PyObject_HEAD_INIT
-Typedefs:
-unaryfunc, binaryfunc, ternaryfunc, inquiry, coercion, intargfunc,
-intintargfunc, intobjargproc, intintobjargproc, objobjargproc,
-destructor, printfunc, getattrfunc, getattrofunc, setattrfunc,
-setattrofunc, cmpfunc, reprfunc, hashfunc
-
\begin{ctypedesc}{PyCFunction}
Type of the functions used to implement most Python callables in C.
Functions of this type take two \ctype{PyObject*} parameters and
\end{cfuncdesc}
+\section{Type Objects \label{type-structs}}
+
+Perhaps one of the most important structures of the Python object
+system is the structure that defines a new type: the
+\ctype{PyTypeObject} structure. Type objects can be handled using any
+of the \cfunction{PyObject_*()} or \cfunction{PyType_*()} functions,
+but do not offer much that's interesting to most Python applications.
+These objects are fundamental to how objects behave, so they are very
+important to the interpreter itself and to any extension module that
+implements new types.
+
+Type objects are fairly large compared to most of the standard types.
+The reason for the size is that each type object stores a large number
+of values, mostly C function pointers, each of which implements a
+small part of the type's functionality. The fields of the type object
+are examined in detail in this section. The fields will be described
+in the order in which they occur in the structure.
+
+Typedefs:
+unaryfunc, binaryfunc, ternaryfunc, inquiry, coercion, intargfunc,
+intintargfunc, intobjargproc, intintobjargproc, objobjargproc,
+destructor, freefunc, printfunc, getattrfunc, getattrofunc, setattrfunc,
+setattrofunc, cmpfunc, reprfunc, hashfunc
+
+The structure definition for \ctype{PyTypeObject} can be found in
+\file{Include/object.h}. For convenience of reference, this repeats
+the definition found there:
+
+\verbatiminput{typestruct.h}
+
+The type object structure extends the \ctype{PyVarObject} structure,
+though it does not actually need the the \member{ob_size} field. The
+inclusion of this field is a historical accident that must be
+maintained to ensure binary compatibility between new versions of
+Python and older compiled extensions.
+
+\begin{cmemberdesc}{PyObject}{PyObject*}{_ob_next}
+\cmemberline{PyObject}{PyObject*}{_ob_prev}
+ These fields are only present when the macro \code{Py_TRACE_REFS} is
+ defined. Their initialization to \NULL{} is taken care of by the
+ \code{PyObject_HEAD_INIT} macro. For statically allocated objects,
+ these fields always remain \NULL. For dynamically allocated
+ objects, these two fields are used to link the object into a
+ doubly-linked list of \emph{all} live objects on the heap. This
+ could be used for various debugging purposes; currently the only use
+ is to print the objects that are still alive at the end of a run
+ when the environment variable \envvar{PYTHONDUMPREFS} is set.
+
+ These fields are not inherited by subtypes.
+\end{cmemberdesc}
+
+\begin{cmemberdesc}{PyObject}{int}{ob_refcnt}
+ This is the type object's reference count, initialized to \code{1}
+ by the \code{PyObject_HEAD_INIT} macro. Note that for statically
+ allocated type objects, the type's instances (objects whose
+ \member{ob_type} points back to the type) do \emph{not} count as
+ references. But for dynamically allocated type objects, the
+ instances \emph{do} count as references.
+
+ This field is not inherited by subtypes.
+\end{cmemberdesc}
+
+\begin{cmemberdesc}{PyObject}{PyTypeObject*}{ob_type}
+ This is the type's type, in other words its metatype. It is
+ initialized by the argument to the \code{PyObject_HEAD_INIT} macro,
+ and its value should normally be \code{\&PyType_Type}. However, for
+ dynamically loadable extension modules that must be usable on
+ Windows (at least), the compiler complains that this is not a valid
+ initializer. Therefore, the convention is to pass \NULL{} to the
+ \code{PyObject_HEAD_INIT} macro and to initialize this field
+ explicitly at the start of the module's initialization function,
+ before doing anything else. This is typically done like this:
+
+\begin{verbatim}
+Foo_Type.ob_type = &PyType_Type;
+\end{verbatim}
+
+ This should be done before any instances of the type are created.
+ \cfunction{PyType_Ready()} checks if \member{ob_type} is \NULL, and
+ if so, initializes it: in Python 2.2, it is set to
+ \code{\&PyType_Type}; in Python 2.2.1 and later it will be
+ initialized to the \member{ob_type} field of the base class.
