The Python Apprentice

A more practical example of the iteration protocols

With for-loops and comprehensions at our fingertips, the utility of these lower level iteration protocols may not be obvious. To demonstrate a more concrete use, here's a little utility function which, when passed an iterable object, returns the first item from that series or, if the series is empty, raises a ValueError:

>>> def first(iterable):...     iterator = iter(iterable)...     try:...         return next(iterator)...     except StopIteration:...         raise ValueError("iterable is empty")...

This works as expected on any iterable object, in this case both a list and a set:

>>> first(["1st", "2nd", "3rd"])'1st'>>> first({"1st", "2nd", "3rd"})'1st'>>> first(set())Traceback (most recent call last): File "./iterable.py", ...