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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
:ctype:`PyObject` and :ctype:`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.
.. ctype:: PyObject
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. It
corresponds to the fields defined by the expansion of the ``PyObject_HEAD``
macro.
.. ctype:: PyVarObject
This is an extension of :ctype:`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. It corresponds to the
fields defined by the expansion of the ``PyObject_VAR_HEAD`` macro.
These macros are used in the definition of :ctype:`PyObject` and
:ctype:`PyVarObject`:
.. cmacro:: PyObject_HEAD
This is a macro which expands to the declarations of the fields of the
:ctype:`PyObject` type; it is used when declaring new types which represent
objects without a varying length. The specific fields it expands to depend
on the definition of :cmacro:`Py_TRACE_REFS`. By default, that macro is
not defined, and :cmacro:`PyObject_HEAD` expands to::
Py_ssize_t ob_refcnt;
PyTypeObject *ob_type;
When :cmacro:`Py_TRACE_REFS` is defined, it expands to::
PyObject *_ob_next, *_ob_prev;
Py_ssize_t ob_refcnt;
PyTypeObject *ob_type;
.. cmacro:: PyObject_VAR_HEAD
This is a macro which expands to the declarations of the fields of the
:ctype:`PyVarObject` type; it is used when declaring new types which
represent objects with a length that varies from instance to instance.
This macro always expands to::
PyObject_HEAD
Py_ssize_t ob_size;
Note that :cmacro:`PyObject_HEAD` is part of the expansion, and that its own
expansion varies depending on the definition of :cmacro:`Py_TRACE_REFS`.
.. cmacro:: PyObject_HEAD_INIT(type)
This is a macro which expands to initialization values for a new
:ctype:`PyObject` type. This macro expands to::
_PyObject_EXTRA_INIT
1, type,
.. cmacro:: PyVarObject_HEAD_INIT(type, size)
This is a macro which expands to initialization values for a new
:ctype:`PyVarObject` type, including the :attr:`ob_size` field.
This macro expands to::
_PyObject_EXTRA_INIT
1, type, size,
.. ctype:: PyCFunction
Type of the functions used to implement most Python callables in C.
Functions of this type take two :ctype:`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.
.. ctype:: PyMethodDef
Structure used to describe a method of an extension type. This structure has
four fields:
+------------------+-------------+-------------------------------+
| Field | C Type | Meaning |
+==================+=============+===============================+
| :attr:`ml_name` | 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` | char \* | points to the contents of the |
| | | docstring |
+------------------+-------------+-------------------------------+
The :attr:`ml_meth` is a C function pointer. The functions may be of different
types, but they always return :ctype:`PyObject\*`. If the function is not of
the :ctype:`PyCFunction`, the compiler will require a cast in the method table.
Even though :ctype:`PyCFunction` defines the first parameter as
:ctype:`PyObject\*`, it is common that the method implementation uses a the
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 (but note that :const:`METH_KEYWORDS`
alone is equivalent to ``METH_VARARGS | METH_KEYWORDS``). Any of the calling
convention flags can be combined with a binding flag.
.. data:: METH_VARARGS
This is the typical calling convention, where the methods have the type
:ctype:`PyCFunction`. The function expects two :ctype:`PyObject\*` values.
The first one is the *self* object for methods; for module functions, it
has the value given to :cfunc:`Py_InitModule4` (or *NULL* if
:cfunc:`Py_InitModule` was used). The second parameter (often called
*args*) is a tuple object representing all arguments. This parameter is
typically processed using :cfunc:`PyArg_ParseTuple` or
:cfunc:`PyArg_UnpackTuple`.
.. data:: METH_KEYWORDS
Methods with these flags must be of type :ctype:`PyCFunctionWithKeywords`.
The function expects three parameters: *self*, *args*, and a dictionary of
all the keyword arguments. The flag is typically combined with
:const:`METH_VARARGS`, and the parameters are typically processed using
:cfunc:`PyArg_ParseTupleAndKeywords`.
.. 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
:ctype:`PyCFunction`. When used with object methods, the first parameter is
typically named ``self`` and will hold a reference to the object instance.
In all cases the second parameter will be *NULL*.
.. data:: METH_O
Methods with a single object argument can be listed with the :const:`METH_O`
flag, instead of invoking :cfunc:`PyArg_ParseTuple` with a ``"O"`` argument.
They have the type :ctype:`PyCFunction`, with the *self* parameter, and a
:ctype:`PyObject\*` parameter representing the single argument.
.. data:: METH_OLDARGS
This calling convention is deprecated. The method must be of type
:ctype:`PyCFunction`. The second argument is *NULL* if no arguments are
given, a single object if exactly one argument is given, and a tuple of
objects if more than one argument is given. There is no way for a function
using this convention to distinguish between a call with multiple arguments
and a call with a tuple as the only argument.
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.
.. versionadded:: 2.3
.. 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.
.. versionadded:: 2.3
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.
.. versionadded:: 2.4
.. ctype:: 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` | 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` | 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 char \*
T_OBJECT PyObject \*
T_OBJECT_EX PyObject \*
T_CHAR char
T_BYTE char
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
=============== ==================
:cmacro:`T_OBJECT` and :cmacro:`T_OBJECT_EX` differ in that
:cmacro:`T_OBJECT` returns ``None`` if the member is *NULL* and
:cmacro:`T_OBJECT_EX` raises an :exc:`AttributeError`. Try to use
:cmacro:`T_OBJECT_EX` over :cmacro:`T_OBJECT` because :cmacro:`T_OBJECT_EX`
handles use of the :keyword:`del` statement on that attribute more correctly
than :cmacro:`T_OBJECT`.
:attr:`flags` can be 0 for write and read access or :cmacro:`READONLY` for
read-only access. Using :cmacro:`T_STRING` for :attr:`type` implies
:cmacro:`READONLY`. Only :cmacro:`T_OBJECT` and :cmacro:`T_OBJECT_EX`
members can be deleted. (They are set to *NULL*).
.. cfunction:: PyObject* Py_FindMethod(PyMethodDef table[], PyObject *ob, char *name)
Return a bound method object for an extension type implemented in C. This
can be useful in the implementation of a :attr:`tp_getattro` or
:attr:`tp_getattr` handler that does not use the
:cfunc:`PyObject_GenericGetAttr` function.
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