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Python/C API Reference Manual |
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10.2 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 PyObject and 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.
- 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 objects reference count and a pointer to the corresponding
type object. It corresponds to the fields defined by the
expansion of the
PyObject_HEAD
macro.
- PyVarObject
-
This is an extension of PyObject that adds the
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 PyObject and
PyVarObject:
- PyObject_HEAD
-
This is a macro which expands to the declarations of the fields of
the 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
Py_TRACE_REFS. By default, that macro is not
defined, and PyObject_HEAD expands to:
int ob_refcnt;
PyTypeObject *ob_type;
When Py_TRACE_REFS is defined, it expands to:
PyObject *_ob_next, *_ob_prev;
int ob_refcnt;
PyTypeObject *ob_type;
- PyObject_VAR_HEAD
-
This is a macro which expands to the declarations of the fields of
the 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
int ob_size;
Note that PyObject_HEAD is part of the expansion, and
that it's own expansion varies depending on the definition of
Py_TRACE_REFS.
PyObject_HEAD_INIT
- PyCFunction
-
Type of the functions used to implement most Python callables in C.
Functions of this type take two 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.
- PyMethodDef
-
Structure used to describe a method of an extension type. This
structure has four fields:
Field |
C Type |
Meaning |
ml_name |
char * |
name of the method |
ml_meth |
PyCFunction |
pointer to the C implementation |
ml_flags |
int |
flag bits indicating how the call should be
constructed |
ml_doc |
char * |
points to the contents of the docstring |
The ml_meth is a C function pointer. The functions may be of
different types, but they always return PyObject*. If the
function is not of the PyCFunction, the compiler will require
a cast in the method table. Even though PyCFunction defines
the first parameter as PyObject*, it is common that the method
implementation uses a the specific C type of the self object.
The 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 METH_VARARGS and METH_KEYWORDS can be
combined (but note that METH_KEYWORDS alone is equivalent
to METH_VARARGS | METH_KEYWORDS
).
Any of the calling convention flags can be combined with a
binding flag.
- METH_VARARGS
-
This is the typical calling convention, where the methods have the
type PyCFunction. The function expects two
PyObject* values. The first one is the self object for
methods; for module functions, it has the value given to
Py_InitModule4() (or NULL if
Py_InitModule() was used). The second parameter
(often called args) is a tuple object representing all
arguments. This parameter is typically processed using
PyArg_ParseTuple() or PyArg_UnpackTuple.
- METH_KEYWORDS
-
Methods with these flags must be of type
PyCFunctionWithKeywords. The function expects three
parameters: self, args, and a dictionary of all the
keyword arguments. The flag is typically combined with
METH_VARARGS, and the parameters are typically processed
using PyArg_ParseTupleAndKeywords().
- METH_NOARGS
-
Methods without parameters don't need to check whether arguments are
given if they are listed with the METH_NOARGS flag. They
need to be of type 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.
- METH_O
-
Methods with a single object argument can be listed with the
METH_O flag, instead of invoking
PyArg_ParseTuple() with a
"O"
argument. They have
the type PyCFunction, with the self parameter, and a
PyObject* parameter representing the single argument.
- METH_OLDARGS
-
This calling convention is deprecated. The method must be of type
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.
- METH_CLASS
-
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
classmethod() built-in
function.
New in version 2.3.
- METH_STATIC
-
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
staticmethod() built-in
function.
New in version 2.3.
One other constant controls whether a method is loaded in place of
another definition with the same method name.
- 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 __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.
New in version 2.4.
PyObject* Py_FindMethod( | PyMethodDef table[],
PyObject *ob, char *name) |
-
Return value:
New reference.
Return a bound method object for an extension type implemented in
C. This can be useful in the implementation of a
tp_getattro or tp_getattr handler that does not
use the PyObject_GenericGetAttr() function.
Release 2.4.2, documentation updated on 28 September 2005.
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