cpython/Doc/c-api/dict.rst

.. highlightlang:: c

.. _dictobjects:

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

.. index:: object: dictionary


.. ctype:: PyDictObject

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


.. cvar:: PyTypeObject PyDict_Type

   .. index::
      single: DictType (in module types)
      single: DictionaryType (in module types)

   This instance of :ctype:`PyTypeObject` represents the Python dictionary
   type.  This is exposed to Python programs as ``dict`` and
   ``types.DictType``.


.. cfunction:: int PyDict_Check(PyObject *p)

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


.. cfunction:: int PyDict_CheckExact(PyObject *p)

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


.. cfunction:: PyObject* PyDict_New()

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


.. cfunction:: PyObject* PyDictProxy_New(PyObject *dict)

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


.. cfunction:: void PyDict_Clear(PyObject *p)

   Empty an existing dictionary of all key-value pairs.


.. cfunction:: 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``.


.. cfunction:: PyObject* PyDict_Copy(PyObject *p)

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


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

   Insert *value* 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.


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

   .. index:: single: PyUnicode_FromString()

   Insert *value* into the dictionary *p* using *key* as a key. *key* should
   be a :ctype:`char\*`.  The key object is created using
   ``PyUnicode_FromString(key)``.  Return ``0`` on success or ``-1`` on
   failure.


.. cfunction:: 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.  Return ``0`` on success or ``-1``
   on failure.


.. cfunction:: int PyDict_DelItemString(PyObject *p, char *key)

   Remove the entry in dictionary *p* which has a key specified by the string
   *key*.  Return ``0`` on success or ``-1`` on failure.


.. cfunction:: 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.


.. cfunction:: PyObject* PyDict_GetItemWithError(PyObject *p, PyObject *key)

   Variant of :cfunc:`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.


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

   This is the same as :cfunc:`PyDict_GetItem`, but *key* is specified as a
   :ctype:`char\*`, rather than a :ctype:`PyObject\*`.


.. cfunction:: PyObject* PyDict_Items(PyObject *p)

   Return a :ctype:`PyListObject` containing all the items from the
   dictionary, as in the dictionary method :meth:`dict.items`.


.. cfunction:: PyObject* PyDict_Keys(PyObject *p)

   Return a :ctype:`PyListObject` containing all the keys from the dictionary,
   as in the dictionary method :meth:`dict.keys`.


.. cfunction:: PyObject* PyDict_Values(PyObject *p)

   Return a :ctype:`PyListObject` containing all the values from the
   dictionary *p*, as in the dictionary method :meth:`dict.values`.


.. cfunction:: 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.


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

   Iterate over all key-value pairs in the dictionary *p*.  The
   :ctype:`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 :ctype:`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);
      }


.. cfunction:: 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 :cfunc:`PyMapping_Keys`
   and :cfunc:`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.


.. cfunction:: int PyDict_Update(PyObject *a, PyObject *b)

   This is the same as ``PyDict_Merge(a, b, 1)`` in C, or ``a.update(b)`` in
   Python.  Return ``0`` on success or ``-1`` if an exception was raised.


.. cfunction:: 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