Common Object Structures (original) (raw)

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 thePyObject 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.

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 andPy_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_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, 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:

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:

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.

New 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.

New 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.

New 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.

New in version 3.10.

PyTypeObject *Py_TYPE(PyObject *o)¶

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.

New in version 3.9.

void Py_SET_TYPE(PyObject *o, PyTypeObject *type)¶

Set the object o type to type.

New in version 3.9.

Py_ssize_t Py_REFCNT(PyObject *o)¶

Get the reference count of the Python object o.

Use the Py_SET_REFCNT() function to set an object reference count.

Changed in version 3.11: The parameter type is no longer const PyObject*.

Changed in version 3.10: Py_REFCNT() is changed to the inline static function.

void Py_SET_REFCNT(PyObject *o, Py_ssize_t refcnt)¶

Set the object o reference counter to refcnt.

New 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.

New in version 3.9.

PyObject_HEAD_INIT(type)¶

This is a macro which expands to initialization values for a newPyObject 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 newPyVarObject 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¶

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¶

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)

New 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 asPyObject*, 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 typePyCFunction. 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 andMETH_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).

New 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 thevectorcall 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.

New 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_KEYWORDSsupporting 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 forMETH_FASTCALL | METH_KEYWORDS with defining_class argument added afterself.

New 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 typePyCFunction. 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_Oflag, instead of invoking PyArg_ParseTuple() with a "O" argument. They have the type PyCFunction, with the self parameter, and aPyObject* 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.

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.

New 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. Its fields are:

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.

T_OBJECT and T_OBJECT_EX differ in thatT_OBJECT returns None if the member is NULL andT_OBJECT_EX raises an AttributeError. Try to useT_OBJECT_EX over T_OBJECT because T_OBJECT_EXhandles 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 impliesREADONLY. T_STRING data is interpreted as UTF-8. Only T_OBJECT and T_OBJECT_EXmembers can be deleted. (They are set to NULL).

Heap allocated types (created using PyType_FromSpec() or similar),PyMemberDef may contain definitions for the special members__dictoffset__, __weaklistoffset__ and __vectorcalloffset__, corresponding totp_dictoffset,tp_weaklistoffset andtp_vectorcall_offset in type objects. These must be defined with T_PYSSIZET and READONLY, for example:

static PyMemberDef spam_type_members[] = { {"dictoffset", T_PYSSIZET, offsetof(Spam_object, dict), READONLY}, {NULL} /* Sentinel */ };

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

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

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

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

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.

The get function takes one PyObject* parameter (the instance) and a user data 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 user data 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.