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.

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*. Access to the members must be done by using the macros Py_REFCNT and Py_TYPE.

type PyVarObject¶: Part of the Limited API. (Only some members are part of the stable ABI.)
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. Access to the members must be done by using the macros Py_REFCNT, Py_TYPE, and Py_SIZE.

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.

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.

Return a borrowed reference.

Use the Py_SET_TYPE() function to set an object type.

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 object o type to type.

Added in version 3.9.

Py_ssize_t Py_SIZE(PyVarObject *o)¶

Get the size of the Python object o.

Use the Py_SET_SIZE() function to set an object size.

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 object o size 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¶: 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¶: 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¶: 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¶

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¶: 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.

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.

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".

Member flags¶

The following flags can be used with PyMemberDef.flags:

Py_READONLY¶: Not writable.

Py_AUDIT_READ¶: Emit an object.__getattr__ audit event before reading.

Py_RELATIVE_OFFSET¶

Indicates that the offset of this PyMemberDef entry indicates an offset from the subclass-specific data, rather than from PyObject.

Can only be used as part of Py_tp_members slot when creating a class using negative basicsize. It is mandatory in that case.

This flag is only used in PyType_Slot. When setting tp_members during class creation, Python clears it and sets PyMemberDef.offset to the offset from the PyObject struct.

Changed in version 3.10: The RESTRICTED, READ_RESTRICTED and WRITE_RESTRICTED macros available with #include "structmember.h" are deprecated. READ_RESTRICTED and RESTRICTED are equivalent to Py_AUDIT_READ; WRITE_RESTRICTED does nothing.

Changed in version 3.12: The READONLY macro was renamed to Py_READONLY. The PY_AUDIT_READ macro was renamed with the Py_ prefix. The new names are now always available. Previously, these required #include "structmember.h". The header is still available and it provides the old names.

Member types¶

PyMemberDef.type can be one of the following macros corresponding to various C types. When the member is accessed in Python, it will be converted to the equivalent Python type. When it is set from Python, it will be converted back to the C type. If that is not possible, an exception such as TypeError or ValueError is raised.

Unless marked (D), attributes defined this way cannot be deleted using e.g. del or delattr().

Macro name	C type	Python type
Py_T_BYTE¶	char	`int`
Py_T_SHORT¶	short	`int`
Py_T_INT¶	int	`int`
Py_T_LONG¶	long	`int`
Py_T_LONGLONG¶	long long	`int`
Py_T_UBYTE¶	unsigned char	`int`
Py_T_UINT¶	unsigned int	`int`
Py_T_USHORT¶	unsigned short	`int`
Py_T_ULONG¶	unsigned long	`int`
Py_T_ULONGLONG¶	unsigned long long	`int`
Py_T_PYSSIZET¶	Py_ssize_t	`int`
Py_T_FLOAT¶	float	`float`
Py_T_DOUBLE¶	double	`float`
Py_T_BOOL¶	char (written as 0 or 1)	`bool`
Py_T_STRING¶	const char* (*)	`str` (RO)
Py_T_STRING_INPLACE¶	const char[] (*)	`str` (RO)
Py_T_CHAR¶	char (0-127)	`str` (**)
Py_T_OBJECT_EX¶	PyObject*	`object` (D)

(*): Zero-terminated, UTF8-encoded C string. With Py_T_STRING the C representation is a pointer; with Py_T_STRING_INPLACE the string is stored directly in the structure.

(**): String of length 1. Only ASCII is accepted.

(RO): Implies Py_READONLY.

(D): Can be deleted, in which case the pointer is set to NULL. Reading a NULL pointer raises AttributeError.

Added in version 3.12: In previous versions, the macros were only available with #include "structmember.h" and were named without the Py_ prefix (e.g. as T_INT). The header is still available and contains the old names, along with the following deprecated types:

T_OBJECT¶: Like Py_T_OBJECT_EX, but NULL is converted to None. This results in surprising behavior in Python: deleting the attribute effectively sets it to None.

T_NONE¶: Always None. Must be used with Py_READONLY.

Defining Getters and Setters¶

type PyGetSetDef¶

Part of the Stable ABI (including all members).

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

const char *name¶: attribute name

getter get¶: C function to get the attribute.

setter set¶: Optional C function to set or delete the attribute. If NULL, the attribute is read-only.

const char *doc¶: optional docstring

void *closure¶: Optional user data pointer, providing additional data for getter and setter.

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

Part of the Stable ABI.

The get function takes one PyObject* parameter (the instance) and a user data pointer (the associated closure):

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

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

Part of the Stable ABI.

set functions take two PyObject* parameters (the instance and the value to be set) and a user data pointer (the associated closure):

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.

Common Object Structures¶

Base object types and macros¶

Implementing functions and methods¶

Accessing attributes of extension types¶

Member flags¶

Member types¶

Defining Getters and Setters¶

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