Python dictionaries are something completely different (Monty Python reference intended)—they are not sequences at all, but are instead known as mappings. Mappings are also collections of other objects, but they store objects by key instead of by relative position. In fact, mappings don’t maintain any reliable left-to-right order; they simply map keys to associated values. Dictionaries, the only mapping type in Python’s core objects set, are also mutable: they may be changed in-place, and can grow and shrink on demand, like lists.

Mapping Operations

When written as literals, dictionaries are coded in curly braces, and consist of a series of “key: value” pairs. Dictionaries are useful anytime we need to associate a set of values with keys—to describe the properties of something, for instance. As an example, consider the following three-item dictionary (with keys “food,” “quantity,” and “color”):

  >>> D = {'food': 'Spam', 'quantity': 4, 'color': 'pink'}

We can index this dictionary by key to fetch and change the keys’ associated values. The dictionary index operation uses the same syntax as that used for sequences, but the item in the square brackets is a key, not a relative position:

  >>> D['food']               # Fetch value of key 'food'
  'Spam '

  >>> D['quantity'] += 1      # Add 1 to 'quantity' valu e
  >>> D
  {'food': 'Spam', 'color': 'pink', 'quantity': 5}

Although the curly-braces literal form does see use, it is perhaps more common to see dictionaries built up in different ways. The following, for example, starts with an empty dictionary, and fills it out one key at a time. Unlike out-of-bounds assignments in lists, which are forbidden, an assignment to new dictionary key creates that key:

  >>> D = {}
  >>> D['name'] = 'Bob'             # Create keys by assignment
  >>> D['job'] = 'dev'
  >>> D['age'] = 40

  >>> D
  {'age': 40, 'job': 'dev', 'name': 'Bob'} 

  >>> print D['name']
  Bob

Here, we’re effectively using dictionary keys as field names in a record that describes someone. In other applications, dictionaries can also be used to replace searching operations—indexing a dictionary by key is often the fastest way to code a search in Python.