cpython/Doc/c-api/object.rst

.. highlight:: c

.. _object:

Object Protocol
===============


.. c:var:: PyObject* Py_NotImplemented

   The ``NotImplemented`` singleton, used to signal that an operation is
   not implemented for the given type combination.


.. c:macro:: Py_RETURN_NOTIMPLEMENTED

   Properly handle returning :c:data:`Py_NotImplemented` from within a C
   function (that is, increment the reference count of NotImplemented and
   return it).


.. c:function:: int PyObject_Print(PyObject *o, FILE *fp, int flags)

   Print an object *o*, on file *fp*.  Returns ``-1`` on error.  The flags argument
   is used to enable certain printing options.  The only option currently supported
   is :const:`Py_PRINT_RAW`; if given, the :func:`str` of the object is written
   instead of the :func:`repr`.


.. c:function:: int PyObject_HasAttr(PyObject *o, PyObject *attr_name)

   Returns ``1`` if *o* has the attribute *attr_name*, and ``0`` otherwise.  This
   is equivalent to the Python expression ``hasattr(o, attr_name)``.  This function
   always succeeds.

   Note that exceptions which occur while calling :meth:`__getattr__` and
   :meth:`__getattribute__` methods will get suppressed.
   To get error reporting use :c:func:`PyObject_GetAttr()` instead.


.. c:function:: int PyObject_HasAttrString(PyObject *o, const char *attr_name)

   Returns ``1`` if *o* has the attribute *attr_name*, and ``0`` otherwise.  This
   is equivalent to the Python expression ``hasattr(o, attr_name)``.  This function
   always succeeds.

   Note that exceptions which occur while calling :meth:`__getattr__` and
   :meth:`__getattribute__` methods and creating a temporary string object
   will get suppressed.
   To get error reporting use :c:func:`PyObject_GetAttrString()` instead.


.. c:function:: PyObject* PyObject_GetAttr(PyObject *o, PyObject *attr_name)

   Retrieve an attribute named *attr_name* from object *o*. Returns the attribute
   value on success, or *NULL* on failure.  This is the equivalent of the Python
   expression ``o.attr_name``.


.. c:function:: PyObject* PyObject_GetAttrString(PyObject *o, const char *attr_name)

   Retrieve an attribute named *attr_name* from object *o*. Returns the attribute
   value on success, or *NULL* on failure. This is the equivalent of the Python
   expression ``o.attr_name``.


.. c:function:: PyObject* PyObject_GenericGetAttr(PyObject *o, PyObject *name)

   Generic attribute getter function that is meant to be put into a type
   object's ``tp_getattro`` slot.  It looks for a descriptor in the dictionary
   of classes in the object's MRO as well as an attribute in the object's
   :attr:`~object.__dict__` (if present).  As outlined in :ref:`descriptors`,
   data descriptors take preference over instance attributes, while non-data
   descriptors don't.  Otherwise, an :exc:`AttributeError` is raised.


.. c:function:: int PyObject_SetAttr(PyObject *o, PyObject *attr_name, PyObject *v)

   Set the value of the attribute named *attr_name*, for object *o*, to the value
   *v*. Raise an exception and return ``-1`` on failure;
   return ``0`` on success.  This is the equivalent of the Python statement
   ``o.attr_name = v``.

   If *v* is *NULL*, the attribute is deleted, however this feature is
   deprecated in favour of using :c:func:`PyObject_DelAttr`.


.. c:function:: int PyObject_SetAttrString(PyObject *o, const char *attr_name, PyObject *v)

   Set the value of the attribute named *attr_name*, for object *o*, to the value
   *v*. Raise an exception and return ``-1`` on failure;
   return ``0`` on success.  This is the equivalent of the Python statement
   ``o.attr_name = v``.

   If *v* is *NULL*, the attribute is deleted, however this feature is
   deprecated in favour of using :c:func:`PyObject_DelAttrString`.


.. c:function:: int PyObject_GenericSetAttr(PyObject *o, PyObject *name, PyObject *value)

   Generic attribute setter and deleter function that is meant
   to be put into a type object's :c:member:`~PyTypeObject.tp_setattro`
   slot.  It looks for a data descriptor in the
   dictionary of classes in the object's MRO, and if found it takes preference
   over setting or deleting the attribute in the instance dictionary. Otherwise, the
   attribute is set or deleted in the object's :attr:`~object.__dict__` (if present).
   On success, ``0`` is returned, otherwise an :exc:`AttributeError`
   is raised and ``-1`` is returned.


