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# 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. 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`(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 the `ob_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. See `PyCFunction` 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 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.

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 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_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` 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).

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