.. 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:

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:

: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 const 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. :c:macro:T_STRING data is interpreted as UTF-8. Only :c:macro:T_OBJECT and :c:macro:T_OBJECT_EX members can be deleted. (They are set to NULL).

.. c:type:: PyGetSetDef

Structure to define property-like access for a type. See also description of the :c:member:PyTypeObject.tp_getset slot.

The get function takes one :c:type:PyObject\* parameter (the instance) and a function pointer (the associated closure)::

  typedef PyObject *(*getter)(PyObject *, void *);

It should return a new reference on success or NULL with a set exception on failure.

set functions take two :c:type:PyObject\* parameters (the instance and the value to be set) and a function pointer (the associated closure)::

  typedef int (*setter)(PyObject *, PyObject *, void *);

In case the attribute should be deleted the second parameter is NULL. Should return 0 on success or -1 with a set exception on failure.