213 lines
8.8 KiB
ReStructuredText
213 lines
8.8 KiB
ReStructuredText
.. 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 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.
|
|
|
|
|
|
.. 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`.
|
|
|
|
PyObject_HEAD_INIT
|
|
|
|
|
|
.. 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
|
|
|
|
|
|
.. 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.
|