8. The dictionary type

Python dictionaries are one of its more powerful built-in types. They are generally used for look-up tables and many similar applications.

A Python dictionary represents a set of zero or more ordered pairs (k_i, v_i) such that:

Another term for this structure is mapping, since it maps the set of keys onto the set of values.

These operations are available on any dictionary object D:

`len(D)`	Returns the number of entries in `D`.
`D[k]`	If `D` has an entry with key `k`, returns the corresponding value. If there is no such key, raises a `KeyError` exception.
`D[k] = v`	Sets the entry for key `k` to value `v`. If there was previously a value for that key, it is replaced.
`del D[k]`	If there is an entry for key `k`, that entry is deleted. If not, a `KeyError` exception is raised.
`D.has_key(k)`	Predicate that returns true if `D` has a key `k`.
`D.items()`	A list of (`key`,`value`) tuples from `D`.
`.keys()`	A list of all keys in `D`.
`D.values()`	A list of all values in `D`, in the same order as `D.keys()`.
`D.update(E)`	Merge dictionary `E` into `D`. If the same key exists in both, use the value from `E`.
`D.get(k,x)`	Same as `D[k]`, but if no entry exists for key `k`, returns `x`, or `None` if `x` is omitted.
`D.setdefault(k[,x])`	If `D[k]` exists, returns that value. If there is no element `D[k]`, sets `D[k]` to `x`, defaulting to `None`, and returns that value.
`D.iteritems()`	Returns an iterator over the (`key`, `value`) pairs of `D`.
`D.iterkeys()`	Returns an iterator over the keys of `D`.
`D.itervalues()`	Returns an iterator over the values of `D`.
`D.popitem()`	Returns an arbitrary entry from `D` as a (`key, value)` tuple, and also removes that entry. If `D` is empty, raises a `KeyError` exception.
`x in D`	True if `x` is a key in `D`.
`x not in D`	True if `x` is not a key in `D`.

Python 2.2 quick reference