An interesting way to implement an abstract base class

[olsongt]

This one made me smile. From:

http://aima.cs.berkeley.edu/python/utils.html#Queue

class Queue:
"""Queue is an abstract class/interface. There are three types:
Stack(): A Last In First Out Queue.
FIFOQueue(): A First In First Out Queue.
PriorityQueue(lt): Queue where items are sorted by lt, (default
<).
Each type supports the following methods and functions:
q.append(item) -- add an item to the queue
q.extend(items) -- equivalent to: for item in items:
q.append(item)
q.pop() -- return the top item from the queue
len(q) -- number of items in q (also q.__len())
Note that isinstance(Stack(), Queue) is false, because we implement
stacks
as lists. If Python ever gets interfaces, Queue will be an
interface."""

def __init__(self):
abstract

def extend(self, items):
for item in items: self.append(item)