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.
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
¶ 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 aPyObject*
. Access to the members must be done by using the macrosPy_REFCNT
andPy_TYPE
.
-
PyVarObject
¶ This is an extension of
PyObject
that adds theob_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 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
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_REFCNT
(o)¶ This macro is used to access the
ob_refcnt
member of a Python object. It expands to:(((PyObject*)(o))->ob_refcnt)
-
Py_SIZE
(o)¶ This macro is used to access the
ob_size
member of a Python object. It expands to:(((PyVarObject*)(o))->ob_size)
-
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 theob_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.
-
PyCFunctionWithKeywords
¶ Type of the functions used to implement Python callables in C that take keyword arguments: they take three
PyObject*
parameters and return one such value. SeePyCFunction
above for the meaning of the return value.
-
PyMethodDef
¶ Structure used to describe a method of an extension type. This structure has four fields:
Field C Type Meaning ml_name
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
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. Of the calling convention flags, only METH_VARARGS
and
METH_KEYWORDS
can be combined. Any of the calling convention flags
can be combined with a binding flag.
-
METH_VARARGS
¶ This is the typical calling convention, where the methods have the type
PyCFunction
. The function expects twoPyObject*
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 usingPyArg_ParseTuple()
orPyArg_UnpackTuple()
.
-
METH_KEYWORDS
¶ Methods with these flags must be of type
PyCFunctionWithKeywords
. The function expects three parameters: self, args, and a dictionary of all the keyword arguments. The flag must be combined withMETH_VARARGS
, and the parameters are typically processed usingPyArg_ParseTupleAndKeywords()
.
-
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.
-
METH_O
¶ Methods with a single object argument can be listed with the
METH_O
flag, instead of invokingPyArg_ParseTuple()
with a"O"
argument. They have the typePyCFunction
, 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.
-
PyMemberDef
¶ Structure which describes an attribute of a type which corresponds to a C struct member. Its fields are:
Field C Type Meaning name
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
char * points to the contents of the docstring type
can be one of manyT_
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 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
andT_OBJECT_EX
differ in thatT_OBJECT
returnsNone
if the member is NULL andT_OBJECT_EX
raises anAttributeError
. Try to useT_OBJECT_EX
overT_OBJECT
becauseT_OBJECT_EX
handles use of thedel
statement on that attribute more correctly thanT_OBJECT
.flags
can be0
for write and read access orREADONLY
for read-only access. UsingT_STRING
fortype
impliesREADONLY
. OnlyT_OBJECT
andT_OBJECT_EX
members can be deleted. (They are set to NULL).