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_REFCNTand- Py_TYPE.
- 
PyVarObject¶
- This is an extension of - PyObjectthat adds the- ob_sizefield. 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 - PyObjectabove.
- 
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 - PyVarObjectabove.
- 
Py_TYPE(o)¶
- This macro is used to access the - ob_typemember of a Python object. It expands to:- (((PyObject*)(o))->ob_type) 
- 
Py_REFCNT(o)¶
- This macro is used to access the - ob_refcntmember of a Python object. It expands to:- (((PyObject*)(o))->ob_refcnt) 
- 
Py_SIZE(o)¶
- This macro is used to access the - ob_sizemember 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 - PyObjecttype. 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 - PyVarObjecttype, including the- ob_sizefield. 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- NULLif 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_FASTCALLsupporting 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- NULLif 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_NOARGSflag. 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_Oflag, 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 - NULLas 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 - typecan 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_OBJECTand- T_OBJECT_EXdiffer in that- T_OBJECTreturns- Noneif the member is- NULLand- T_OBJECT_EXraises an- AttributeError. Try to use- T_OBJECT_EXover- T_OBJECTbecause- T_OBJECT_EXhandles use of the- delstatement on that attribute more correctly than- T_OBJECT.- flagscan be- 0for write and read access or- READONLYfor read-only access. Using- T_STRINGfor- typeimplies- READONLY.- T_STRINGdata is interpreted as UTF-8. Only- T_OBJECTand- T_OBJECT_EXmembers can be deleted. (They are set to- NULL).
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PyGetSetDef¶
- Structure to define property-like access for a type. See also description of the - PyTypeObject.tp_getsetslot.- 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 - getfunction 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 - NULLwith a set exception on failure.- setfunctions 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- 0on success or- -1with a set exception on failure.
