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
and
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
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
, but every pointer to a Python object can be cast to a
. Access to the members must be done by using the macros
and
.
PyVarObject
This is an extension of
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
,
, and
.
PyObject_HEAD
This is a macro used when declaring new types which represent objects without a varying length. The PyObject_HEAD macro expands to:
PyObjectob_base;See documentation of
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:
PyVarObjectob_base;See documentation of
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) int Py_IS_TYPE(
*o,
*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(
*o,
*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) void Py_SET_REFCNT(
*o,
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) void Py_SET_SIZE(
*o,
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
type. This macro expands to:
_PyObject_EXTRA_INIT1,type,PyVarObject_HEAD_INIT(type, size)
This is a macro which expands to initialization values for a new
type, including the ob_size field. This macro expands to:
_PyObject_EXTRA_INIT1,type,size,Implementing functions and methods
PyCFunction
Type of the functions used to implement most Python callables in C. Functions of this type take two
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
. The function signature is:
PyObject*_PyCFunctionFast(PyObject*self,PyObject*const*args,Py_ssize_tnargs);_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_tnargs,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_tnargs,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
ml_name
const char *
name of the method
ml_meth
PyCFunction
pointer to the C implementation
ml_flags
int
flag bits indicating how the call should be constructed
ml_doc
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
. If the function is not of the
, the compiler will require a cast in the method table. Even though
defines the first parameter as
, 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
. The function expects two
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
or
.
METH_VARARGS | METH_KEYWORDSMethods with these flags must be of type
. 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
.
METH_FASTCALL
Fast calling convention supporting only positional arguments. The methods have the type
. The first parameter is self, the second parameter is a C array of
values indicating the arguments and the third parameter is the number of arguments (the length of the array).
This is not part of the
.
New in version 3.7.
METH_FASTCALL | METH_KEYWORDSExtension of
supporting also keyword arguments, with methods of type
. Keyword arguments are passed the same way as in the
: there is an additional fourth
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
.
New in version 3.7.
METH_METHOD | METH_FASTCALL | METH_KEYWORDSExtension 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
, the same as for METH_FASTCALL|METH_KEYWORDS with defining_class argument added after self.
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
flag. They need to be of type
. 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
Methods with a single object argument can be listed with the
flag, instead of invoking
with a "O" argument. They have the type
, with the self parameter, and a
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
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
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
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.
Accessing attributes of extension types
PyMemberDef
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 type’s 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
. Try to use T_OBJECT_EX over T_OBJECT because T_OBJECT_EX handles use of the
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
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
or similar), PyMemberDef may contain definitions for the special members __dictoffset__, __weaklistoffset__ and __vectorcalloffset__, corresponding to
,
and
in type objects. These must be defined with T_PYSSIZET and READONLY, for example:
staticPyMemberDefspam_type_members[]={{"__dictoffset__",T_PYSSIZET,offsetof(Spam_object,dict),READONLY},{NULL}/* Sentinel */};
* PyMember_GetOne(const char *obj_addr, struct
*m)
Get an attribute belonging to the object at address obj_addr. The attribute is described by PyMemberDefm. Returns NULL on error.
int PyMember_SetOne(char *obj_addr, struct
*m,
*o)
Set an attribute belonging to the object at address obj_addr to object o. The attribute to set is described by PyMemberDefm. Returns 0 if successful and a negative value on failure.
PyGetSetDef
Structure to define property-like access for a type. See also description of the
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
parameter (the instance) and a function pointer (the associated closure):
typedefPyObject*(*getter)(PyObject*,void*);It should return a new reference on success or NULL with a set exception on failure.
set functions take two
parameters (the instance and the value to be set) and a function pointer (the associated closure):
typedefint(*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.