cpython/Doc/c-api/dict.rst

.. highlight:: c

.. _dictobjects:

Dictionary Objects
------------------

.. index:: object: dictionary


.. c:type:: PyDictObject

   This subtype of :c:type:`PyObject` represents a Python dictionary object.


.. c:var:: PyTypeObject PyDict_Type

   This instance of :c:type:`PyTypeObject` represents the Python dictionary
   type.  This is the same object as :class:`dict` in the Python layer.


.. c:function:: int PyDict_Check(PyObject *p)

   Return true if *p* is a dict object or an instance of a subtype of the dict
   type.  This function always succeeds.


.. c:function:: int PyDict_CheckExact(PyObject *p)

   Return true if *p* is a dict object, but not an instance of a subtype of
   the dict type.  This function always succeeds.


.. c:function:: PyObject* PyDict_New()

   Return a new empty dictionary, or ``NULL`` on failure.


.. c:function:: PyObject* PyDictProxy_New(PyObject *mapping)

   Return a :class:`types.MappingProxyType` object for a mapping which
   enforces read-only behavior.  This is normally used to create a view to
   prevent modification of the dictionary for non-dynamic class types.


.. c:function:: void PyDict_Clear(PyObject *p)

   Empty an existing dictionary of all key-value pairs.


.. c:function:: int PyDict_Contains(PyObject *p, PyObject *key)

   Determine if dictionary *p* contains *key*.  If an item in *p* is matches
   *key*, return ``1``, otherwise return ``0``.  On error, return ``-1``.
   This is equivalent to the Python expression ``key in p``.


.. c:function:: PyObject* PyDict_Copy(PyObject *p)

   Return a new dictionary that contains the same key-value pairs as *p*.


.. c:function:: int PyDict_SetItem(PyObject *p, PyObject *key, PyObject *val)

   Insert *val* into the dictionary *p* with a key of *key*.  *key* must be
   :term:`hashable`; if it isn't, :exc:`TypeError` will be raised. Return
   ``0`` on success or ``-1`` on failure.  This function *does not* steal a
   reference to *val*.


.. c:function:: int PyDict_SetItemString(PyObject *p, const char *key, PyObject *val)

   .. index:: single: PyUnicode_FromString()

   Insert *val* into the dictionary *p* using *key* as a key. *key* should
   be a :c:expr:`const char*`.  The key object is created using
   ``PyUnicode_FromString(key)``.  Return ``0`` on success or ``-1`` on
   failure.  This function *does not* steal a reference to *val*.


.. c:function:: int PyDict_DelItem(PyObject *p, PyObject *key)

   Remove the entry in dictionary *p* with key *key*. *key* must be hashable;
   if it isn't, :exc:`TypeError` is raised.
   If *key* is not in the dictionary, :exc:`KeyError` is raised.
   Return ``0`` on success or ``-1`` on failure.


.. c:function:: int PyDict_DelItemString(PyObject *p, const char *key)

   Remove the entry in dictionary *p* which has a key specified by the string *key*.
   If *key* is not in the dictionary, :exc:`KeyError` is raised.
   Return ``0`` on success or ``-1`` on failure.


.. c:function:: PyObject* PyDict_GetItem(PyObject *p, PyObject *key)

   Return the object from dictionary *p* which has a key *key*.  Return ``NULL``
   if the key *key* is not present, but *without* setting an exception.

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

   .. versionchanged:: 3.10
      Calling this API without :term:`GIL` held had been allowed for historical
      reason. It is no longer allowed.


.. c:function:: PyObject* PyDict_GetItemWithError(PyObject *p, PyObject *key)

   Variant of :c:func:`PyDict_GetItem` that does not suppress
   exceptions. Return ``NULL`` **with** an exception set if an exception
   occurred.  Return ``NULL`` **without** an exception set if the key
   wasn't present.


