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 macrosPy_REFCNTandPy_TYPE.
- 
type PyVarObject¶
-  Part of the Limited API. (Only some members are part of the stable ABI.)This is an extension of PyObjectthat adds theob_sizefield. 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 macrosPy_REFCNT,Py_TYPE, andPy_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 - PyObjectabove.
- 
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 - PyVarObjectabove.
- 
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 yin 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 Nonesingleton, the same asx is Nonein 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 Truesingleton, the same asx is Truein 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 Falsesingleton, the same asx is Falsein Python.Added in version 3.10. 
- 
PyTypeObject *Py_TYPE(PyObject *o)¶
- Return value: Borrowed reference.Get the type of the Python object o. Return a borrowed reference. Use the Py_SET_TYPE()function to set an object type.
- 
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 - PyObjecttype. 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 - PyVarObjecttype, including the- ob_sizefield. 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 notNULL, 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. 
 
- 
const char *ml_name¶
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- NULLif 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_FASTCALLis now part of the stable ABI.
- METH_FASTCALL | METH_KEYWORDS
- Extension of - METH_FASTCALLsupporting 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- NULLif 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_KEYWORDSwith- defining_classargument 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_NOARGSflag. 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_UNUSEDcan be used to prevent a compiler warning.
- 
METH_O¶
- Methods with a single object argument can be listed with the - METH_Oflag, 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 - NULLas 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_methwhen invoked. self can beNULL.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 toNoneorNULL.See also The cls parameter will be passed as the defining_class argument to the C function. Must be set if METH_METHODis set onml->ml_flags.Added in version 3.9. 
- 
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).
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_membersslot.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 flagsis0), members allow both read and write access. Use thePy_READONLYflag for read-only access. Certain types, likePy_T_STRING, implyPy_READONLY. OnlyPy_T_OBJECT_EX(and legacyT_OBJECT) members can be deleted.For heap-allocated types (created using PyType_FromSpec()or similar),PyMemberDefmay contain a definition for the special member"__vectorcalloffset__", corresponding totp_vectorcall_offsetin type objects. These must be defined withPy_T_PYSSIZETandPy_READONLY, 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>foroffsetof().)The legacy offsets tp_dictoffsetandtp_weaklistoffsetcan be defined similarly using"__dictoffset__"and"__weaklistoffset__"members, but extensions are strongly encouraged to usePy_TPFLAGS_MANAGED_DICTandPy_TPFLAGS_MANAGED_WEAKREFinstead.Changed in version 3.12: PyMemberDefis always available. Previously, it required including"structmember.h".
- 
const char *name¶
- 
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 PyMemberDefm. ReturnsNULLon error.Changed in version 3.12: PyMember_GetOneis 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 PyMemberDefm. Returns0if successful and a negative value on failure.Changed in version 3.12: PyMember_SetOneis 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 - offsetof this- PyMemberDefentry indicates an offset from the subclass-specific data, rather than from- PyObject.- Can only be used as part of - Py_tp_members- slotwhen creating a class using negative- basicsize. It is mandatory in that case.- This flag is only used in - PyType_Slot. When setting- tp_membersduring class creation, Python clears it and sets- PyMemberDef.offsetto the offset from the- PyObjectstruct.
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 | 
|---|---|---|
| 
 | char | |
| 
 | short | |
| 
 | int | |
| 
 | long | |
| 
 | long long | |
| 
 | unsigned char | |
| 
 | unsigned int | |
| 
 | unsigned short | |
| 
 | unsigned long | |
| 
 | unsigned long long | |
| 
 | ||
| 
 | float | |
| 
 | double | |
| 
 | char (written as 0 or 1) | |
| 
 | const char* (*) | 
 | 
| 
 | const char[] (*) | 
 | 
| 
 | char (0-127) | 
 | 
| 
 | 
 | 
(*): Zero-terminated, UTF8-encoded C string. With
Py_T_STRINGthe C representation is a pointer; withPy_T_STRING_INPLACEthe 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 aNULLpointer raisesAttributeError.
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- NULLis 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_getsetslot.- 
const char *name¶
- attribute name 
 - 
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. 
 
- 
const char *name¶
- 
typedef PyObject *(*getter)(PyObject*, void*)¶
-  Part of the Stable ABI.The getfunction takes one PyObject* parameter (the instance) and a user data pointer (the associatedclosure):It should return a new reference on success or NULLwith a set exception on failure.
- 
typedef int (*setter)(PyObject*, PyObject*, void*)¶
-  Part of the Stable ABI.setfunctions take two PyObject* parameters (the instance and the value to be set) and a user data pointer (the associatedclosure):In case the attribute should be deleted the second parameter is NULL. Should return0on success or-1with a set exception on failure.