Edit File: structures.txt
.. 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. It corresponds to the fields defined by the expansion of the ``PyObject_HEAD`` macro. .. 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. It corresponds to the fields defined by the expansion of the ``PyObject_VAR_HEAD`` macro. These macros are used in the definition of :c:type:`PyObject` and :c:type:`PyVarObject`: .. c:macro:: PyObject_HEAD This is a macro which expands to the declarations of the fields of the :c:type:`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 :c:macro:`Py_TRACE_REFS`. By default, that macro is not defined, and :c:macro:`PyObject_HEAD` expands to:: Py_ssize_t ob_refcnt; PyTypeObject *ob_type; When :c:macro:`Py_TRACE_REFS` is defined, it expands to:: PyObject *_ob_next, *_ob_prev; Py_ssize_t ob_refcnt; PyTypeObject *ob_type; .. c:macro:: PyObject_VAR_HEAD This is a macro which expands to the declarations of the fields of the :c:type:`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 :c:macro:`PyObject_HEAD` is part of the expansion, and that its own expansion varies depending on the definition of :c:macro:`Py_TRACE_REFS`. .. 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:: 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 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 :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 is typically 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. .. data:: METH_OLDARGS This calling convention is deprecated. The method must be of type :c: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. .. 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 .. 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*). .. c:function:: 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 :c:func:`PyObject_GenericGetAttr` function.