+ \cfunction{PyType_Ready()} will not change this field if it is
+ nonzero.
+
+ In Python 2.2, this field is not inherited by subtypes. In 2.2.1,
+ and in 2.3 and beyond, it is inherited by subtypes.
+\end{cmemberdesc}
+
+\begin{cmemberdesc}{PyVarObject}{int}{ob_size}
+ For statically allocated type objects, this should be initialized
+ to zero. For dynamically allocated type objects, this field has a
+ special internal meaning.
+
+ This field is not inherited by subtypes.
+\end{cmemberdesc}
+
+\begin{cmemberdesc}{PyTypeObject}{char*}{tp_name}
+ Pointer to a NUL-terminated string containing the name of the type.
+ For types that are accessible as module globals, the string should
+ be the full module name, followed by a dot, followed by the type
+ name; for built-in types, it should be just the type name. If the
+ module is a submodule of a package, the full package name is part of
+ the full module name. For example, a type named \class{T} defined
+ in module \module{M} in subpackage \module{Q} in package \module{P}
+ should have the \member{tp_name} initializer \code{"P.Q.M.T"}.
+
+ For dynamically allocated type objects, this may be just the type
+ name, if the module name is explicitly stored in the type dict as
+ the value for key \code{'__module__'}.
+
+ If the tp_name field contains a dot, everything before the last dot
+ is made accessible as the \member{__module__} attribute, and
+ everything after the last dot is made accessible as the
+ \member{__name__} attribute. If no dot is present, the entire
+ \member{tp_name} field is made accessible as the \member{__name__}
+ attribute, and the \member{__module__} attribute is undefined
+ (unless explicitly set in the dictionary, as explained above).
+
+ This field is not inherited by subtypes.
+\end{cmemberdesc}
+
+\begin{cmemberdesc}{PyTypeObject}{int}{tp_basicsize}
+\cmemberline{PyTypeObject}{int}{tp_itemsize}
+ These fields allow calculating the size in byte of instances of
+ the type.
+
+ There are two kinds of types: types with fixed-length instances have
+ a zero \member{tp_itemsize} field, types with variable-length
+ instances have a non-zero \member{tp_itemsize} field. For a type
+ with fixed-length instances, all instances have the same size,
+ given in \member{tp_basicsize}.
+
+ For a type with variable-length instances, the instances must have
+ an \member{ob_size} field, and the instance size is
+ \member{tp_basicsize} plus N times \member{tp_itemsize}, where N is
+ the ``length'' of the object. The value of N is typically stored in
+ the instance's \member{ob_size} field. There are exceptions: for
+ example, long ints use a negative \member{ob_size} to indicate a
+ negative number, and N is \code{abs(\member{ob_size})} there. Also,
+ the presence of an \member{ob_size} field in the instance layout
+ doesn't mean that the type is variable-length (for example, the list
+ type has fixed-length instances, yet those instances have a
+ meaningful \member{ob_size} field).
+
+ The basic size includes the fields in the instance declared by the
+ macro \csimplemacro{PyObject_HEAD} or
+ \csimplemacro{PyObject_VAR_HEAD} (whichever is used to declare the
+ instance struct) and this in turn includes the \member{_ob_prev} and
+ \member{_ob_next} fields if they are present. This means that the
+ only correct way to get an initializer for the \member{tp_basicsize}
+ is to use the \keyword{sizeof} operator on the struct used to
+ declare the instance layout. The basic size does not include the GC
+ header size (this is new in Python 2.2; in 2.1 and 2.0, the GC
+ header size was included in \member{tp_basicsize}).
+
+ These fields are inherited by subtypes.
+\end{cmemberdesc}
+
+\begin{cmemberdesc}{PyTypeObject}{destructor}{tp_dealloc}
+ A pointer to the instance destructor function. This function must
+ be defined unless the type guarantees that its instances will never
+ be deallocated (as is the case for the singletons \code{None} and
+ \code{Ellipsis}).
