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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 :cPyObject and :cPyVarObject 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 :cPyObject and :cPyVarObject:
The ml_meth is a C function pointer. The functions may be of different types, but they always return :cPyObject\*. If the function is not of the :cPyCFunction, the compiler will require a cast in the method table. Even though :cPyCFunction defines the first parameter as :cPyObject\*, 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 :cPyCFunction. The function expects two :cPyObject\* 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 :cPyArg_ParseTuple or :cPyArg_UnpackTuple.
METH_KEYWORDS
Methods with these flags must be of type :cPyCFunctionWithKeywords. 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 :cPyArg_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 :cPyCFunction. 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 invoking :cPyArg_ParseTuple with a "O" argument. They have the type :cPyCFunction, with the self parameter, and a :cPyObject\* 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
builtin: classmethod
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
builtin: staticmethod
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.