Iterators

In typical design pattern parlance, an iterator is an object with a next() method and a done() method; the latter returns True if there are no items left in the sequence. In a programming language without built-in support for iterators, the iterator would be looped over like this:

while not iterator.done():
    item = iterator.next()
    # do something with the item

In Python, iteration is a special feature, so the method gets a special name, __next__. This method can be accessed using the next(iterator) built-in. Rather than a done method, the iterator protocol raises StopIteration to notify the loop that it has completed. Finally, we have the much more readable for item in iterator syntax to actually access items in an iterator instead of messing around with a while loop. Let's look at these in more detail.

The iterator protocol

The abstract base class Iterator, in the collections.abc module, defines the iterator protocol in Python. As mentioned, it must have a __next__ method that...