Base object types and macros

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. Additional macros can be found under reference counting.

type PyObject

Part of the Limited API. (Only some members are part of the stable ABI.)

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 PyObject, but every pointer to a Python object can be cast to a PyObject*.

The members must not be accessed directly; instead use macros such as Py_REFCNT and Py_TYPE.

Py_ssize_t ob_refcnt

Part of the Stable ABI.

The object’s reference count, as returned by Py_REFCNT. Do not use this field directly; instead use functions and macros such as Py_REFCNT, Py_INCREF() and Py_DecRef().

The field type may be different from Py_ssize_t, depending on build configuration and platform.

PyTypeObject *ob_type

Part of the Stable ABI.

The object’s type. Do not use this field directly; use Py_TYPE and Py_SET_TYPE() instead.

type PyVarObject

Part of the Limited API. (Only some members are part of the stable ABI.)

An extension of PyObject that adds the ob_size field. This is intended for objects that have some notion of length.

As with PyObject, the members must not be accessed directly; instead use macros such as Py_SIZE, Py_REFCNT and Py_TYPE.

Py_ssize_t ob_size

Part of the Stable ABI.

A size field, whose contents should be considered an object’s internal implementation detail.

Do not use this field directly; use Py_SIZE instead.

Object creation functions such as PyObject_NewVar() will generally set this field to the requested size (number of items). After creation, arbitrary values can be stored in ob_size using Py_SET_SIZE.

To get an object’s publicly exposed length, as returned by the Python function len(), use PyObject_Length() instead.

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 PyObject above.

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 PyVarObject above.

PyTypeObject PyBaseObject_Type

Part of the Stable ABI.

The base class of all other objects, the same as object in Python.

int Py_Is(PyObject *x, PyObject *y)

Part of the Stable ABI since version 3.10.

Test if the x object is the y object, the same as x is y in Python.

Added in version 3.10.

int Py_IsNone(PyObject *x)

Part of the Stable ABI since version 3.10.

Test if an object is the None singleton, the same as x is None in Python.

Added in version 3.10.

int Py_IsTrue(PyObject *x)

Part of the Stable ABI since version 3.10.

Test if an object is the True singleton, the same as x is True in Python.

Added in version 3.10.

int Py_IsFalse(PyObject *x)

Part of the Stable ABI since version 3.10.

Test if an object is the False singleton, the same as x is False in Python.

Added in version 3.10.

PyTypeObject *Py_TYPE(PyObject *o)

Return value: Borrowed reference. Part of the Stable ABI since version 3.14.

Get the type of the Python object o.

The returned reference is borrowed from o. Do not release it with Py_DECREF() or similar.

Changed in version 3.11: Py_TYPE() is changed to an inline static function. The parameter type is no longer const PyObject*.

int Py_IS_TYPE(PyObject *o, PyTypeObject *type)

Return non-zero if the object o type is type. Return zero otherwise. Equivalent to: Py_TYPE(o) == type.

Added in version 3.9.

void Py_SET_TYPE(PyObject *o, PyTypeObject *type)

Set the type of object o to type, without any checking or reference counting.

This is a very low-level operation. Consider instead setting the Python attribute __class__ using PyObject_SetAttrString() or similar.

Note that assigning an incompatible type can lead to undefined behavior.

If type is a heap type, the caller must create a new reference to it. Similarly, if the old type of o is a heap type, the caller must release a reference to that type.

Added in version 3.9.

Py_ssize_t Py_SIZE(PyVarObject *o)

Get the ob_size field of o.

Changed in version 3.11: Py_SIZE() is changed to an inline static function. The parameter type is no longer const PyVarObject*.

void Py_SET_SIZE(PyVarObject *o, Py_ssize_t size)

Set the ob_size field of o to size.

Added in version 3.9.

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,

Implementing functions and methods

type PyCFunction

Part of the Stable ABI.

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.

The function signature is:

PyObject *PyCFunction(PyObject *self,
                      PyObject *args);

type PyCFunctionWithKeywords

Part of the Stable ABI.

Type of the functions used to implement Python callables in C with signature METH_VARARGS | METH_KEYWORDS. The function signature is:

PyObject *PyCFunctionWithKeywords(PyObject *self,
                                  PyObject *args,
                                  PyObject *kwargs);

type PyCFunctionFast

Part of the Stable ABI since version 3.13.

Type of the functions used to implement Python callables in C with signature METH_FASTCALL. The function signature is:

PyObject *PyCFunctionFast(PyObject *self,
                          PyObject *const *args,
                          Py_ssize_t nargs);

type PyCFunctionFastWithKeywords

Part of the Stable ABI since version 3.13.

