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.
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;
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.
Structure used to describe a method of an extension type. This structure has four fields:
|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.
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.
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.
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.
Structure which describes an attribute of a type which corresponds to a C struct member. Its fields are:
|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 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_ULONGLONG||unsigned long long|
T_OBJECT and T_OBJECT_EX differ in that T_OBJECT returns None if the member is NULL and T_OBJECT_EX raises an AttributeError.
flags can be 0 for write and read access or READONLY for read-only access. Using T_STRING for type implies READONLY. Only T_OBJECT and T_OBJECT_EX members can be deleted. (They are set to NULL).