.. c:function:: PyObject* PyDict_GetItemString(PyObject *p, const char *key)

   This is the same as :c:func:`PyDict_GetItem`, but *key* is specified as a
   :c:expr:`const char*`, rather than a :c:expr:`PyObject*`.

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


.. c:function:: PyObject* PyDict_SetDefault(PyObject *p, PyObject *key, PyObject *defaultobj)

   This is the same as the Python-level :meth:`dict.setdefault`.  If present, it
   returns the value corresponding to *key* from the dictionary *p*.  If the key
   is not in the dict, it is inserted with value *defaultobj* and *defaultobj*
   is returned.  This function evaluates the hash function of *key* only once,
   instead of evaluating it independently for the lookup and the insertion.

   .. versionadded:: 3.4

.. c:function:: PyObject* PyDict_Items(PyObject *p)

   Return a :c:type:`PyListObject` containing all the items from the dictionary.


.. c:function:: PyObject* PyDict_Keys(PyObject *p)

   Return a :c:type:`PyListObject` containing all the keys from the dictionary.


.. c:function:: PyObject* PyDict_Values(PyObject *p)

   Return a :c:type:`PyListObject` containing all the values from the dictionary
   *p*.


.. c:function:: Py_ssize_t PyDict_Size(PyObject *p)

   .. index:: builtin: len

   Return the number of items in the dictionary.  This is equivalent to
   ``len(p)`` on a dictionary.


.. c:function:: int PyDict_Next(PyObject *p, Py_ssize_t *ppos, PyObject **pkey, PyObject **pvalue)

   Iterate over all key-value pairs in the dictionary *p*.  The
   :c:type:`Py_ssize_t` referred to by *ppos* must be initialized to ``0``
   prior to the first call to this function to start the iteration; the
   function returns true for each pair in the dictionary, and false once all
   pairs have been reported.  The parameters *pkey* and *pvalue* should either
   point to :c:expr:`PyObject*` variables that will be filled in with each key
   and value, respectively, or may be ``NULL``.  Any references returned through
   them are borrowed.  *ppos* should not be altered during iteration. Its
   value represents offsets within the internal dictionary structure, and
   since the structure is sparse, the offsets are not consecutive.

   For example::

      PyObject *key, *value;
      Py_ssize_t pos = 0;

      while (PyDict_Next(self->dict, &pos, &key, &value)) {
          /* do something interesting with the values... */
          ...
      }

   The dictionary *p* should not be mutated during iteration.  It is safe to
   modify the values of the keys as you iterate over the dictionary, but only
   so long as the set of keys does not change.  For example::

      PyObject *key, *value;
      Py_ssize_t pos = 0;

      while (PyDict_Next(self->dict, &pos, &key, &value)) {
          long i = PyLong_AsLong(value);
          if (i == -1 && PyErr_Occurred()) {
              return -1;
          }
          PyObject *o = PyLong_FromLong(i + 1);
          if (o == NULL)
              return -1;
          if (PyDict_SetItem(self->dict, key, o) < 0) {
              Py_DECREF(o);
              return -1;
          }
          Py_DECREF(o);
      }


.. c:function:: int PyDict_Merge(PyObject *a, PyObject *b, int override)

   Iterate over mapping object *b* adding key-value pairs to dictionary *a*.
   *b* may be a dictionary, or any object supporting :c:func:`PyMapping_Keys`
   and :c:func:`PyObject_GetItem`. If *override* is true, existing pairs in *a*
   will be replaced if a matching key is found in *b*, otherwise pairs will
   only be added if there is not a matching key in *a*. Return ``0`` on
   success or ``-1`` if an exception was raised.


.. c:function:: int PyDict_Update(PyObject *a, PyObject *b)

   This is the same as ``PyDict_Merge(a, b, 1)`` in C, and is similar to
   ``a.update(b)`` in Python except that :c:func:`PyDict_Update` doesn't fall
   back to the iterating over a sequence of key value pairs if the second
   argument has no "keys" attribute.  Return ``0`` on success or ``-1`` if an
   exception was raised.


