cpython/Doc/c-api/structures.rst

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.. highlightlang:: c
.. _common-structs:
Common Object Structures
========================
There are a large number of structures which are used in the definition of
object types for Python. This section describes these structures and how they
are used.
All Python objects ultimately share a small number of fields at the beginning
of the object's representation in memory. These are represented by the
:c:type:`PyObject` and :c:type:`PyVarObject` types, which are defined, in turn,
by the expansions of some macros also used, whether directly or indirectly, in
the definition of all other Python objects.
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.. c:type:: PyObject
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All object types are extensions of this type. This is a type which
contains the information Python needs to treat a pointer to an object as an
object. In a normal "release" build, it contains only the object's
reference count and a pointer to the corresponding type object.
Nothing is actually declared to be a :c:type:`PyObject`, but every pointer
to a Python object can be cast to a :c:type:`PyObject*`. Access to the
members must be done by using the macros :c:macro:`Py_REFCNT` and
:c:macro:`Py_TYPE`.
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.. c:type:: PyVarObject
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This is an extension of :c:type:`PyObject` that adds the :attr:`ob_size`
field. This is only used for objects that have some notion of *length*.
This type does not often appear in the Python/C API.
Access to the members must be done by using the macros
:c:macro:`Py_REFCNT`, :c:macro:`Py_TYPE`, and :c:macro:`Py_SIZE`.
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.. c:macro:: PyObject_HEAD
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This is a macro used when declaring new types which represent objects
without a varying length. The PyObject_HEAD macro expands to::
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PyObject ob_base;
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See documentation of :c:type:`PyObject` above.
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.. c:macro:: PyObject_VAR_HEAD
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This is a macro used when declaring new types which represent objects
with a length that varies from instance to instance.
The PyObject_VAR_HEAD macro expands to::
PyVarObject ob_base;
See documentation of :c:type:`PyVarObject` above.
.. c:macro:: Py_TYPE(o)
This macro is used to access the :attr:`ob_type` member of a Python object.
It expands to::
(((PyObject*)(o))->ob_type)
.. c:macro:: Py_REFCNT(o)
This macro is used to access the :attr:`ob_refcnt` member of a Python
object.
It expands to::
(((PyObject*)(o))->ob_refcnt)
.. c:macro:: Py_SIZE(o)
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This macro is used to access the :attr:`ob_size` member of a Python object.
It expands to::
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(((PyVarObject*)(o))->ob_size)
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.. c:macro:: PyObject_HEAD_INIT(type)
This is a macro which expands to initialization values for a new
:c:type:`PyObject` type. This macro expands to::
_PyObject_EXTRA_INIT
1, type,
.. c:macro:: PyVarObject_HEAD_INIT(type, size)
This is a macro which expands to initialization values for a new
:c:type:`PyVarObject` type, including the :attr:`ob_size` field.
This macro expands to::
_PyObject_EXTRA_INIT
1, type, size,
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.. c:type:: PyCFunction
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Type of the functions used to implement most Python callables in C.
Functions of this type take two :c:type:`PyObject\*` parameters and return
one such value. If the return value is *NULL*, an exception shall have
been set. If not *NULL*, the return value is interpreted as the return
value of the function as exposed in Python. The function must return a new
reference.
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.. c:type:: PyCFunctionWithKeywords
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Type of the functions used to implement Python callables in C that take
keyword arguments: they take three :c:type:`PyObject\*` parameters and return
one such value. See :c:type:`PyCFunction` above for the meaning of the return
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value.
.. c:type:: PyMethodDef
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Structure used to describe a method of an extension type. This structure has
four fields:
+------------------+---------------+-------------------------------+
| Field | C Type | Meaning |
+==================+===============+===============================+
| :attr:`ml_name` | const char \* | name of the method |
+------------------+---------------+-------------------------------+
| :attr:`ml_meth` | PyCFunction | pointer to the C |
| | | implementation |
+------------------+---------------+-------------------------------+
| :attr:`ml_flags` | int | flag bits indicating how the |
| | | call should be constructed |
+------------------+---------------+-------------------------------+
| :attr:`ml_doc` | const char \* | points to the contents of the |
| | | docstring |
+------------------+---------------+-------------------------------+
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The :attr:`ml_meth` is a C function pointer. The functions may be of different
types, but they always return :c:type:`PyObject\*`. If the function is not of
the :c:type:`PyCFunction`, the compiler will require a cast in the method table.
