# 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. Nothing is actually declared to be a `PyObject`, but every pointer to a Python object can be cast to a `PyObject*`. Access to the members must be done by using the macros `Py_REFCNT` and `Py_TYPE`.

`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. Access to the members must be done by using the macros `Py_REFCNT`, `Py_TYPE`, and `Py_SIZE`.

`PyObject_HEAD`

  This is a macro used when declaring new types which represent objects without a varying length. The PyObject_HEAD macro expands to:

      PyObject ob_base;

  See documentation of `PyObject` above.

`PyObject_VAR_HEAD`

  This is a macro used when declaring new types which represent objects with a length that varies from instance to instance. The PyObject_VAR_HEAD macro expands to:

      PyVarObject ob_base;

  See documentation of `PyVarObject` above.

`Py_TYPE`(o)

  This macro is used to access the `ob_type` member of a Python object. It expands to:

      (((PyObject*)(o))->ob_type)

`Py_REFCNT`(o)

  This macro is used to access the `ob_refcnt` member of a Python object. It expands to:

      (((PyObject*)(o))->ob_refcnt)

`Py_SIZE`(o)

  This macro is used to access the `ob_size` member of a Python object. It expands to:

      (((PyVarObject*)(o))->ob_size)

`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 with signature `METH_VARARGS | METH_KEYWORDS`.

`_PyCFunctionFast`

  Type of the functions used to implement Python callables in C with signature `METH_FASTCALL`.

`_PyCFunctionFastWithKeywords`

  Type of the functions used to implement Python callables in C with signature `METH_FASTCALL | METH_KEYWORDS`.

`PyMethodDef`

  Structure used to describe a method of an extension type. This structure has four fields:

  Field | C Type | Meaning
  --- | --- | ---
  `ml_name` | const 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` | const 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 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.

There are four basic calling conventions for positional arguments and two of them can be combined with `METH_KEYWORDS` to support also keyword arguments. So there are a total of 6 calling conventions:

`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 is the module object. 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_VARARGS | METH_KEYWORDS`

  Methods with these flags must be of type `PyCFunctionWithKeywords`. The function expects three parameters: self, args, kwargs where kwargs is a dictionary of all the keyword arguments or possibly `NULL` if there are no keyword arguments. The parameters are typically processed using `PyArg_ParseTupleAndKeywords()`.

`METH_FASTCALL`

  Fast calling convention supporting only positional arguments. The methods have the type `_PyCFunctionFast`. The first parameter is self, the second parameter is a C array of `PyObject*` values indicating the arguments and the third parameter is the number of arguments (the length of the array).

  This is not part of the limited API.

  New in version 3.7.

`METH_FASTCALL | METH_KEYWORDS`

  Extension of `METH_FASTCALL` supporting also keyword arguments, with methods of type `_PyCFunctionFastWithKeywords`. Keyword arguments are passed the same way as in the vectorcall protocol: there is an additional fourth `PyObject*` parameter which is a tuple representing the names of the keyword arguments or possibly `NULL` if there are no keywords. The values of the keyword arguments are stored in the args array, after the positional arguments.

  This is not part of the limited API.

  New in version 3.7.

`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`. 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 `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` | const 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` | const 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 | const 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`. Try to use `T_OBJECT_EX` over `T_OBJECT` because `T_OBJECT_EX` handles use of the `del` statement on that attribute more correctly than `T_OBJECT`.

  `flags` can be `0` for write and read access or `READONLY` for read-only access. Using `T_STRING` for `type` implies `READONLY`. `T_STRING` data is interpreted as UTF-8. Only `T_OBJECT` and `T_OBJECT_EX` members can be deleted. (They are set to `NULL`).

`PyGetSetDef`

  Structure to define property-like access for a type. See also description of the `PyTypeObject.tp_getset` slot.

  Field | C Type | Meaning
  --- | --- | ---
  name | const char * | attribute name
  get | getter | C Function to get the attribute
  set | setter | optional C function to set or delete the attribute, if omitted the attribute is readonly
  doc | const char * | optional docstring
  closure | void * | optional function pointer, providing additional data for getter and setter

  The `get` function takes one `PyObject*` parameter (the instance) and a function pointer (the associated `closure`):

      typedef PyObject *(*getter)(PyObject *, void *);

  It should return a new reference on success or `NULL` with a set exception on failure.

  `set` functions take two `PyObject*` parameters (the instance and the value to be set) and a function pointer (the associated `closure`):

      typedef int (*setter)(PyObject *, PyObject *, void *);

  In case the attribute should be deleted the second parameter is `NULL`. Should return `0` on success or `-1` with a set exception on failure.