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.

Py_TYPE(o)

This macro is used to access the ob_type member of a Python object. It expands to:

(((PyObject*)(o))->ob_type)

New in version 2.6.

Py_REFCNT(o)

This macro is used to access the ob_refcnt member of a Python object. It expands to:

(((PyObject*)(o))->ob_refcnt)

New in version 2.6.

Py_SIZE(o)

This macro is used to access the ob_size member of a Python object. It expands to:

(((PyVarObject*)(o))->ob_size)

New in version 2.6.

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.
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 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. 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 is the module object. 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. 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.
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.
METH_OLDARGS
This calling convention is deprecated. The method must be of 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.

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.

New in version 2.3.

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.

New in version 2.3.

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.

New in version 2.4.

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. Try to use T_OBJECT_EX over T_OBJECT because T_OBJECT_EX handles use of the del statement on that attribute more correctly than T_OBJECT.

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).

PyGetSetDef

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

Field

C Type

Meaning

name

char *

attribute name

get

getter

C Function to get the attribute

set

setter

optional C function to set or

delete the attribute, if omitted

the attribute is readonly

doc

char *

optional docstring

closure

void *

optional function pointer,

providing additional data for

getter and setter

The get function takes one 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 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.

PyObject* Py_FindMethod(PyMethodDef table[], PyObject *ob, char *name)

Return value: New reference.

Return a bound method object for an extension type implemented in C. This can be useful in the implementation of a tp_getattro or tp_getattr handler that does not use the PyObject_GenericGetAttr() function.