tuple items;
:c:func:`PySequence_SetItem` and :c:func:`PyObject_SetItem` refuse to do this
since tuples are an immutable data type. You should only use
:cfunc:`PyTuple_SetItem` for tuples that you are creating yourself.
:c:func:`PyTuple_SetItem` for tuples that you are creating yourself.
Equivalent code for populating a list can be written using :cfunc:`PyList_New`
and :cfunc:`PyList_SetItem`.
Equivalent code for populating a list can be written using :c:func:`PyList_New`
and :c:func:`PyList_SetItem`.
However, in practice, you will rarely use these ways of creating and populating
a tuple or list. There's a generic function, :cfunc:`Py_BuildValue`, that can
a tuple or list. There's a generic function, :c:func:`Py_BuildValue`, that can
create most common objects from C values, directed by a :dfn:`format string`.
For example, the above two blocks of code could be replaced by the following
(which also takes care of the error checking)::
@ -236,7 +236,7 @@ For example, the above two blocks of code could be replaced by the following
tuple = Py_BuildValue("(iis)", 1, 2, "three");
list = Py_BuildValue("[iis]", 1, 2, "three");
It is much more common to use :cfunc:`PyObject_SetItem` and friends with items
It is much more common to use :c:func:`PyObject_SetItem` and friends with items
whose references you are only borrowing, like arguments that were passed in to
the function you are writing. In that case, their behaviour regarding reference
counts is much saner, since you don't have to increment a reference count so you
@ -270,15 +270,15 @@ for that reference, many functions that return a reference to an object give
you ownership of the reference. The reason is simple: in many cases, the
returned object is created on the fly, and the reference you get is the only
reference to the object. Therefore, the generic functions that return object
references, like :cfunc:`PyObject_GetItem` and :cfunc:`PySequence_GetItem`,
references, like :c:func:`PyObject_GetItem` and :c:func:`PySequence_GetItem`,
always return a new reference (the caller becomes the owner of the reference).
It is important to realize that whether you own a reference returned by a
function depends on which function you call only --- *the plumage* (the type of
the object passed as an argument to the function) *doesn't enter into it!*
Thus, if you extract an item from a list using :cfunc:`PyList_GetItem`, you
Thus, if you extract an item from a list using :c:func:`PyList_GetItem`, you
don't own the reference --- but if you obtain the same item from the same list
using :cfunc:`PySequence_GetItem` (which happens to take exactly the same
using :c:func:`PySequence_GetItem` (which happens to take exactly the same
arguments), you do own a reference to the returned object.
.. index::
@ -286,8 +286,8 @@ arguments), you do own a reference to the returned object.
single: PySequence_GetItem()
Here is an example of how you could write a function that computes the sum of
the items in a list of integers; once using :cfunc:`PyList_GetItem`, and once
using :cfunc:`PySequence_GetItem`. ::
the items in a list of integers; once using :c:func:`PyList_GetItem`, and once
using :c:func:`PySequence_GetItem`. ::
long
sum_list(PyObject *list)
@ -340,8 +340,8 @@ Types
-----
There are few other data types that play a significant role in the Python/C
API; most are simple C types such as :ctype:`int`, :ctype:`long`,
:ctype:`double` and :ctype:`char\*`. A few structure types are used to
API; most are simple C types such as :c:type:`int`, :c:type:`long`,
:c:type:`double` and :c:type:`char\*`. A few structure types are used to
describe static tables used to list the functions exported by a module or the
data attributes of a new object type, and another is used to describe the value
of a complex number. These will be discussed together with the functions that
@ -369,7 +369,7 @@ it owns, and returns an error indicator --- usually *NULL* or ``-1``. A few
functions return a Boolean true/false result, with false indicating an error.
Very few functions return no explicit error indicator or have an ambiguous
return value, and require explicit testing for errors with
:cfunc:`PyErr_Occurred`.
:c:func:`PyErr_Occurred`.
.. index::
single: PyErr_SetString()
@ -378,11 +378,11 @@ return value, and require explicit testing for errors with
Exception state is maintained in per-thread storage (this is equivalent to
using global storage in an unthreaded application). A thread can be in one of
two states: an exception has occurred, or not. The function
:cfunc:`PyErr_Occurred` can be used to check for this: it returns a borrowed
:c:func:`PyErr_Occurred` can be used to check for this: it returns a borrowed
reference to the exception type object when an exception has occurred, and
*NULL* otherwise. There are a number of functions to set the exception state:
:cfunc:`PyErr_SetString` is the most common (though not the most general)
function to set the exception state, and :cfunc:`PyErr_Clear` clears the
:c:func:`PyErr_SetString` is the most common (though not the most general)
function to set the exception state, and :c:func:`PyErr_Clear` clears the
exception state.
The full exception state consists of three objects (all of which can be
@ -418,7 +418,7 @@ and lose important information about the exact cause of the error.
.. index:: single: sum_sequence()
A simple example of detecting exceptions and passing them on is shown in the
:cfunc:`sum_sequence` example above. It so happens that that example doesn't
:c:func:`sum_sequence` example above. It so happens that that example doesn't
need to clean up any owned references when it detects an error. The following
example function shows some error cleanup. First, to remind you why you like
Python, we show the equivalent Python code::
@ -485,10 +485,10 @@ Here is the corresponding C code, in all its glory::
single: Py_XDECREF()
This example represents an endorsed use of the ``goto`` statement in C!
