cpython/Doc/c-api/object.rst

.. highlightlang:: c

.. _object:

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


.. 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.


.. 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.


.. 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:`__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*. Returns ``-1`` on failure.  This is the equivalent of the Python statement
   ``o.attr_name = v``.


.. 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*. Returns ``-1`` on failure.  This is the equivalent of the Python statement
   ``o.attr_name = v``.


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

   Generic attribute setter function that is meant to be put into a type
   object's ``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 the attribute in the instance dictionary. Otherwise, the
   attribute is set in the object's :attr:`__dict__` (if present).  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_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:: int PyObject_Cmp(PyObject *o1, PyObject *o2, int *result)

   .. index:: builtin: cmp

   Compare the values of *o1* and *o2* using a routine provided by *o1*, if one
   exists, otherwise with a routine provided by *o2*.  The result of the comparison
   is returned in *result*.  Returns ``-1`` on failure.  This is the equivalent of
   the Python statement ``result = cmp(o1, o2)``.


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

   .. index:: builtin: cmp

   Compare the values of *o1* and *o2* using a routine provided by *o1*, if one
   exists, otherwise with a routine provided by *o2*.  Returns the result of the
   comparison on success.  On error, the value returned is undefined; use
   :c:func:`PyErr_Occurred` to detect an error.  This is equivalent to the Python
   expression ``cmp(o1, o2)``.


.. 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 and
   by reverse quotes.


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

   .. index:: builtin: str

   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
   by the :keyword:`print` statement.


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

   .. index:: builtin: bytes

   Compute a bytes representation of object *o*.  In 2.x, this is just a alias
   for :c:func:`PyObject_Str`.


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

   .. index:: builtin: unicode

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


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

   Returns ``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 type object rather than a class object, :c:func:`PyObject_IsInstance`
   returns ``1`` if *inst* is of type *cls*.  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 *inst* is not a
   class instance and *cls* is neither a type object, nor a class object, nor a
   tuple, *inst* must have a :attr:`__class__` attribute --- the class relationship
   of the value of that attribute with *cls* will be used to determine the result
   of this function.

   .. versionadded:: 2.1

   .. versionchanged:: 2.2
      Support for a tuple as the second argument added.

Subclass determination is done in a fairly straightforward way, but includes a
wrinkle that implementors of extensions to the class system may want to be aware
of.  If :class:`A` and :class:`B` are class objects, :class:`B` is a subclass of
:class:`A` if it inherits from :class:`A` either directly or indirectly.  If
either is not a class object, a more general mechanism is used to determine the
class relationship of the two objects.  When testing if *B* is a subclass of
*A*, if *A* is *B*, :c:func:`PyObject_IsSubclass` returns true.  If *A* and *B*
are different objects, *B*'s :attr:`__bases__` attribute is searched in a
depth-first fashion for *A* --- the presence of the :attr:`__bases__` attribute
is considered sufficient for this determination.


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

   Returns ``1`` if the class *derived* is identical to or derived from the class
   *cls*, otherwise returns ``0``.  In case of an error, returns ``-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 either *derived* or *cls* is not an actual class object (or tuple),
   this function uses the generic algorithm described above.

   .. versionadded:: 2.1

   .. versionchanged:: 2.3
      Older versions of Python did not support a tuple as the second argument.


.. 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_object, PyObject *args, PyObject *kw)

   .. index:: builtin: apply

   Call a callable Python object *callable_object*, with arguments given by the
   tuple *args*, and named arguments given by the dictionary *kw*. If no named
   arguments are needed, *kw* may be *NULL*. *args* must not be *NULL*, use an
   empty tuple if no arguments are needed. Returns the result of the call on
   success, or *NULL* on failure.  This is the equivalent of the Python expression
   ``apply(callable_object, args, kw)`` or ``callable_object(*args, **kw)``.

   .. versionadded:: 2.2


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

   .. index:: builtin: apply

   Call a callable Python object *callable_object*, with arguments given by the
   tuple *args*.  If no arguments are needed, then *args* may be *NULL*.  Returns
   the result of the call on success, or *NULL* on failure.  This is the equivalent
   of the Python expression ``apply(callable_object, args)`` or
   ``callable_object(*args)``.


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

   .. index:: builtin: apply

   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 may be *NULL*, indicating that no arguments are provided.
   Returns the result of the call on success, or *NULL* on failure.  This is the
   equivalent of the Python expression ``apply(callable, args)`` or
   ``callable(*args)``. Note that if you only pass :c:type:`PyObject \*` args,
   :c:func:`PyObject_CallFunctionObjArgs` is a faster alternative.


.. c:function:: PyObject* PyObject_CallMethod(PyObject *o, char *method, char *format, ...)

   Call the method named *method* of object *o* 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 may be *NULL*, indicating that
   no arguments are provided. Returns the result of the call on success, or *NULL*
   on failure.  This is the equivalent of the Python expression ``o.method(args)``.
   Note that if you only pass :c:type:`PyObject \*` args,
   :c:func:`PyObject_CallMethodObjArgs` is a faster alternative.


.. 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*. Returns the result of the call on success, or
   *NULL* on failure.

   .. versionadded:: 2.2


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

   Calls a method of the object *o*, 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*. Returns the result of the call on success, or
   *NULL* on failure.

   .. versionadded:: 2.2


.. c:function:: long 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)``.


.. c:function:: long 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.

   .. versionadded:: 2.6


.. 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*.

   .. versionadded:: 2.2


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

   .. versionchanged:: 2.5
      These functions returned an :c:type:`int` type. This might require
      changes in your code for properly supporting 64-bit systems.


.. 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*.  Returns ``-1`` on failure.  This is the
   equivalent of the Python statement ``o[key] = v``.


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

   Delete the mapping for *key* from *o*.  Returns ``-1`` on failure. This is the
   equivalent of the Python statement ``del o[key]``.


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

   Derives a file descriptor from a Python object.  If the object is an integer or
   long integer, its value is returned.  If not, the object's :meth:`fileno` method
   is called if it exists; the method must return an integer or long integer, which
   is returned as the file descriptor value.  Returns ``-1`` on failure.


.. 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.