Python in a Nutshell

Dictionary Operations

Python provides a variety of operations that can be applied 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.

Dictionary Membership

In Python 2.2 and later, the k in D operator tests to see whether object k is one of the keys of the dictionary D. It returns True if it is and False if it isn’t. Similarly, the k not in D operator 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 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 that adds the key and value as a new item in the dictionary. For instance:

d = { 'x':42, 'y':3.14, 'z':7 } 
d['a'] = 16                      # d is now {'x':42,'y':3.14,'z':7,'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 several methods, as shown in Table 4-4. Non-mutating methods return a result without altering the object to which they apply, while mutating methods may alter the object ...