It illustrates the use of :cfunc:`PyErr_ExceptionMatches` and
:cfunc:`PyErr_Clear` to handle specific exceptions, and the use of
:cfunc:`Py_XDECREF` to dispose of owned references that may be *NULL* (note the
``'X'`` in the name; :cfunc:`Py_DECREF` would crash when confronted with a
It illustrates the use of :c:func:`PyErr_ExceptionMatches` and
:c:func:`PyErr_Clear` to handle specific exceptions, and the use of
:c:func:`Py_XDECREF` to dispose of owned references that may be *NULL* (note the
``'X'`` in the name; :c:func:`Py_DECREF` would crash when confronted with a
*NULL* reference). It is important that the variables used to hold owned
references are initialized to *NULL* for this to work; likewise, the proposed
return value is initialized to ``-1`` (failure) and only set to success after
@ -514,20 +514,20 @@ interpreter can only be used after the interpreter has been initialized.
triple: module; search; path
single: path (in module sys)
The basic initialization function is :cfunc:`Py_Initialize`. This initializes
The basic initialization function is :c:func:`Py_Initialize`. This initializes
the table of loaded modules, and creates the fundamental modules
:mod:`builtins`, :mod:`__main__`, :mod:`sys`, and :mod:`exceptions`. It also
initializes the module search path (``sys.path``).
.. index:: single: PySys_SetArgvEx()
:cfunc:`Py_Initialize` does not set the "script argument list" (``sys.argv``).
:c:func:`Py_Initialize` does not set the "script argument list" (``sys.argv``).
If this variable is needed by Python code that will be executed later, it must
be set explicitly with a call to ``PySys_SetArgvEx(argc, argv, updatepath)``
after the call to :cfunc:`Py_Initialize`.
after the call to :c:func:`Py_Initialize`.
On most systems (in particular, on Unix and Windows, although the details are
slightly different), :cfunc:`Py_Initialize` calculates the module search path
slightly different), :c:func:`Py_Initialize` calculates the module search path
based upon its best guess for the location of the standard Python interpreter
executable, assuming that the Python library is found in a fixed location
relative to the Python interpreter executable. In particular, it looks for a
@ -551,22 +551,22 @@ front of the standard path by setting :envvar:`PYTHONPATH`.
single: Py_GetProgramFullPath()
The embedding application can steer the search by calling
``Py_SetProgramName(file)`` *before* calling :cfunc:`Py_Initialize`. Note that
``Py_SetProgramName(file)`` *before* calling :c:func:`Py_Initialize`. Note that
:envvar:`PYTHONHOME` still overrides this and :envvar:`PYTHONPATH` is still
inserted in front of the standard path. An application that requires total
control has to provide its own implementation of :cfunc:`Py_GetPath`,
:cfunc:`Py_GetPrefix`, :cfunc:`Py_GetExecPrefix`, and
:cfunc:`Py_GetProgramFullPath` (all defined in :file:`Modules/getpath.c`).
control has to provide its own implementation of :c:func:`Py_GetPath`,
:c:func:`Py_GetPrefix`, :c:func:`Py_GetExecPrefix`, and
:c:func:`Py_GetProgramFullPath` (all defined in :file:`Modules/getpath.c`).
.. index:: single: Py_IsInitialized()
Sometimes, it is desirable to "uninitialize" Python. For instance, the
application may want to start over (make another call to
:cfunc:`Py_Initialize`) or the application is simply done with its use of
:c:func:`Py_Initialize`) or the application is simply done with its use of
Python and wants to free memory allocated by Python. This can be accomplished
by calling :cfunc:`Py_Finalize`. The function :cfunc:`Py_IsInitialized` returns
by calling :c:func:`Py_Finalize`. The function :c:func:`Py_IsInitialized` returns
true if Python is currently in the initialized state. More information about
these functions is given in a later chapter. Notice that :cfunc:`Py_Finalize`
these functions is given in a later chapter. Notice that :c:func:`Py_Finalize`
does *not* free all memory allocated by the Python interpreter, e.g. memory
allocated by extension modules currently cannot be released.
@ -586,11 +586,11 @@ available that support tracing of reference counts, debugging the memory
allocator, or low-level profiling of the main interpreter loop. Only the most
frequently-used builds will be described in the remainder of this section.
Compiling the interpreter with the :cmacro:`Py_DEBUG` macro defined produces
what is generally meant by "a debug build" of Python. :cmacro:`Py_DEBUG` is
Compiling the interpreter with the :c:macro:`Py_DEBUG` macro defined produces
what is generally meant by "a debug build" of Python. :c:macro:`Py_DEBUG` is
enabled in the Unix build by adding :option:`--with-pydebug` to the
:file:`configure` command. It is also implied by the presence of the
not-Python-specific :cmacro:`_DEBUG` macro. When :cmacro:`Py_DEBUG` is enabled
not-Python-specific :c:macro:`_DEBUG` macro. When :c:macro:`Py_DEBUG` is enabled
in the Unix build, compiler optimization is disabled.
In addition to the reference count debugging described below, the following
@ -619,11 +619,11 @@ extra checks are performed:
There may be additional checks not mentioned here.
Defining :cmacro:`Py_TRACE_REFS` enables reference tracing. When defined, a
Defining :c:macro:`Py_TRACE_REFS` enables reference tracing. When defined, a
circular doubly linked list of active objects is maintained by adding two extra
fields to every :ctype:`PyObject`. Total allocations are tracked as well. Upon
fields to every :c:type:`PyObject`. Total allocations are tracked as well. Upon
exit, all existing references are printed. (In interactive mode this happens
after every statement run by the interpreter.) Implied by :cmacro:`Py_DEBUG`.
after every statement run by the interpreter.) Implied by :c:macro:`Py_DEBUG`.
Please refer to :file:`Misc/SpecialBuilds.txt` in the Python source distribution
for more detailed information.