Type of the functions used to implement Python callables in C with signature METH_FASTCALL | METH_KEYWORDS. The function signature is:

PyObject *PyCFunctionFastWithKeywords(PyObject *self,
                                      PyObject *const *args,
                                      Py_ssize_t nargs,
                                      PyObject *kwnames);

type PyCMethod

Type of the functions used to implement Python callables in C with signature METH_METHOD | METH_FASTCALL | METH_KEYWORDS. The function signature is:

PyObject *PyCMethod(PyObject *self,
                    PyTypeObject *defining_class,
                    PyObject *const *args,
                    Py_ssize_t nargs,
                    PyObject *kwnames)

Added in version 3.9.

type PyMethodDef

Part of the Stable ABI (including all members).

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

const char *ml_name

Name of the method.

PyCFunction ml_meth

Pointer to the C implementation.

int ml_flags

Flags bits indicating how the call should be constructed.

const char *ml_doc

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.

There are these calling conventions:

METH_VARARGS

Part of the Stable ABI.

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

Can only be used in certain combinations with other flags: METH_VARARGS | METH_KEYWORDS, METH_FASTCALL | METH_KEYWORDS and METH_METHOD | METH_FASTCALL | METH_KEYWORDS.

METH_VARARGS | METH_KEYWORDS

Methods with these flags must be of type PyCFunctionWithKeywords. The function expects three parameters: self, args, kwargs where kwargs is a dictionary of all the keyword arguments or possibly NULL if there are no keyword arguments. The parameters are typically processed using PyArg_ParseTupleAndKeywords().

METH_FASTCALL

Part of the Stable ABI since version 3.7.

Fast calling convention supporting only positional arguments. The methods have the type PyCFunctionFast. The first parameter is self, the second parameter is a C array of PyObject* values indicating the arguments and the third parameter is the number of arguments (the length of the array).

Added in version 3.7.

Changed in version 3.10: METH_FASTCALL is now part of the stable ABI.

METH_FASTCALL | METH_KEYWORDS

Extension of METH_FASTCALL supporting also keyword arguments, with methods of type PyCFunctionFastWithKeywords. Keyword arguments are passed the same way as in the vectorcall protocol: there is an additional fourth PyObject* parameter which is a tuple representing the names of the keyword arguments (which are guaranteed to be strings) or possibly NULL if there are no keywords. The values of the keyword arguments are stored in the args array, after the positional arguments.

Added in version 3.7.

METH_METHOD

Part of the Stable ABI since version 3.7.

Can only be used in the combination with other flags: METH_METHOD | METH_FASTCALL | METH_KEYWORDS.

METH_METHOD | METH_FASTCALL | METH_KEYWORDS

Extension of METH_FASTCALL | METH_KEYWORDS supporting the defining class, that is, the class that contains the method in question. The defining class might be a superclass of Py_TYPE(self).

The method needs to be of type PyCMethod, the same as for METH_FASTCALL | METH_KEYWORDS with defining_class argument added after self.

Added in version 3.9.

METH_NOARGS

Part of the Stable ABI.

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.

The function must have 2 parameters. Since the second parameter is unused, Py_UNUSED can be used to prevent a compiler warning.

METH_O

Part of the Stable ABI.

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 used 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

Part of the Stable ABI.

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

Part of the Stable ABI.

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

Part of the Stable ABI.

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.

PyTypeObject PyCMethod_Type

The type object corresponding to Python C method objects. This is available as types.BuiltinMethodType in the Python layer.

int PyCMethod_Check(PyObject *op)

Return true if op is an instance of the PyCMethod_Type type or a subtype of it. This function always succeeds.

int PyCMethod_CheckExact(PyObject *op)

This is the same as PyCMethod_Check(), but does not account for subtypes.

PyObject *PyCMethod_New(PyMethodDef *ml, PyObject *self, PyObject *module, PyTypeObject *cls)

Return value: New reference. Part of the Stable ABI since version 3.9.

Turn ml into a Python callable object. The caller must ensure that ml outlives the callable. Typically, ml is defined as a static variable.

The self parameter will be passed as the self argument to the C function in ml->ml_meth when invoked. self can be NULL.

The callable object’s __module__ attribute can be set from the given module argument. module should be a Python string, which will be used as name of the module the function is defined in. If unavailable, it can be set to None or NULL.

See also

function.__module__

The cls parameter will be passed as the defining_class argument to the C function. Must be set if METH_METHOD is set on ml->ml_flags.

Added in version 3.9.

PyTypeObject PyCFunction_Type

Part of the Stable ABI.

The type object corresponding to Python C function objects. This is available as types.BuiltinFunctionType in the Python layer.

int PyCFunction_Check(PyObject *op)

Return true if op is an instance of the PyCFunction_Type type or a subtype of it. This function always succeeds.

int PyCFunction_CheckExact(PyObject *op)

This is the same as PyCFunction_Check(), but does not account for subtypes.

