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284 lines
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ReStructuredText
284 lines
12 KiB
ReStructuredText
.. highlightlang:: c
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.. _common-structs:
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Common Object Structures
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========================
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There are a large number of structures which are used in the definition of
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object types for Python. This section describes these structures and how they
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are used.
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All Python objects ultimately share a small number of fields at the beginning
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of the object's representation in memory. These are represented by the
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:c:type:`PyObject` and :c:type:`PyVarObject` types, which are defined, in turn,
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by the expansions of some macros also used, whether directly or indirectly, in
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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
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contains the information Python needs to treat a pointer to an object as an
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object. In a normal "release" build, it contains only the object's
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reference count and a pointer to the corresponding type object. It
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corresponds to the fields defined by the expansion of the ``PyObject_HEAD``
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macro.
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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`
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field. This is only used for objects that have some notion of *length*.
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This type does not often appear in the Python/C API. It corresponds to the
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fields defined by the expansion of the ``PyObject_VAR_HEAD`` macro.
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These macros are used in the definition of :c:type:`PyObject` and
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:c:type:`PyVarObject`:
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.. XXX need to document PEP 3123 changes here
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.. c:macro:: PyObject_HEAD
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This is a macro which expands to the declarations of the fields of the
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:c:type:`PyObject` type; it is used when declaring new types which represent
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objects without a varying length. The specific fields it expands to depend
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on the definition of :c:macro:`Py_TRACE_REFS`. By default, that macro is
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not defined, and :c:macro:`PyObject_HEAD` expands to::
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Py_ssize_t ob_refcnt;
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PyTypeObject *ob_type;
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When :c:macro:`Py_TRACE_REFS` is defined, it expands to::
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PyObject *_ob_next, *_ob_prev;
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Py_ssize_t ob_refcnt;
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PyTypeObject *ob_type;
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.. c:macro:: PyObject_VAR_HEAD
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This is a macro which expands to the declarations of the fields of the
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:c:type:`PyVarObject` type; it is used when declaring new types which
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represent objects with a length that varies from instance to instance.
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This macro always expands to::
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PyObject_HEAD
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Py_ssize_t ob_size;
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Note that :c:macro:`PyObject_HEAD` is part of the expansion, and that its own
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expansion varies depending on the definition of :c:macro:`Py_TRACE_REFS`.
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.. c:macro:: PyObject_HEAD_INIT(type)
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This is a macro which expands to initialization values for a new
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:c:type:`PyObject` type. This macro expands to::
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_PyObject_EXTRA_INIT
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1, type,
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.. c:macro:: PyVarObject_HEAD_INIT(type, size)
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This is a macro which expands to initialization values for a new
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:c:type:`PyVarObject` type, including the :attr:`ob_size` field.
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This macro expands to::
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_PyObject_EXTRA_INIT
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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.
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Functions of this type take two :c:type:`PyObject\*` parameters and return
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one such value. If the return value is *NULL*, an exception shall have
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been set. If not *NULL*, the return value is interpreted as the return
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value of the function as exposed in Python. The function must return a new
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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
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keyword arguments: they take three :c:type:`PyObject\*` parameters and return
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one such value. See :c:type:`PyCFunction` above for the meaning of the return
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value.
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.. c:type:: PyMethodDef
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Structure used to describe a method of an extension type. This structure has
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four fields:
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+------------------+-------------+-------------------------------+
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| Field | C Type | Meaning |
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+==================+=============+===============================+
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| :attr:`ml_name` | char \* | name of the method |
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+------------------+-------------+-------------------------------+
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| :attr:`ml_meth` | PyCFunction | pointer to the C |
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| | | implementation |
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+------------------+-------------+-------------------------------+
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| :attr:`ml_flags` | int | flag bits indicating how the |
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| | | call should be constructed |
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+------------------+-------------+-------------------------------+
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| :attr:`ml_doc` | char \* | points to the contents of the |
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| | | docstring |
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+------------------+-------------+-------------------------------+
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The :attr:`ml_meth` is a C function pointer. The functions may be of different
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types, but they always return :c:type:`PyObject\*`. If the function is not of
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the :c:type:`PyCFunction`, the compiler will require a cast in the method table.
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Even though :c:type:`PyCFunction` defines the first parameter as
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:c:type:`PyObject\*`, it is common that the method implementation uses a the
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specific C type of the *self* object.
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The :attr:`ml_flags` field is a bitfield which can include the following flags.
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The individual flags indicate either a calling convention or a binding
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convention. Of the calling convention flags, only :const:`METH_VARARGS` and
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:const:`METH_KEYWORDS` can be combined (but note that :const:`METH_KEYWORDS`
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alone is equivalent to ``METH_VARARGS | METH_KEYWORDS``). Any of the calling
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convention flags can be combined with a binding flag.
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.. data:: METH_VARARGS
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This is the typical calling convention, where the methods have the type
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:c:type:`PyCFunction`. The function expects two :c:type:`PyObject\*` values.