Even though :c:type:`PyCFunction` defines the first parameter as
:c:type:`PyObject\*`, it is common that the method implementation uses the
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specific C type of the *self* object.
The :attr:`ml_flags` field is a bitfield which can include the following flags.
The individual flags indicate either a calling convention or a binding
convention. Of the calling convention flags, only :const:`METH_VARARGS` and
:const:`METH_KEYWORDS` can be combined. Any of the calling convention flags
can be combined with a binding flag.
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.. data:: METH_VARARGS
This is the typical calling convention, where the methods have the type
:c:type:`PyCFunction`. The function expects two :c:type:`PyObject\*` values.
The first one is the *self* object for methods; for module functions, it is
the module object. The second parameter (often called *args*) is a tuple
object representing all arguments. This parameter is typically processed
using :c:func:`PyArg_ParseTuple` or :c:func:`PyArg_UnpackTuple`.
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.. data:: METH_KEYWORDS
Methods with these flags must be of type :c:type:`PyCFunctionWithKeywords`.
The function expects three parameters: *self*, *args*, and a dictionary of
all the keyword arguments. The flag must be combined with
:const:`METH_VARARGS`, and the parameters are typically processed using
:c:func:`PyArg_ParseTupleAndKeywords`.
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.. data:: METH_NOARGS
Methods without parameters don't need to check whether arguments are given if
they are listed with the :const:`METH_NOARGS` flag. They need to be of type
:c:type:`PyCFunction`. The first parameter is typically named *self* and will
hold a reference to the module or object instance. In all cases the second
parameter will be *NULL*.
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.. data:: METH_O
Methods with a single object argument can be listed with the :const:`METH_O`
flag, instead of invoking :c:func:`PyArg_ParseTuple` with a ``"O"`` argument.
They have the type :c:type:`PyCFunction`, with the *self* parameter, and a
:c:type:`PyObject\*` parameter representing the single argument.
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These two constants are not used to indicate the calling convention but the
binding when use with methods of classes. These may not be used for functions
defined for modules. At most one of these flags may be set for any given
method.
.. data:: METH_CLASS
.. index:: builtin: classmethod
The method will be passed the type object as the first parameter rather
than an instance of the type. This is used to create *class methods*,
similar to what is created when using the :func:`classmethod` built-in
function.
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.. data:: METH_STATIC
.. index:: builtin: staticmethod
The method will be passed *NULL* as the first parameter rather than an
instance of the type. This is used to create *static methods*, similar to
what is created when using the :func:`staticmethod` built-in function.
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One other constant controls whether a method is loaded in place of another
definition with the same method name.
.. data:: METH_COEXIST
The method will be loaded in place of existing definitions. Without
*METH_COEXIST*, the default is to skip repeated definitions. Since slot
wrappers are loaded before the method table, the existence of a
*sq_contains* slot, for example, would generate a wrapped method named
:meth:`__contains__` and preclude the loading of a corresponding
PyCFunction with the same name. With the flag defined, the PyCFunction
will be loaded in place of the wrapper object and will co-exist with the
slot. This is helpful because calls to PyCFunctions are optimized more
than wrapper object calls.
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.. c:type:: PyMemberDef
Structure which describes an attribute of a type which corresponds to a C
struct member. Its fields are:
+------------------+---------------+-------------------------------+
| Field | C Type | Meaning |
+==================+===============+===============================+
| :attr:`name` | const char \* | name of the member |
+------------------+---------------+-------------------------------+
| :attr:`!type` | int | the type of the member in the |
| | | C struct |
+------------------+---------------+-------------------------------+
| :attr:`offset` | Py_ssize_t | the offset in bytes that the |
| | | member is located on the |
| | | type's object struct |
+------------------+---------------+-------------------------------+
| :attr:`flags` | int | flag bits indicating if the |
| | | field should be read-only or |
| | | writable |
+------------------+---------------+-------------------------------+
| :attr:`doc` | const char \* | points to the contents of the |
| | | docstring |
+------------------+---------------+-------------------------------+
:attr:`!type` can be one of many ``T_`` macros corresponding to various C
types. When the member is accessed in Python, it will be converted to the
equivalent Python type.