+
+ The destructor function is called by the \cfunction{Py_DECREF()} and
+ \cfunction{Py_XDECREF()} macros when the new reference count is
+ zero. At this point, the instance is still in existance, but there
+ are no references to it. The destructor function should free all
+ references which the instance owns, free all memory buffers owned by
+ the instance (using the freeing function corresponding to the
+ allocation function used to allocate the buffer), and finally (as
+ its last action) call the type's \member{tp_free} slot. If the type
+ is not subtypable (doesn't have the \constant{Py_TPFLAGS_BASETYPE}
+ flag bit set), it is permissible to call the object deallocator
+ directly instead of via \member{tp_free}. The object deallocator
+ should be the one used to allocate the instance; this is normally
+ \cfunction{PyObject_Del()} if the instance was allocated using
+ \cfunction{PyObject_New()} or \cfunction{PyOject_VarNew()}, or
+ \cfunction{PyObject_GC_Del()} if the instance was allocated using
+ \cfunction{PyObject_GC_New()} or \cfunction{PyObject_GC_VarNew()}.
+
+ This field is inherited by subtypes.
+\end{cmemberdesc}
+
+\begin{cmemberdesc}{PyTypeObject}{printfunc}{tp_print}
+ An optional pointer to the instance print function.
+
+ The print function is only called when the instance is printed to a
+ \emph{real} file; when it is printed to a pseudo-file (like a
+ \class{StringIO} instance), the instance's \member{tp_repr} or
+ \member{tp_str} function is called to convert it to a string. These
+ are also called when the type's \member{tp_print} field is \NULL.
+
+ The print function is called with the same signature as
+ \cfunction{PyObject_Print()}: \code{tp_print(PyObject *self, FILE
+ *file, int flags)}. The \var{self} argument is the instance to be
+ printed. The \var{file} argument is the stdio file to which it is
+ to be printed. The \var{flags} argument is composed of flag bits.
+ The only flag bit currently defined is \constant{Py_PRINT_RAW}.
+ When the \constant{Py_PRINT_RAW} flag bit is set, the instance
+ should be printed the same way as \member{tp_str} would format it;
+ when the \constant{Py_PRINT_RAW} flag bit is clear, the instance
+ should be printed the same was as \member{tp_repr} would format it.
+
+ It is possible that the \member{tp_print} field will be deprecated.
+ In any case, it is recommended not to define \member{tp_print}, but
+ instead to rely on \member{tp_repr} and \member{tp_str} for
+ printing.
+
+ This field is inherited by subtypes.
+\end{cmemberdesc}
+
+\begin{cmemberdesc}{PyTypeObject}{getattrfunc}{tp_getattr}
+ An optional pointer to the get-attribute-string function.
+
+ This field is deprecated. When it is defined, it should point to a
+ function that acts the same as the \member{tp_getattro} function,
+ but taking a C string instead of a Python string object to give the
+ attribute name. The signature is the same as for
+ \cfunction{PyObject_GetAttrString()}.
+
+ This field is inherited by subtypes together with
+ \member{tp_getattro}: a subtype inherits both \member{tp_getattr}
+ and \member{tp_getattro} from its base type when the subtype's
+ \member{tp_getattr} and \member{tp_getattro} are both \NULL.
+\end{cmemberdesc}
+
+\begin{cmemberdesc}{PyTypeObject}{setattrfunc}{tp_setattr}
+ An optional pointer to the set-attribute-string function.
+
+ This field is deprecated. When it is defined, it should point to a
+ function that acts the same as the \member{tp_setattro} function,
+ but taking a C string instead of a Python string object to give the
+ attribute name. The signature is the same as for
+ \cfunction{PyObject_SetAttrString()}.
+
+ This field is inherited by subtypes together with
+ \member{tp_setattro}: a subtype inherits both \member{tp_setattr}
+ and \member{tp_setattro} from its base type when the subtype's
+ \member{tp_setattr} and \member{tp_setattro} are both \NULL.
+\end{cmemberdesc}
+
+\begin{cmemberdesc}{PyTypeObject}{cmpfunc}{tp_compare}
+ An optional pointer to the three-way comparison function.
+
+ The signature is the same as for \cfunction{PyObject_Compare()}.
+ The function should return \code{1} if \var{self} greater than
+ \var{other}, \code{0} if \var{self} is equal to \var{other}, and
+ \code{-1} if \var{self} less than \var{other}. It should return
+ \code{-1} and set an exception condition when an error occurred
+ during the comparison.
+
+ This field is inherited by subtypes together with
+ \member{tp_richcompare} and \member{tp_hash}: a subtypes inherits
+ all three of \member{tp_compare}, \member{tp_richcompare}, and
+ \member{tp_hash} when the subtype's \member{tp_compare},
+ \member{tp_richcompare}, and \member{tp_hash} are all \NULL.
+\end{cmemberdesc}
+
+
+
\section{Mapping Object Structures \label{mapping-structs}}
\begin{ctypedesc}{PyMappingMethods}
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