As usual, a demonstration at the Python REPL will help all of these concepts crystallize into something you can work with. We start with a list of the names of the seasons as our iterable object:

>>> iterable = ['Spring', 'Summer', 'Autumn', 'Winter']

We then ask our iterable object to give us an iterator using the iter() built-in:

>>> iterator = iter(iterable)

Next we request a value from the iterator object using the next() built-in:

>>> next(iterator)
'Spring'

Each call to next() moves the iterator through the sequence:

>>> next(iterator)
'Summer'
>>> next(iterator)
'Autumn'
>>> next(iterator)
'Winter'

But what happens when we reach the end?

>>> next(iterator)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

In a spectacular display of liberalism, Python raises a StopIteration exception. Those of you coming from other programming languages with a more straight-laced approach to exceptions may find this mildly outrageous, but, really, what could be more exceptional than reaching the end of a collection? It only has one end after all!

This attempt at rationalizing the Python language design decision makes even more sense when one considers that the iterable series may be a potentially infinite stream of data. Reaching the end in that case really would be something to write home about, or indeed raise an exception for.