297 lines
11 KiB
ReStructuredText
297 lines
11 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
|
|
: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.
|
|
|
|
|
|
.. c:type:: 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.
|
|
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`.
|
|
|
|
|
|
.. c:type:: PyVarObject
|
|
|
|
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`.
|
|
|
|
|
|
.. c:macro:: PyObject_HEAD
|
|
|
|
This is a macro used when declaring new types which represent objects
|
|
without a varying length. The PyObject_HEAD macro expands to::
|
|
|
|
PyObject ob_base;
|
|
|
|
See documentation of :c:type:`PyObject` above.
|
|
|
|
|
|
.. c:macro:: PyObject_VAR_HEAD
|
|
|
|
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)
|
|
|
|
This macro is used to access the :attr:`ob_size` member of a Python object.
|
|
It expands to::
|
|
|
|
(((PyVarObject*)(o))->ob_size)
|
|
|
|
|
|
.. 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,
|
|
|
|
|
|
.. c:type:: PyCFunction
|
|
|
|
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.
|
|
|
|
|
|
.. c:type:: PyCFunctionWithKeywords
|
|
|
|
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
|
|
value.
|
|
|
|
|
|
.. c:type:: 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 :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
|
|
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.
|
|
|
|
|
|
.. 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`.
|
|
|
|
|
|
.. 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`.
|
|
|
|
|
|
.. 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*.
|
|
|
|
|
|
.. 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.
|
|
|
|
|
|
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.
|
|
|
|
|
|
.. 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.
|
|
|
|
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.
|
|
|
|
|
|
.. 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` | 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
|
|
=============== ==================
|
|
|
|
: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*).
|