PyObject *PyCFunction_NewEx(PyMethodDef *ml, PyObject *self, PyObject *module)

Return value: New reference. Part of the Stable ABI.

Equivalent to PyCMethod_New(ml, self, module, NULL).

PyObject *PyCFunction_New(PyMethodDef *ml, PyObject *self)

Return value: New reference. Part of the Stable ABI since version 3.4.

Equivalent to PyCMethod_New(ml, self, NULL, NULL).

int PyCFunction_GetFlags(PyObject *func)

Part of the Stable ABI.

Get the function’s flags on func as they were passed to ml_flags.

If func is not a C function object, this fails with an exception. func must not be NULL.

This function returns the function’s flags on success, and -1 with an exception set on failure.

int PyCFunction_GET_FLAGS(PyObject *func)

This is the same as PyCFunction_GetFlags(), but without error or type checking.

PyCFunction PyCFunction_GetFunction(PyObject *func)

Part of the Stable ABI.

Get the function pointer on func as it was passed to ml_meth.

If func is not a C function object, this fails with an exception. func must not be NULL.

This function returns the function pointer on success, and NULL with an exception set on failure.

int PyCFunction_GET_FUNCTION(PyObject *func)

This is the same as PyCFunction_GetFunction(), but without error or type checking.

PyObject *PyCFunction_GetSelf(PyObject *func)

Part of the Stable ABI.

Get the “self” object on func. This is the object that would be passed to the first argument of a PyCFunction. For C function objects created through a PyMethodDef on a PyModuleDef, this is the resulting module object.

If func is not a C function object, this fails with an exception. func must not be NULL.

This function returns a borrowed reference to the “self” object on success, and NULL with an exception set on failure.

PyObject *PyCFunction_GET_SELF(PyObject *func)

This is the same as PyCFunction_GetSelf(), but without error or type checking.

Accessing attributes of extension types

type PyMemberDef

Part of the Stable ABI (including all members).

Structure which describes an attribute of a type which corresponds to a C struct member. When defining a class, put a NULL-terminated array of these structures in the tp_members slot.

Its fields are, in order:

const char *name

Name of the member. A NULL value marks the end of a PyMemberDef[] array.

The string should be static, no copy is made of it.

int type

The type of the member in the C struct. See Member types for the possible values.

Py_ssize_t offset

The offset in bytes that the member is located on the type’s object struct.

int flags

Zero or more of the Member flags, combined using bitwise OR.

const char *doc

The docstring, or NULL. The string should be static, no copy is made of it. Typically, it is defined using PyDoc_STR.

By default (when flags is 0), members allow both read and write access. Use the Py_READONLY flag for read-only access. Certain types, like Py_T_STRING, imply Py_READONLY. Only Py_T_OBJECT_EX (and legacy T_OBJECT) members can be deleted.

For heap-allocated types (created using PyType_FromSpec() or similar), PyMemberDef may contain a definition for the special member "__vectorcalloffset__", corresponding to tp_vectorcall_offset in type objects. This member must be defined with Py_T_PYSSIZET, and either Py_READONLY or Py_READONLY | Py_RELATIVE_OFFSET. For example:

static PyMemberDef spam_type_members[] = {
    {"__vectorcalloffset__", Py_T_PYSSIZET,
     offsetof(Spam_object, vectorcall), Py_READONLY},
    {NULL}  /* Sentinel */
};

(You may need to #include <stddef.h> for offsetof().)

The legacy offsets tp_dictoffset and tp_weaklistoffset can be defined similarly using "__dictoffset__" and "__weaklistoffset__" members, but extensions are strongly encouraged to use Py_TPFLAGS_MANAGED_DICT and Py_TPFLAGS_MANAGED_WEAKREF instead.

Changed in version 3.12: PyMemberDef is always available. Previously, it required including "structmember.h".

Changed in version 3.14: Py_RELATIVE_OFFSET is now allowed for "__vectorcalloffset__", "__dictoffset__" and "__weaklistoffset__".

PyObject *PyMember_GetOne(const char *obj_addr, struct PyMemberDef *m)

Part of the Stable ABI.

Get an attribute belonging to the object at address obj_addr. The attribute is described by PyMemberDef m. Returns NULL on error.

Changed in version 3.12: PyMember_GetOne is always available. Previously, it required including "structmember.h".

int PyMember_SetOne(char *obj_addr, struct PyMemberDef *m, PyObject *o)

Part of the Stable ABI.

Set an attribute belonging to the object at address obj_addr to object o. The attribute to set is described by PyMemberDef m. Returns 0 if successful and a negative value on failure.

Changed in version 3.12: PyMember_SetOne is always available. Previously, it required including "structmember.h".