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 :ctype:`PyObject` and :ctype:`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.
These macros are used in the definition of :ctype:`PyObject` and :ctype:`PyVarObject`:
The ml_meth
is a C function pointer. The functions may be of different
types, but they always return :ctype:`PyObject\*`. If the function is not of
the :ctype:`PyCFunction`, the compiler will require a cast in the method table.
Even though :ctype:`PyCFunction` defines the first parameter as
:ctype:`PyObject\*`, it is common that the method implementation uses a 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 (but note that METH_KEYWORDS
alone is equivalent to METH_VARARGS | METH_KEYWORDS
). 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 :ctype:`PyCFunction`. The function expects two :ctype:`PyObject\*` values. The first one is the self object for methods; for module functions, it has the value given to :cfunc:`Py_InitModule4` (or NULL if :cfunc:`Py_InitModule` was used). The second parameter (often called args) is a tuple object representing all arguments. This parameter is typically processed using :cfunc:`PyArg_ParseTuple` or :cfunc:`PyArg_UnpackTuple`.
-
METH_KEYWORDS
¶ Methods with these flags must be of type :ctype:`PyCFunctionWithKeywords`. The function expects three parameters: self, args, and a dictionary of all the keyword arguments. The flag is typically combined with
METH_VARARGS
, and the parameters are typically processed using :cfunc:`PyArg_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 type :ctype:`PyCFunction`. When used with object methods, the first parameter is typically namedself
and will hold a reference to the 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 invoking :cfunc:`PyArg_ParseTuple` with a"O"
argument. They have the type :ctype:`PyCFunction`, with the self parameter, and a :ctype:`PyObject\*` parameter representing the single argument.
-
METH_OLDARGS
¶ This calling convention is deprecated. The method must be of type :ctype:`PyCFunction`. The second argument is NULL if no arguments are given, a single object if exactly one argument is given, and a tuple of objects if more than one argument is given. There is no way for a function using this convention to distinguish between a call with multiple arguments and a call with a tuple as the only 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.New in version 2.3.
-
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.New in version 2.3.
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.New in version 2.4.