74 – Peek at an iterable

To know the first element of a list, you index with [0]. However, not all iterables can be indexed.

In general, to get the first element of an iterable (to “peek” at it), you can use next to fetch that element and then you can use chain from the module itertools to “glue it back”.

Here is an example of how to do this:

from itertools import chain

def peek(iterable):
    iterator = iter(iterable)
    first = next(iterator)
    return first, chain([first], iterator)

This specific implementation raises a StopIteration error if the iterable is empty. Other behaviours can be easily implemented.