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 PyObject and 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.

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.

PyVarObject

This is an extension of PyObject that adds the 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 PyObject and PyVarObject:

PyObject_HEAD

This is a macro which expands to the declarations of the fields of the 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 Py_TRACE_REFS. By default, that macro is not defined, and PyObject_HEAD expands to:

Py_ssize_t ob_refcnt;
PyTypeObject *ob_type;

When Py_TRACE_REFS is defined, it expands to:

PyObject *_ob_next, *_ob_prev;
Py_ssize_t ob_refcnt;
PyTypeObject *ob_type;

PyObject_VAR_HEAD

This is a macro which expands to the declarations of the fields of the 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 PyObject_HEAD is part of the expansion, and that its own expansion varies depending on the definition of Py_TRACE_REFS.

PyObject_HEAD_INIT(type)

This is a macro which expands to initialization values for a new PyObject type. This macro expands to:

_PyObject_EXTRA_INIT
1, type,

PyVarObject_HEAD_INIT(type, size)

This is a macro which expands to initialization values for a new PyVarObject type, including the ob_size field. This macro expands to:

_PyObject_EXTRA_INIT
1, type, size,

PyCFunction

Type of the functions used to implement most Python callables in C. Functions of this type take two 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.

PyCFunctionWithKeywords

Type of the functions used to implement Python callables in C that take keyword arguments: they take three PyObject* parameters and return one such value. See PyCFunction above for the meaning of the return value.

PyMethodDef

Structure used to describe a method of an extension type. This structure has four fields:

Field	C Type	Meaning
ml_name	char *	name of the method
ml_meth	PyCFunction	pointer to the C implementation
ml_flags	int	flag bits indicating how the call should be constructed
ml_doc	char *	points to the contents of the docstring

The ml_meth is a C function pointer. The functions may be of different types, but they always return PyObject*. If the function is not of the PyCFunction, the compiler will require a cast in the method table. Even though PyCFunction defines the first parameter as PyObject*, it is common that the method implementation uses a the specific C type of the self object.

The 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 METH_VARARGS and METH_KEYWORDS can be combined (but note that METH_KEYWORDS alone is equivalent to METH_VARARGS | METH_KEYWORDS). Any of the calling convention flags can be combined with a binding flag.

METH_VARARGS

This is the typical calling convention, where the methods have the type PyCFunction. The function expects two PyObject* values. The first one is the self object for methods; for module functions, it has the value given to Py_InitModule4 (or NULL if Py_InitModule was used). The second parameter (often called args) is a tuple object representing all arguments. This parameter is typically processed using PyArg_ParseTuple or PyArg_UnpackTuple.

METH_KEYWORDS

Methods with these flags must be of type PyCFunctionWithKeywords. The function expects three parameters: self, args, and a dictionary of all the keyword arguments. The flag is typically combined with METH_VARARGS, and the parameters are typically processed using PyArg_ParseTupleAndKeywords.

METH_NOARGS

Methods without parameters don’t need to check whether arguments are given if they are listed with the METH_NOARGS flag. They need to be of type 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.

METH_O

Methods with a single object argument can be listed with the METH_O flag, instead of invoking PyArg_ParseTuple with a "O" argument. They have the type PyCFunction, with the self parameter, and a 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.

METH_CLASS

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 classmethod() built-in function.

METH_STATIC

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 staticmethod() built-in function.

One other constant controls whether a method is loaded in place of another definition with the same method name.

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 __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.

PyMemberDef

Structure which describes an attribute of a type which corresponds to a C struct member. Its fields are:

Field	C Type	Meaning
name	char *	name of the member
type	int	the type of the member in the C struct
offset	Py_ssize_t	the offset in bytes that the member is located on the type’s object struct
flags	int	flag bits indicating if the field should be read-only or writable
doc	char *	points to the contents of the docstring

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

T_OBJECT and T_OBJECT_EX differ in that T_OBJECT returns None if the member is NULL and T_OBJECT_EX raises an AttributeError.

flags can be 0 for write and read access or READONLY for read-only access. Using T_STRING for type implies READONLY. Only T_OBJECT and T_OBJECT_EX members can be deleted. (They are set to NULL).