10.2 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 objects reference count and a pointer to the corresponding type object. It corresponds to the fields defined by the expansion of the PyObject_HEAD macro.

PyVarObject: This is an extension of 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. It corresponds to the fields defined by the expansion of the PyObject_VAR_HEAD macro.

These macros are used in the definition of PyObject and PyVarObject:

PyObject_HEAD

This is a macro which expands to the declarations of the fields of the PyObject type; it is used when declaring new types which represent objects without a varying length. The specific fields it expands to depend on the definition of Py_TRACE_REFS. By default, that macro is not defined, and PyObject_HEAD expands to:

    Py_ssize_t ob_refcnt;
    PyTypeObject *ob_type;

When Py_TRACE_REFS is defined, it expands to:

    PyObject *_ob_next, *_ob_prev;
    Py_ssize_t ob_refcnt;
    PyTypeObject *ob_type;

PyObject_VAR_HEAD

This is a macro which expands to the declarations of the fields of the PyVarObject type; it is used when declaring new types which represent objects with a length that varies from instance to instance. This macro always expands to:

    PyObject_HEAD
    Py_ssize_t ob_size;

Note that PyObject_HEAD is part of the expansion, and that its own expansion varies depending on the definition of Py_TRACE_REFS.

PyObject_HEAD_INIT

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.

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 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 PyCFunction. The function expects two PyObject* values. The first one is the self object for methods; for module functions, it has the value given to Py_InitModule4() (or NULL if Py_InitModule() was used). 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_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 is typically combined with METH_VARARGS, and the parameters are typically processed using 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 PyCFunction. When used with object methods, the first parameter is typically named self 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 PyArg_ParseTuple() with a "O" argument. They have the type PyCFunction, with the self parameter, and a PyObject* parameter representing the single argument.

METH_OLDARGS: This calling convention is deprecated. The method must be of type 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.

PyObject* Py_FindMethod( PyMethodDef table[], PyObject *ob, char *name): Return value: New reference.
Return a bound method object for an extension type implemented in C. This can be useful in the implementation of a tp_getattro or tp_getattr handler that does not use the PyObject_GenericGetAttr() function.

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