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Building Robust Object-Oriented Python Applications and Libraries

Iterators

Explore how Python uses the iterator protocol to manage data sequences. Understand the roles of __iter__ and __next__ methods, the StopIteration exception, and how for loops simplify iteration. This lesson helps you master implementing custom iterators and leveraging Python’s iterable classes for cleaner and more efficient code.

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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 used like this:

Python 3.10.4
while not iterator.done():
  item = iterator.next()
  # do something with the item

In Python, iteration is available across many language features, so the method gets a special name, __next__. This method can be accessed using the next(iterator) built- in. Rather than a done() method, Python’s iterator protocol raises the StopIteration exception to notify the client that the iterator 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 statement. Let’s look at each these in more detail.

The iterator protocol

The Iterator ...