.. c:function:: int PyObject_DelAttr(PyObject *o, PyObject *attr_name)

   Delete attribute named *attr_name*, for object *o*. Returns ``-1`` on failure.
   This is the equivalent of the Python statement ``del o.attr_name``.


.. c:function:: int PyObject_DelAttrString(PyObject *o, const char *attr_name)

   Delete attribute named *attr_name*, for object *o*. Returns ``-1`` on failure.
   This is the equivalent of the Python statement ``del o.attr_name``.


.. c:function:: PyObject* PyObject_GenericGetDict(PyObject *o, void *context)

   A generic implementation for the getter of a ``__dict__`` descriptor. It
   creates the dictionary if necessary.

   .. versionadded:: 3.3


.. c:function:: int PyObject_GenericSetDict(PyObject *o, void *context)

   A generic implementation for the setter of a ``__dict__`` descriptor. This
   implementation does not allow the dictionary to be deleted.

   .. versionadded:: 3.3


.. c:function:: PyObject* PyObject_RichCompare(PyObject *o1, PyObject *o2, int opid)

   Compare the values of *o1* and *o2* using the operation specified by *opid*,
   which must be one of :const:`Py_LT`, :const:`Py_LE`, :const:`Py_EQ`,
   :const:`Py_NE`, :const:`Py_GT`, or :const:`Py_GE`, corresponding to ``<``,
   ``<=``, ``==``, ``!=``, ``>``, or ``>=`` respectively. This is the equivalent of
   the Python expression ``o1 op o2``, where ``op`` is the operator corresponding
   to *opid*. Returns the value of the comparison on success, or *NULL* on failure.


.. c:function:: int PyObject_RichCompareBool(PyObject *o1, PyObject *o2, int opid)

   Compare the values of *o1* and *o2* using the operation specified by *opid*,
   which must be one of :const:`Py_LT`, :const:`Py_LE`, :const:`Py_EQ`,
   :const:`Py_NE`, :const:`Py_GT`, or :const:`Py_GE`, corresponding to ``<``,
   ``<=``, ``==``, ``!=``, ``>``, or ``>=`` respectively. Returns ``-1`` on error,
   ``0`` if the result is false, ``1`` otherwise. This is the equivalent of the
   Python expression ``o1 op o2``, where ``op`` is the operator corresponding to
   *opid*.

.. note::
   If *o1* and *o2* are the same object, :c:func:`PyObject_RichCompareBool`
   will always return ``1`` for :const:`Py_EQ` and ``0`` for :const:`Py_NE`.

.. c:function:: PyObject* PyObject_Repr(PyObject *o)

   .. index:: builtin: repr

   Compute a string representation of object *o*.  Returns the string
   representation on success, *NULL* on failure.  This is the equivalent of the
   Python expression ``repr(o)``.  Called by the :func:`repr` built-in function.

   .. versionchanged:: 3.4
      This function now includes a debug assertion to help ensure that it
      does not silently discard an active exception.

.. c:function:: PyObject* PyObject_ASCII(PyObject *o)

   .. index:: builtin: ascii

   As :c:func:`PyObject_Repr`, compute a string representation of object *o*, but
   escape the non-ASCII characters in the string returned by
   :c:func:`PyObject_Repr` with ``\x``, ``\u`` or ``\U`` escapes.  This generates
   a string similar to that returned by :c:func:`PyObject_Repr` in Python 2.
   Called by the :func:`ascii` built-in function.

   .. index:: string; PyObject_Str (C function)


.. c:function:: PyObject* PyObject_Str(PyObject *o)

   Compute a string representation of object *o*.  Returns the string
   representation on success, *NULL* on failure.  This is the equivalent of the
   Python expression ``str(o)``.  Called by the :func:`str` built-in function
   and, therefore, by the :func:`print` function.

   .. versionchanged:: 3.4
      This function now includes a debug assertion to help ensure that it
      does not silently discard an active exception.

