How can I count the number of items in an arbitrary iterable (such as a generator)?

Question

Suppose I have an arbitrary iterable - for example, a generator that iterates over lines of a file and yields the ones matching a regex.

How can I count the number of items in that iterable, supposing that I don't care about the elements themselves?

Answer 1

Calls to itertools.imap() in Python 2 or map() in Python 3 can be replaced by equivalent generator expressions:

sum(1 for dummy in it)

This also uses a lazy generator, so it avoids materializing a full list of all iterator elements in memory.