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.
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.
PyObject
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
.PyVarObject
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.
Py_TYPE
(o)This macro is used to access the ob_type
member of a Python object.
It expands to:
(((PyObject*)(o))->ob_type)
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.
Py_SET_TYPE
(PyObject *o, PyTypeObject *type)Set the object o type to type.
New in version 3.9.
Py_REFCNT
(o)This macro is used to access the ob_refcnt
member of a Python
object.
It expands to:
(((PyObject*)(o))->ob_refcnt)
Py_SET_REFCNT
(PyObject *o, Py_ssize_t refcnt)Set the object o reference counter to refcnt.
New in version 3.9.
Py_SIZE
(o)This macro is used to access the ob_size
member of a Python object.
It expands to:
(((PyVarObject*)(o))->ob_size)
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 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,
PyCFunction
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);
PyCFunctionWithKeywords
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);
_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);
_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);
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.
PyMethodDef
Structure used to describe a method of an extension type. This structure has four fields:
Field |
C Type |
Meaning |
---|---|---|
|
const char * |
name of the method |
|
PyCFunction |
pointer to the C implementation |
|
int |
flag bits indicating how the call should be constructed |
|
const char * |
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
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_VARARGS | METH_KEYWORDS
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).
This is not part of the limited API.
New in version 3.7.
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.
This is not part of the limited API.
New in version 3.7.
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.
METH_NOARGS
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
.METH_O
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
classmethod()
built-in function.METH_STATIC
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
__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.
PyMemberDef
Structure which describes an attribute of a type which corresponds to a C struct member. Its fields are:
Field |
C Type |
Meaning |
---|---|---|
|
const char * |
name of the member |
|
int |
the type of the member in the C struct |
|
Py_ssize_t |
the offset in bytes that the member is located on the type’s object struct |
|
int |
flag bits indicating if the field should be read-only or writable |
|
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 */
};
PyGetSetDef
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 function pointer, providing additional data for getter and setter |
The get
function takes one PyObject*
parameter (the
instance) and a function 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 function 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.