.. c:function:: PyObject* PyObject_Bytes(PyObject *o)

   .. index:: builtin: bytes

   Compute a bytes representation of object *o*.  *NULL* is returned on
   failure and a bytes object on success.  This is equivalent to the Python
   expression ``bytes(o)``, when *o* is not an integer.  Unlike ``bytes(o)``,
   a TypeError is raised when *o* is an integer instead of a zero-initialized
   bytes object.


.. c:function:: int PyObject_IsSubclass(PyObject *derived, PyObject *cls)

   Return ``1`` if the class *derived* is identical to or derived from the class
   *cls*, otherwise return ``0``.  In case of an error, return ``-1``.

   If *cls* is a tuple, the check will be done against every entry in *cls*.
   The result will be ``1`` when at least one of the checks returns ``1``,
   otherwise it will be ``0``.

   If *cls* has a :meth:`~class.__subclasscheck__` method, it will be called to
   determine the subclass status as described in :pep:`3119`.  Otherwise,
   *derived* is a subclass of *cls* if it is a direct or indirect subclass,
   i.e. contained in ``cls.__mro__``.

   Normally only class objects, i.e. instances of :class:`type` or a derived
   class, are considered classes.  However, objects can override this by having
   a :attr:`__bases__` attribute (which must be a tuple of base classes).


.. c:function:: int PyObject_IsInstance(PyObject *inst, PyObject *cls)

   Return ``1`` if *inst* is an instance of the class *cls* or a subclass of
   *cls*, or ``0`` if not.  On error, returns ``-1`` and sets an exception.

   If *cls* is a tuple, the check will be done against every entry in *cls*.
   The result will be ``1`` when at least one of the checks returns ``1``,
   otherwise it will be ``0``.

   If *cls* has a :meth:`~class.__instancecheck__` method, it will be called to
   determine the subclass status as described in :pep:`3119`.  Otherwise, *inst*
   is an instance of *cls* if its class is a subclass of *cls*.

   An instance *inst* can override what is considered its class by having a
   :attr:`__class__` attribute.

   An object *cls* can override if it is considered a class, and what its base
   classes are, by having a :attr:`__bases__` attribute (which must be a tuple
   of base classes).


.. c:function:: int PyCallable_Check(PyObject *o)

   Determine if the object *o* is callable.  Return ``1`` if the object is callable
   and ``0`` otherwise.  This function always succeeds.


.. c:function:: PyObject* PyObject_Call(PyObject *callable, PyObject *args, PyObject *kwargs)

   Call a callable Python object *callable*, with arguments given by the
   tuple *args*, and named arguments given by the dictionary *kwargs*.

   *args* must not be *NULL*, use an empty tuple if no arguments are needed.
   If no named arguments are needed, *kwargs* can be *NULL*.

   Return the result of the call on success, or raise an exception and return
   *NULL* on failure.

   This is the equivalent of the Python expression:
   ``callable(*args, **kwargs)``.


.. c:function:: PyObject* PyObject_CallObject(PyObject *callable, PyObject *args)

   Call a callable Python object *callable*, with arguments given by the
   tuple *args*.  If no arguments are needed, then *args* can be *NULL*.

   Return the result of the call on success, or raise an exception and return
   *NULL* on failure.

   This is the equivalent of the Python expression: ``callable(*args)``.


.. c:function:: PyObject* PyObject_CallFunction(PyObject *callable, const char *format, ...)

   Call a callable Python object *callable*, with a variable number of C arguments.
   The C arguments are described using a :c:func:`Py_BuildValue` style format
   string.  The format can be *NULL*, indicating that no arguments are provided.

   Return the result of the call on success, or raise an exception and return
   *NULL* on failure.

   This is the equivalent of the Python expression: ``callable(*args)``.

   Note that if you only pass :c:type:`PyObject \*` args,
   :c:func:`PyObject_CallFunctionObjArgs` is a faster alternative.

   .. versionchanged:: 3.4
      The type of *format* was changed from ``char *``.


.. c:function:: PyObject* PyObject_CallMethod(PyObject *obj, const char *name, const char *format, ...)

   Call the method named *name* of object *obj* with a variable number of C
   arguments.  The C arguments are described by a :c:func:`Py_BuildValue` format
   string that should  produce a tuple.

   The format can be *NULL*, indicating that no arguments are provided.

   Return the result of the call on success, or raise an exception and return
   *NULL* on failure.

