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 objectâs 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.
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 âreleaseâ build, it contains only the objectâs 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.
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.
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.
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.
Test if the x object is the y object, the same as x is y in Python.
New in version 3.10.
Test if an object is the None singleton, the same as x is None in Python.
New in version 3.10.
Test if an object is the True singleton, the same as x is True in Python.
New in version 3.10.
Test if an object is the False singleton, the same as x is False in Python.
New in version 3.10.
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*.
Return non-zero if the object o type is type. Return zero otherwise. Equivalent to: Py_TYPE(o) == type.
New in version 3.9.
Set the object o type to type.
New in version 3.9.
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.
Set the object o reference counter to refcnt.
New in version 3.9.
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*.
Set the object o size to size.
New in version 3.9.
This is a macro which expands to initialization values for a new PyObject type. This macro expands to:
_PyObject_EXTRA_INIT 1, type,
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,
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 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 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 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 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.
Structure used to describe a method of an extension type. This structure has four fields:
Name of the method.
Pointer to the C implementation.
Flags bits indicating how the call should be constructed.
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:
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().
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.
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().
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.
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.
New in version 3.7.
Can only be used in the combination with other flags: 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.
New in version 3.9.
Methods without parameters donât 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.
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.
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.
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.
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.
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 objectâs __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.
Equivalent to PyCMethod_New(ml, self, module, NULL).
Equivalent to PyCMethod_New(ml, self, NULL, NULL).
Structure which describes an attribute of a type which corresponds to a C struct member. Its fields are:
Field
C Type
Meaning
name
const 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 typeâs object struct
flags
int
flag bits indicating if the field should be read-only or writable
doc
const 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
const 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. T_STRING data is interpreted as UTF-8. Only T_OBJECT and T_OBJECT_EX members 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 to tp_dictoffset, tp_weaklistoffset and tp_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 */
};
Get an attribute belonging to the object at address obj_addr. The attribute is described by PyMemberDef m. Returns NULL on error.
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.
Structure to define property-like access for a type. See also description of the PyTypeObject.tp_getset slot.
Field
C Type
Meaning
name
const 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
const char *
optional docstring
closure
void *
optional user data pointer, providing additional data for getter and setter
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.
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