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The first one is the *self* object for methods; for module functions, it is
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the module object. The second parameter (often called *args*) is a tuple
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object representing all arguments. This parameter is typically processed
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using :c:func:`PyArg_ParseTuple` or :c:func:`PyArg_UnpackTuple`.
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.. data:: METH_KEYWORDS
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Methods with these flags must be of type :c:type:`PyCFunctionWithKeywords`.
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The function expects three parameters: *self*, *args*, and a dictionary of
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all the keyword arguments. The flag is typically combined with
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:const:`METH_VARARGS`, and the parameters are typically processed using
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:c:func:`PyArg_ParseTupleAndKeywords`.
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.. data:: METH_NOARGS
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Methods without parameters don't need to check whether arguments are given if
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they are listed with the :const:`METH_NOARGS` flag. They need to be of type
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:c:type:`PyCFunction`. The first parameter is typically named *self* and will
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hold a reference to the module or object instance. In all cases the second
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parameter will be *NULL*.
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.. data:: METH_O
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Methods with a single object argument can be listed with the :const:`METH_O`
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flag, instead of invoking :c:func:`PyArg_ParseTuple` with a ``"O"`` argument.
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They have the type :c:type:`PyCFunction`, with the *self* parameter, and a
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: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
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binding when use with methods of classes. These may not be used for functions
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defined for modules. At most one of these flags may be set for any given
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method.
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.. data:: METH_CLASS
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.. index:: builtin: classmethod
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The method will be passed the type object as the first parameter rather
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than an instance of the type. This is used to create *class methods*,
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similar to what is created when using the :func:`classmethod` built-in
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function.
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.. data:: METH_STATIC
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.. index:: builtin: staticmethod
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The method will be passed *NULL* as the first parameter rather than an
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instance of the type. This is used to create *static methods*, similar to
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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
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definition with the same method name.
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.. data:: METH_COEXIST
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The method will be loaded in place of existing definitions. Without
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*METH_COEXIST*, the default is to skip repeated definitions. Since slot
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wrappers are loaded before the method table, the existence of a
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*sq_contains* slot, for example, would generate a wrapped method named
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:meth:`__contains__` and preclude the loading of a corresponding
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PyCFunction with the same name. With the flag defined, the PyCFunction
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will be loaded in place of the wrapper object and will co-exist with the
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slot. This is helpful because calls to PyCFunctions are optimized more
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than wrapper object calls.
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.. c:type:: PyMemberDef
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Structure which describes an attribute of a type which corresponds to a C
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struct member. Its fields are:
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+------------------+-------------+-------------------------------+
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| Field | C Type | Meaning |
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+==================+=============+===============================+
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| :attr:`name` | char \* | name of the member |
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+------------------+-------------+-------------------------------+
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| :attr:`type` | int | the type of the member in the |
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| | | C struct |
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+------------------+-------------+-------------------------------+
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| :attr:`offset` | Py_ssize_t | the offset in bytes that the |
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| | | member is located on the |
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| | | type's object struct |
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+------------------+-------------+-------------------------------+
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| :attr:`flags` | int | flag bits indicating if the |
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| | | field should be read-only or |
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| | | writable |
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+------------------+-------------+-------------------------------+
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| :attr:`doc` | char \* | points to the contents of the |
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| | | docstring |
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+------------------+-------------+-------------------------------+
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:attr:`type` can be one of many ``T_`` macros corresponding to various C
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types. When the member is accessed in Python, it will be converted to the
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equivalent Python type.
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=============== ==================
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Macro name C type
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=============== ==================
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T_SHORT short
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T_INT int
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T_LONG long
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T_FLOAT float
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T_DOUBLE double
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T_STRING char \*
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T_OBJECT PyObject \*
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T_OBJECT_EX PyObject \*
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T_CHAR char
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T_BYTE char
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T_UBYTE unsigned char
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T_UINT unsigned int
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T_USHORT unsigned short
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T_ULONG unsigned long
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T_BOOL char
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T_LONGLONG long long
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T_ULONGLONG unsigned long long
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T_PYSSIZET Py_ssize_t
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=============== ==================
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:c:macro:`T_OBJECT` and :c:macro:`T_OBJECT_EX` differ in that
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:c:macro:`T_OBJECT` returns ``None`` if the member is *NULL* and
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:c:macro:`T_OBJECT_EX` raises an :exc:`AttributeError`. Try to use
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:c:macro:`T_OBJECT_EX` over :c:macro:`T_OBJECT` because :c:macro:`T_OBJECT_EX`
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handles use of the :keyword:`del` statement on that attribute more correctly
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than :c:macro:`T_OBJECT`.
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:attr:`flags` can be 0 for write and read access or :c:macro:`READONLY` for
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read-only access. Using :c:macro:`T_STRING` for :attr:`type` implies
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:c:macro:`READONLY`. Only :c:macro:`T_OBJECT` and :c:macro:`T_OBJECT_EX`
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members can be deleted. (They are set to *NULL*).
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