   This is the equivalent of the Python expression:
   ``obj.name(arg1, arg2, ...)``.

   Note that if you only pass :c:type:`PyObject \*` args,
   :c:func:`PyObject_CallMethodObjArgs` is a faster alternative.

   .. versionchanged:: 3.4
      The types of *name* and *format* were changed from ``char *``.


.. c:function:: PyObject* PyObject_CallFunctionObjArgs(PyObject *callable, ..., NULL)

   Call a callable Python object *callable*, with a variable number of
   :c:type:`PyObject\*` arguments.  The arguments are provided as a variable number
   of parameters followed by *NULL*.

   Return the result of the call on success, or raise an exception and return
   *NULL* on failure.

   This is the equivalent of the Python expression:
   ``callable(arg1, arg2, ...)``.


.. c:function:: PyObject* PyObject_CallMethodObjArgs(PyObject *obj, PyObject *name, ..., NULL)

   Calls a method of the Python object *obj*, where the name of the method is given as a
   Python string object in *name*.  It is called with a variable number of
   :c:type:`PyObject\*` arguments.  The arguments are provided as a variable number
   of parameters followed by *NULL*.

   Return the result of the call on success, or raise an exception and return
   *NULL* on failure.


.. c:function:: PyObject* _PyObject_Vectorcall(PyObject *callable, PyObject *const *args, size_t nargsf, PyObject *kwnames)

   Call a callable Python object *callable*, using
   :c:data:`vectorcall <PyTypeObject.tp_vectorcall_offset>` if possible.

   *args* is a C array with the positional arguments.

   *nargsf* is the number of positional arguments plus optionally the flag
   :const:`PY_VECTORCALL_ARGUMENTS_OFFSET` (see below).
   To get actual number of arguments, use
   :c:func:`PyVectorcall_NARGS(nargsf) <PyVectorcall_NARGS>`.

   *kwnames* can be either NULL (no keyword arguments) or a tuple of keyword
   names. In the latter case, the values of the keyword arguments are stored
   in *args* after the positional arguments.
   The number of keyword arguments does not influence *nargsf*.

   *kwnames* must contain only objects of type ``str`` (not a subclass),
   and all keys must be unique.

   Return the result of the call on success, or raise an exception and return
   *NULL* on failure.

   This uses the vectorcall protocol if the callable supports it;
   otherwise, the arguments are converted to use
   :c:member:`~PyTypeObject.tp_call`.

   .. note::

      This function is provisional and expected to become public in Python 3.9,
      with a different name and, possibly, changed semantics.
      If you use the function, plan for updating your code for Python 3.9.

   .. versionadded:: 3.8

.. c:var:: PY_VECTORCALL_ARGUMENTS_OFFSET

   If set in a vectorcall *nargsf* argument, the callee is allowed to
   temporarily change ``args[-1]``. In other words, *args* points to
   argument 1 (not 0) in the allocated vector.
   The callee must restore the value of ``args[-1]`` before returning.

   Whenever they can do so cheaply (without additional allocation), callers
   are encouraged to use :const:`PY_VECTORCALL_ARGUMENTS_OFFSET`.
   Doing so will allow callables such as bound methods to make their onward
   calls (which include a prepended *self* argument) cheaply.

   .. versionadded:: 3.8

.. c:function:: Py_ssize_t PyVectorcall_NARGS(size_t nargsf)

   Given a vectorcall *nargsf* argument, return the actual number of
   arguments.
   Currently equivalent to ``nargsf & ~PY_VECTORCALL_ARGUMENTS_OFFSET``.

   .. versionadded:: 3.8

.. c:function:: PyObject* _PyObject_FastCallDict(PyObject *callable, PyObject *const *args, size_t nargsf, PyObject *kwdict)

   Same as :c:func:`_PyObject_Vectorcall` except that the keyword arguments
   are passed as a dictionary in *kwdict*. This may be *NULL* if there
   are no keyword arguments.

   For callables supporting :c:data:`vectorcall <PyTypeObject.tp_vectorcall_offset>`,
   the arguments are internally converted to the vectorcall convention.
   Therefore, this function adds some overhead compared to
   :c:func:`_PyObject_Vectorcall`.
   It should only be used if the caller already has a dictionary ready to use.

   .. note::

      This function is provisional and expected to become public in Python 3.9,
      with a different name and, possibly, changed semantics.
      If you use the function, plan for updating your code for Python 3.9.

