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5.5 Dictionaries

Another useful data type built into Python is the dictionary. Dictionaries are sometimes found in other languages as "associative arrays" or "hashes". Unlike sequences, which are indexed by a range of numbers, dictionaries are indexed by keys, which can be any immutable type; strings and numbers can always be keys. Tuples can be used as keys if they contain only strings, numbers, or tuples; if a tuple contains any mutable object either directly or indirectly, it cannot be used as a key. You can't use lists as keys, since lists can be modified in place using index assignments, slice assignments, or methods like append() and extend().

It is best to think of a dictionary as an unordered set of key: value pairs, with the requirement that the keys are unique (within one dictionary). A pair of braces creates an empty dictionary: {}. Placing a comma-separated list of key:value pairs within the braces adds initial key:value pairs to the dictionary; this is also the way dictionaries are written on output.

The main operations on a dictionary are storing a value with some key and extracting the value given the key. It is also possible to delete a key:value pair with del. If you store using a key that is already in use, the old value associated with that key is forgotten. It is an error to extract a value using a non-existent key.

The keys() method of a dictionary object returns a list of all the keys used in the dictionary, in arbitrary order (if you want it sorted, just apply the sort() method to the list of keys). To check whether a single key is in the dictionary, either use the dictionary's has_key() method or the in keyword.

Here is a small example using a dictionary:

    >>> tel = {'jack': 4098, 'sape': 4139}
    >>> tel['guido'] = 4127
    >>> tel
    {'sape': 4139, 'guido': 4127, 'jack': 4098}
    >>> tel['jack']
    4098
    >>> del tel['sape']
    >>> tel['irv'] = 4127
    >>> tel
    {'guido': 4127, 'irv': 4127, 'jack': 4098}
    >>> tel.keys()
    ['guido', 'irv', 'jack']
    >>> tel.has_key('guido')
    True
    >>> 'guido' in tel
    True

The dict() constructor builds dictionaries directly from lists of key-value pairs stored as tuples. When the pairs form a pattern, list comprehensions can compactly specify the key-value list.

    >>> dict([('sape', 4139), ('guido', 4127),
              ('jack', 4098)])
    {'sape': 4139, 'jack': 4098, 'guido': 4127}
    >>> dict([(x, x**2) for x in (2, 4, 6)]) # use a list
                                             # comprehension
    {2: 4, 4: 16, 6: 36}

Later in the tutorial, we will learn about generator expressions which are even better suited for the task of supplying key-value pairs to the dict() constructor.

When the keys are simple strings, it is sometimes easier to specify pairs using keyword arguments:

    >>> dict(sape=4139, guido=4127, jack=4098)
    {'sape': 4139, 'jack': 4098, 'guido': 4127}