.. c:function:: int PyDict_MergeFromSeq2(PyObject *a, PyObject *seq2, int override)

   Update or merge into dictionary *a*, from the key-value pairs in *seq2*.
   *seq2* must be an iterable object producing iterable objects of length 2,
   viewed as key-value pairs.  In case of duplicate keys, the last wins if
   *override* is true, else the first wins. Return ``0`` on success or ``-1``
   if an exception was raised. Equivalent Python (except for the return
   value)::

      def PyDict_MergeFromSeq2(a, seq2, override):
          for key, value in seq2:
              if override or key not in a:
                  a[key] = value

.. c:function:: int PyDict_AddWatcher(PyDict_WatchCallback callback)

   Register *callback* as a dictionary watcher. Return a non-negative integer
   id which must be passed to future calls to :c:func:`PyDict_Watch`. In case
   of error (e.g. no more watcher IDs available), return ``-1`` and set an
   exception.

   .. versionadded:: 3.12

.. c:function:: int PyDict_ClearWatcher(int watcher_id)

   Clear watcher identified by *watcher_id* previously returned from
   :c:func:`PyDict_AddWatcher`. Return ``0`` on success, ``-1`` on error (e.g.
   if the given *watcher_id* was never registered.)

   .. versionadded:: 3.12

.. c:function:: int PyDict_Watch(int watcher_id, PyObject *dict)

   Mark dictionary *dict* as watched. The callback granted *watcher_id* by
   :c:func:`PyDict_AddWatcher` will be called when *dict* is modified or
   deallocated. Return ``0`` on success or ``-1`` on error.

   .. versionadded:: 3.12

.. c:function:: int PyDict_Unwatch(int watcher_id, PyObject *dict)

   Mark dictionary *dict* as no longer watched. The callback granted
   *watcher_id* by :c:func:`PyDict_AddWatcher` will no longer be called when
   *dict* is modified or deallocated. The dict must previously have been
   watched by this watcher. Return ``0`` on success or ``-1`` on error.

   .. versionadded:: 3.12

.. c:type:: PyDict_WatchEvent

   Enumeration of possible dictionary watcher events: ``PyDict_EVENT_ADDED``,
   ``PyDict_EVENT_MODIFIED``, ``PyDict_EVENT_DELETED``, ``PyDict_EVENT_CLONED``,
   ``PyDict_EVENT_CLEARED``, or ``PyDict_EVENT_DEALLOCATED``.

   .. versionadded:: 3.12

.. c:type:: int (*PyDict_WatchCallback)(PyDict_WatchEvent event, PyObject *dict, PyObject *key, PyObject *new_value)

   Type of a dict watcher callback function.

   If *event* is ``PyDict_EVENT_CLEARED`` or ``PyDict_EVENT_DEALLOCATED``, both
   *key* and *new_value* will be ``NULL``. If *event* is ``PyDict_EVENT_ADDED``
   or ``PyDict_EVENT_MODIFIED``, *new_value* will be the new value for *key*.
   If *event* is ``PyDict_EVENT_DELETED``, *key* is being deleted from the
   dictionary and *new_value* will be ``NULL``.

   ``PyDict_EVENT_CLONED`` occurs when *dict* was previously empty and another
   dict is merged into it. To maintain efficiency of this operation, per-key
   ``PyDict_EVENT_ADDED`` events are not issued in this case; instead a
   single ``PyDict_EVENT_CLONED`` is issued, and *key* will be the source
   dictionary.

   The callback may inspect but must not modify *dict*; doing so could have
   unpredictable effects, including infinite recursion.

   Callbacks occur before the notified modification to *dict* takes place, so
   the prior state of *dict* can be inspected.

   If the callback returns with an exception set, it must return ``-1``; this
   exception will be printed as an unraisable exception using
   :c:func:`PyErr_WriteUnraisable`. Otherwise it should return ``0``.

   .. versionadded:: 3.12