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 objectâs 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 âreleaseâ build, it contains only the objectâs 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.
Py_TYPE(o)¶
This macro is used to access the ob_type member of a Python object. It expands to:
(((PyObject*)(o))->ob_type)
New in version 2.6.
Py_REFCNT(o)¶
This macro is used to access the ob_refcnt member of a Python object. It expands to:
(((PyObject*)(o))->ob_refcnt)
New in version 2.6.
Py_SIZE(o)¶
This macro is used to access the ob_size member of a Python object. It expands to:
(((PyVarObject*)(o))->ob_size)
New in version 2.6.
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.
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 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. 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 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_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 donât 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.
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.
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 typeâs 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. 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. 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
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
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.
Py_FindMethod(PyMethodDef table[], PyObject *ob, char *name)¶
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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