   .. versionadded:: 3.8


.. c:function:: Py_hash_t PyObject_Hash(PyObject *o)

   .. index:: builtin: hash

   Compute and return the hash value of an object *o*.  On failure, return ``-1``.
   This is the equivalent of the Python expression ``hash(o)``.

   .. versionchanged:: 3.2
      The return type is now Py_hash_t.  This is a signed integer the same size
      as Py_ssize_t.


.. c:function:: Py_hash_t PyObject_HashNotImplemented(PyObject *o)

   Set a :exc:`TypeError` indicating that ``type(o)`` is not hashable and return ``-1``.
   This function receives special treatment when stored in a ``tp_hash`` slot,
   allowing a type to explicitly indicate to the interpreter that it is not
   hashable.


.. c:function:: int PyObject_IsTrue(PyObject *o)

   Returns ``1`` if the object *o* is considered to be true, and ``0`` otherwise.
   This is equivalent to the Python expression ``not not o``.  On failure, return
   ``-1``.


.. c:function:: int PyObject_Not(PyObject *o)

   Returns ``0`` if the object *o* is considered to be true, and ``1`` otherwise.
   This is equivalent to the Python expression ``not o``.  On failure, return
   ``-1``.


.. c:function:: PyObject* PyObject_Type(PyObject *o)

   .. index:: builtin: type

   When *o* is non-*NULL*, returns a type object corresponding to the object type
   of object *o*. On failure, raises :exc:`SystemError` and returns *NULL*.  This
   is equivalent to the Python expression ``type(o)``. This function increments the
   reference count of the return value. There's really no reason to use this
   function instead of the common expression ``o->ob_type``, which returns a
   pointer of type :c:type:`PyTypeObject\*`, except when the incremented reference
   count is needed.


.. c:function:: int PyObject_TypeCheck(PyObject *o, PyTypeObject *type)

   Return true if the object *o* is of type *type* or a subtype of *type*.  Both
   parameters must be non-*NULL*.


.. c:function:: Py_ssize_t PyObject_Size(PyObject *o)
               Py_ssize_t PyObject_Length(PyObject *o)

   .. index:: builtin: len

   Return the length of object *o*.  If the object *o* provides either the sequence
   and mapping protocols, the sequence length is returned.  On error, ``-1`` is
   returned.  This is the equivalent to the Python expression ``len(o)``.


.. c:function:: Py_ssize_t PyObject_LengthHint(PyObject *o, Py_ssize_t default)

   Return an estimated length for the object *o*. First try to return its
   actual length, then an estimate using :meth:`~object.__length_hint__`, and
   finally return the default value. On error return ``-1``. This is the
   equivalent to the Python expression ``operator.length_hint(o, default)``.

   .. versionadded:: 3.4


.. c:function:: PyObject* PyObject_GetItem(PyObject *o, PyObject *key)

   Return element of *o* corresponding to the object *key* or *NULL* on failure.
   This is the equivalent of the Python expression ``o[key]``.


.. c:function:: int PyObject_SetItem(PyObject *o, PyObject *key, PyObject *v)

   Map the object *key* to the value *v*.  Raise an exception and
   return ``-1`` on failure; return ``0`` on success.  This is the
   equivalent of the Python statement ``o[key] = v``.


.. c:function:: int PyObject_DelItem(PyObject *o, PyObject *key)

   Remove the mapping for the object *key* from the object *o*.  Return ``-1``
   on failure.  This is equivalent to the Python statement ``del o[key]``.


.. c:function:: PyObject* PyObject_Dir(PyObject *o)

   This is equivalent to the Python expression ``dir(o)``, returning a (possibly
   empty) list of strings appropriate for the object argument, or *NULL* if there
   was an error.  If the argument is *NULL*, this is like the Python ``dir()``,
   returning the names of the current locals; in this case, if no execution frame
   is active then *NULL* is returned but :c:func:`PyErr_Occurred` will return false.


.. c:function:: PyObject* PyObject_GetIter(PyObject *o)

   This is equivalent to the Python expression ``iter(o)``. It returns a new
   iterator for the object argument, or the object  itself if the object is already
   an iterator.  Raises :exc:`TypeError` and returns *NULL* if the object cannot be
   iterated.