Python in a Nutshell, 2nd Edition

The collections Module

The collections module (introduced in Python 2.4) is intended to eventually supply several interesting types that are collections (i.e., containers).

deque

In Python 2.4, the collections module supplies only one type, deque, whose instances are “double-ended queues” (i.e., sequence-like containers suitable for additions and removals at both ends). Call deque with a single argument, any iterable, to obtain a new deque instance whose items are those of the iterable in the same order, or call deque without arguments to obtain a new empty deque instance. A deque instance d is a mutable sequence and thus can be indexed and iterated on (however, d cannot be sliced, only indexed one item at a time, whether for access, rebinding, or deletion). A deque instance d supplies the following methods.

append	d.append(item) Appends item at the right (end) of d.
appendleft	d.appendleft(item) Appends item at the left (start) of d.
clear	d.clear( ) Removes all items from d.
extend	d.extend(iterable) Appends all items of iterable at the right (end) of d.
extendleft	d.extendleft(item) Appends all items of iterable at the left (start) of d.
pop	d.pop( ) Removes and returns the last (rightmost) item from d. If d is empty, raises IndexError.
popleft	d.popleft( ) Removes and returns the first (leftmost) item from d. If d is empty, raises IndexError.
rotate	d.rotate(n=1) Rotates d n steps to the right (if n<0, rotates left).

defaultdict

In Python 2.5, the collections module also supplies type defaultdict ...