Python in a Nutshell, 2nd Edition

Dictionary Operations

Python provides a variety of operations applicable to dictionaries. Since dictionaries are containers, the built-in len function can take a dictionary as its single argument and return the number of items (key/value pairs) in the dictionary object. A dictionary is iterable, so you can pass it to any function or method that takes an iterable argument. In this case, only the keys of the dictionary are iterated upon, in some arbitrary order. For example, for any dictionary D, min(D) returns the smallest key in D.

Dictionary Membership

The k in D operator checks whether object k is one of the keys of the dictionary D. It returns True if it is and False if it isn’t. k not in D is just like not (k in D).

Indexing a Dictionary

The value in a dictionary D that is currently associated with key k is denoted by an indexing: D[k]. Indexing with a key that is not present in the dictionary raises an exception. For example:

d = { 'x':42, 'y':3.14, 'z':7 }
d['x']                           # 42
d['z']                           # 7
d['a']                           # raises KeyError exception

Plain assignment to a dictionary indexed with a key that is not yet in the dictionary (e.g., D[newkey]=value) is a valid operation and adds the key and value as a new item in the dictionary. For instance:

d = { 'x':42, 'y':3.14}
d['a'] = 16                      # d is now {'x':42, 'y':3.14,'a':16}

The del statement, in the form del D[k], removes from the dictionary the item whose key is k. If k is not a key in dictionary D, del D[k] raises an exception.

Dictionary Methods

Dictionary objects provide ...