7.4.1 Dictionary Objects

Python PEP

previous page next page

7.4.1 Dictionary Objects

PyDictObject: This subtype of PyObject represents a Python dictionary object.

PyTypeObject PyDict_Type: This instance of PyTypeObject represents the Python dictionary type. This is exposed to Python programs as types.DictType and types.DictionaryType.

int PyDict_Check( PyObject *p): Returns true if p is a dict object or an instance of a subtype of the dict type. Changed in version 2.2: Allowed subtypes to be accepted.

int PyDict_CheckExact( PyObject *p): Return true if p is a dict object, but not an instance of a subtype of the dict type. New in version 2.4.

PyObject* PyDict_New( ): Return value: New reference.
Returns a new empty dictionary, or NULL on failure.

PyObject* PyDictProxy_New( PyObject *dict): Return value: New reference.
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. New in version 2.2.

void PyDict_Clear( PyObject *p): Empties an existing dictionary of all key-value pairs.

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". New in version 2.4.

PyObject* PyDict_Copy( PyObject *p): Return value: New reference.
Returns a new dictionary that contains the same key-value pairs as p. New in version 1.6.

int PyDict_SetItem( PyObject *p, PyObject *key, PyObject *val): Inserts value into the dictionary p with a key of key. key must be hashable; if it isn't, TypeError will be raised. Returns 0 on success or -1 on failure.

int PyDict_SetItemString( PyObject *p, char *key, PyObject *val): Inserts value into the dictionary p using key as a key. key should be a char*. The key object is created using PyString_FromString(key). Returns 0 on success or -1 on failure.

int PyDict_DelItem( PyObject *p, PyObject *key): Removes the entry in dictionary p with key key. key must be hashable; if it isn't, TypeError is raised. Returns 0 on success or -1 on failure.

int PyDict_DelItemString( PyObject *p, char *key): Removes the entry in dictionary p which has a key specified by the string key. Returns 0 on success or -1 on failure.

PyObject* PyDict_GetItem( PyObject *p, PyObject *key): Return value: Borrowed reference.
Returns the object from dictionary p which has a key key. Returns NULL if the key key is not present, but without setting an exception.

PyObject* PyDict_GetItemString( PyObject *p, char *key): Return value: Borrowed reference.
This is the same as PyDict_GetItem(), but key is specified as a char*, rather than a PyObject*.

PyObject* PyDict_Items( PyObject *p): Return value: New reference.
Returns a PyListObject containing all the items from the dictionary, as in the dictionary method items() (see the Python Library Reference).

PyObject* PyDict_Keys( PyObject *p): Return value: New reference.
Returns a PyListObject containing all the keys from the dictionary, as in the dictionary method keys() (see the Python Library Reference).

PyObject* PyDict_Values( PyObject *p): Return value: New reference.
Returns a PyListObject containing all the values from the dictionary p, as in the dictionary method values() (see the Python Library Reference).

int PyDict_Size( PyObject *p): Returns the number of items in the dictionary. This is equivalent to "len(p)" on a dictionary.

int PyDict_Next( PyObject *p, int *ppos, PyObject **pkey, PyObject **pvalue)

Iterate over all key-value pairs in the dictionary p. The int 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 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;
int 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 (since Python 2.1) 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;
int pos = 0;

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

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 PyMapping_Keys() and 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. New in version 2.2.

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. New in version 2.2.

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

New in version 2.2.

See About this document... for information on suggesting changes.

previous page start next page