=============== ==================
Macro name C type
=============== ==================
T_SHORT short
T_INT int
T_LONG long
T_FLOAT float
T_DOUBLE double
T_STRING const char \*
T_OBJECT PyObject \*
T_OBJECT_EX PyObject \*
T_CHAR char
T_BYTE char
Merged revisions 70712,70714,70764-70765,70769-70771,70773,70776-70777,70788-70789,70824,70828,70832,70836,70842,70851,70855,70857,70866-70872,70883,70885,70893-70894,70896-70897,70903,70905-70907,70915,70927,70933,70951,70960,70962-70964,70998,71001,71006,71008,71010-71011,71019,71037,71056,71094,71101-71103,71106,71119,71123,71149-71150,71203,71212,71214-71217,71221,71240 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r70712 | benjamin.peterson | 2009-03-30 10:15:38 -0500 (Mon, 30 Mar 2009) | 1 line don't rely on the order dict repr #5605 ........ r70714 | brett.cannon | 2009-03-30 10:20:53 -0500 (Mon, 30 Mar 2009) | 1 line Add an entry to developers.txt. ........ r70764 | martin.v.loewis | 2009-03-30 17:06:33 -0500 (Mon, 30 Mar 2009) | 2 lines Add several VM developers. ........ r70765 | georg.brandl | 2009-03-30 17:09:34 -0500 (Mon, 30 Mar 2009) | 1 line #5199: make warning about vars() assignment more visible. ........ r70769 | andrew.kuchling | 2009-03-30 17:29:53 -0500 (Mon, 30 Mar 2009) | 1 line Remove comment ........ r70770 | andrew.kuchling | 2009-03-30 17:30:20 -0500 (Mon, 30 Mar 2009) | 1 line Add several items and placeholders ........ r70771 | andrew.kuchling | 2009-03-30 17:31:11 -0500 (Mon, 30 Mar 2009) | 1 line Many edits ........ r70773 | georg.brandl | 2009-03-30 17:43:00 -0500 (Mon, 30 Mar 2009) | 1 line #5039: make it clear that the impl. note refers to CPython. ........ r70776 | andrew.kuchling | 2009-03-30 18:08:24 -0500 (Mon, 30 Mar 2009) | 1 line typo fix ........ r70777 | andrew.kuchling | 2009-03-30 18:09:46 -0500 (Mon, 30 Mar 2009) | 1 line Add more items ........ r70788 | andrew.kuchling | 2009-03-30 20:21:01 -0500 (Mon, 30 Mar 2009) | 1 line Add various items ........ r70789 | georg.brandl | 2009-03-30 20:25:15 -0500 (Mon, 30 Mar 2009) | 1 line Fix a wrong struct field assignment (docstring as closure). ........ r70824 | georg.brandl | 2009-03-31 10:43:20 -0500 (Tue, 31 Mar 2009) | 1 line #5519: remove reference to Kodos, which seems dead. ........ r70828 | georg.brandl | 2009-03-31 10:50:16 -0500 (Tue, 31 Mar 2009) | 1 line #5581: fget argument of abstractproperty is optional as well. ........ r70832 | georg.brandl | 2009-03-31 11:31:11 -0500 (Tue, 31 Mar 2009) | 1 line #1386675: specify WindowsError as the exception, because it has a winerror attribute that EnvironmentError doesnt have. ........ r70836 | georg.brandl | 2009-03-31 11:50:25 -0500 (Tue, 31 Mar 2009) | 1 line #5417: replace references to undocumented functions by ones to documented functions. ........ r70842 | georg.brandl | 2009-03-31 12:13:06 -0500 (Tue, 31 Mar 2009) | 1 line #970783: document PyObject_Generic[GS]etAttr. ........ r70851 | georg.brandl | 2009-03-31 13:26:55 -0500 (Tue, 31 Mar 2009) | 1 line #837577: note cryptic return value of spawn*e on invalid env dicts. ........ r70855 | georg.brandl | 2009-03-31 13:30:37 -0500 (Tue, 31 Mar 2009) | 1 line #5245: note that PyRun_SimpleString doesnt return on SystemExit. ........ r70857 | georg.brandl | 2009-03-31 13:33:10 -0500 (Tue, 31 Mar 2009) | 1 line #5227: note that Py_Main doesnt return on SystemExit. ........ r70866 | georg.brandl | 2009-03-31 14:06:57 -0500 (Tue, 31 Mar 2009) | 1 line #4882: document named group behavior a bit better. ........ r70867 | georg.brandl | 2009-03-31 14:10:35 -0500 (Tue, 31 Mar 2009) | 1 line #1096310: document usage of sys.__std*__ a bit better. ........ r70868 | georg.brandl | 2009-03-31 14:12:17 -0500 (Tue, 31 Mar 2009) | 1 line #5190: export make_option in __all__. ........ r70869 | georg.brandl | 2009-03-31 14:14:42 -0500 (Tue, 31 Mar 2009) | 1 line Fix-up unwanted change. ........ r70870 | georg.brandl | 2009-03-31 14:26:24 -0500 (Tue, 31 Mar 2009) | 1 line #4411: document mro() and __mro__. (I hope I got it right.) ........ r70871 | georg.brandl | 2009-03-31 14:30:56 -0500 (Tue, 31 Mar 2009) | 1 line #5618: fix typo. ........ r70872 | r.david.murray | 2009-03-31 14:31:17 -0500 (Tue, 31 Mar 2009) | 3 lines Delete out-of-date and little-known README from the test directory by consensus of devs at pycon sprint. ........ r70883 | georg.brandl | 2009-03-31 15:41:08 -0500 (Tue, 31 Mar 2009) | 1 line #1674032: return value of flag from Event.wait(). OKed by Guido. ........ r70885 | tarek.ziade | 2009-03-31 15:48:31 -0500 (Tue, 31 Mar 2009) | 1 line using log.warn for sys.stderr ........ r70893 | georg.brandl | 2009-03-31 15:56:32 -0500 (Tue, 31 Mar 2009) | 1 line #1530012: move TQS section before raw strings. ........ r70894 | benjamin.peterson | 2009-03-31 16:06:30 -0500 (Tue, 31 Mar 2009) | 1 line take the usual lock precautions around _active_limbo_lock ........ r70896 | georg.brandl | 2009-03-31 16:15:33 -0500 (Tue, 31 Mar 2009) | 1 line #5598: document DocFileSuite *args argument. ........ r70897 | benjamin.peterson | 2009-03-31 16:34:42 -0500 (Tue, 31 Mar 2009) | 1 line fix Thread.ident when it is the main thread or a dummy thread #5632 ........ r70903 | georg.brandl | 2009-03-31 16:45:18 -0500 (Tue, 31 Mar 2009) | 1 line #1676135: remove trailing slashes from --prefix argument. ........ r70905 | georg.brandl | 2009-03-31 17:03:40 -0500 (Tue, 31 Mar 2009) | 1 line #5563: more documentation for bdist_msi. ........ r70906 | georg.brandl | 2009-03-31 17:11:53 -0500 (Tue, 31 Mar 2009) | 1 line #1651995: fix _convert_ref for non-ASCII characters. ........ r70907 | georg.brandl | 2009-03-31 17:18:19 -0500 (Tue, 31 Mar 2009) | 1 line #3427: document correct return type for urlopen().info(). ........ r70915 | georg.brandl | 2009-03-31 17:40:16 -0500 (Tue, 31 Mar 2009) | 1 line #5018: remove confusing paragraph. ........ r70927 | georg.brandl | 2009-03-31 18:01:27 -0500 (Tue, 31 Mar 2009) | 1 line Dont shout to users. ........ r70933 | georg.brandl | 2009-03-31 19:04:33 -0500 (Tue, 31 Mar 2009) | 2 lines Issue #5635: Fix running test_sys with tracing enabled. ........ r70951 | georg.brandl | 2009-04-01 09:02:27 -0500 (Wed, 01 Apr 2009) | 1 line Add Maksim, who worked on several issues at the sprint. ........ r70960 | jesse.noller | 2009-04-01 11:42:19 -0500 (Wed, 01 Apr 2009) | 1 line Issue 3270: document Listener address restrictions on windows ........ r70962 | brett.cannon | 2009-04-01 12:07:16 -0500 (Wed, 01 Apr 2009) | 2 lines Ron DuPlain was given commit privileges at PyCon 2009 to work on 3to2. ........ r70963 | georg.brandl | 2009-04-01 12:46:01 -0500 (Wed, 01 Apr 2009) | 1 line #5655: fix docstring oversight. ........ r70964 | brett.cannon | 2009-04-01 12:52:13 -0500 (Wed, 01 Apr 2009) | 2 lines Paul Kippes was given commit privileges to work on 3to2. ........ r70998 | georg.brandl | 2009-04-01 16:54:21 -0500 (Wed, 01 Apr 2009) | 1 line In Pdb, stop assigning values to __builtin__._ which interferes with the one commonly installed by gettext. ........ r71001 | brett.cannon | 2009-04-01 18:01:12 -0500 (Wed, 01 Apr 2009) | 3 lines Add my initials to Misc/developers.txt. Names are now sorted by number of characters in the person's name. ........ r71006 | georg.brandl | 2009-04-01 18:32:17 -0500 (Wed, 01 Apr 2009) | 1 line Cache the f_locals dict of the current frame, since every access to frame.f_locals overrides its contents with the real locals which undoes modifications made by the debugging user. ........ r71008 | andrew.kuchling | 2009-04-01 19:02:14 -0500 (Wed, 01 Apr 2009) | 1 line Typo fix ........ r71010 | benjamin.peterson | 2009-04-01 19:11:52 -0500 (Wed, 01 Apr 2009) | 1 line fix markup ........ r71011 | benjamin.peterson | 2009-04-01 19:12:47 -0500 (Wed, 01 Apr 2009) | 1 line this should be :noindex: ........ r71019 | georg.brandl | 2009-04-01 21:00:01 -0500 (Wed, 01 Apr 2009) | 1 line Fix test_doctest, missed two assignments to curframe. ........ r71037 | r.david.murray | 2009-04-01 23:34:04 -0500 (Wed, 01 Apr 2009) | 6 lines Clarify that datetime strftime does not produce leap seconds and datetime strptime does not accept it in the strftime behavior section of the datetime docs. Closes issue 2568. ........ r71056 | georg.brandl | 2009-04-02 12:43:07 -0500 (Thu, 02 Apr 2009) | 2 lines Actually the displayhook should print the repr. ........ r71094 | vinay.sajip | 2009-04-03 05:23:18 -0500 (Fri, 03 Apr 2009) | 1 line Added warning about logging use from asynchronous signal handlers. ........ r71101 | andrew.kuchling | 2009-04-03 16:43:00 -0500 (Fri, 03 Apr 2009) | 1 line Add some items ........ r71102 | andrew.kuchling | 2009-04-03 16:44:49 -0500 (Fri, 03 Apr 2009) | 1 line Fix 'the the'; grammar fix ........ r71103 | andrew.kuchling | 2009-04-03 16:45:29 -0500 (Fri, 03 Apr 2009) | 1 line Fix 'the the' duplication ........ r71106 | vinay.sajip | 2009-04-03 16:58:16 -0500 (Fri, 03 Apr 2009) | 1 line Clarified warning about logging use from asynchronous signal handlers. ........ r71119 | raymond.hettinger | 2009-04-04 00:37:47 -0500 (Sat, 04 Apr 2009) | 1 line Add helpful link. ........ r71123 | r.david.murray | 2009-04-04 01:39:56 -0500 (Sat, 04 Apr 2009) | 2 lines Fix error in description of 'oct' (issue 5678). ........ r71149 | georg.brandl | 2009-04-04 08:42:39 -0500 (Sat, 04 Apr 2009) | 1 line #5642: clarify map() compatibility to the builtin. ........ r71150 | georg.brandl | 2009-04-04 08:45:49 -0500 (Sat, 04 Apr 2009) | 1 line #5601: clarify that webbrowser is not meant for file names. ........ r71203 | benjamin.peterson | 2009-04-04 18:46:34 -0500 (Sat, 04 Apr 2009) | 1 line note how using iter* are unsafe while mutating and document iter(dict) ........ r71212 | georg.brandl | 2009-04-05 05:24:20 -0500 (Sun, 05 Apr 2009) | 1 line #1742837: expand HTTP server docs, and fix SocketServer ones to document methods as methods, not functions. ........ r71214 | georg.brandl | 2009-04-05 05:29:57 -0500 (Sun, 05 Apr 2009) | 1 line Normalize spelling of Mac OS X. ........ r71215 | georg.brandl | 2009-04-05 05:32:26 -0500 (Sun, 05 Apr 2009) | 1 line Avoid sure signs of a diseased mind. ........ r71216 | georg.brandl | 2009-04-05 05:41:02 -0500 (Sun, 05 Apr 2009) | 1 line #1718017: document the relation of os.path and the posixpath, ntpath etc. modules better. ........ r71217 | georg.brandl | 2009-04-05 05:48:47 -0500 (Sun, 05 Apr 2009) | 1 line #1726172: dont raise an unexpected IndexError if a voidresp() call has an empty response. ........ r71221 | vinay.sajip | 2009-04-05 06:06:24 -0500 (Sun, 05 Apr 2009) | 1 line Issue #5695: Moved logging.captureWarnings() call inside with statement in WarningsTest.test_warnings. ........ r71240 | georg.brandl | 2009-04-05 09:40:06 -0500 (Sun, 05 Apr 2009) | 1 line #5370: doc update about unpickling objects with custom __getattr__ etc. methods. ........
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T_UBYTE unsigned char
T_UINT unsigned int
T_USHORT unsigned short
T_ULONG unsigned long
T_BOOL char
T_LONGLONG long long
T_ULONGLONG unsigned long long
T_PYSSIZET Py_ssize_t
=============== ==================
:c:macro:`T_OBJECT` and :c:macro:`T_OBJECT_EX` differ in that
:c:macro:`T_OBJECT` returns ``None`` if the member is *NULL* and
:c:macro:`T_OBJECT_EX` raises an :exc:`AttributeError`. Try to use
:c:macro:`T_OBJECT_EX` over :c:macro:`T_OBJECT` because :c:macro:`T_OBJECT_EX`
handles use of the :keyword:`del` statement on that attribute more correctly
than :c:macro:`T_OBJECT`.
:attr:`flags` can be ``0`` for write and read access or :c:macro:`READONLY` for
read-only access. Using :c:macro:`T_STRING` for :attr:`type` implies
:c:macro:`READONLY`. Only :c:macro:`T_OBJECT` and :c:macro:`T_OBJECT_EX`
members can be deleted. (They are set to *NULL*).
.. c:type:: PyGetSetDef
Structure to define property-like access for a type. See also description of
the :c:member:`PyTypeObject.tp_getset` slot.
+-------------+------------------+-----------------------------------+
| Field | C Type | Meaning |
+=============+==================+===================================+
| name | const char \* | attribute name |
+-------------+------------------+-----------------------------------+
| get | getter | C Function to get the attribute |
+-------------+------------------+-----------------------------------+
| set | setter | optional C function to set or |
| | | delete the attribute, if omitted |
| | | the attribute is readonly |
+-------------+------------------+-----------------------------------+
| doc | const char \* | optional docstring |
+-------------+------------------+-----------------------------------+
| closure | void \* | optional function pointer, |
| | | providing additional data for |
| | | getter and setter |
+-------------+------------------+-----------------------------------+
The ``get`` function takes one :c:type:`PyObject\*` parameter (the
instance) and a function pointer (the associated ``closure``)::
typedef PyObject *(*getter)(PyObject *, void *);
It should return a new reference on success or *NULL* with a set exception
on failure.
``set`` functions take two :c:type:`PyObject\*` parameters (the instance and
the value to be set) and a function pointer (the associated ``closure``)::
typedef int (*setter)(PyObject *, PyObject *, void *);
In case the attribute should be deleted the second parameter is *NULL*.
Should return ``0`` on success or ``-1`` with a set exception on failure.