Length of generator output

python generator iterable
Maxim picture Maxim · Dec 25, 2008 · Viewed 75.8k times · Source

Python provides a nice method for getting length of an eager iterable, len(x) that is. But I couldn't find anything similar for lazy iterables represented by generator comprehensions and functions. Of course, it is not hard to write something like:

def iterlen(x):
  n = 0
  try:
    while True:
      next(x)
      n += 1
  except StopIteration: pass
  return n

But I can't get rid of a feeling that I'm reimplementing a bicycle.

(While I was typing the function, a thought struck my mind: maybe there really is no such function, because it "destroys" its argument. Not an issue for my case, though).

P.S.: concerning the first answers - yes, something like len(list(x)) would work too, but that drastically increases the usage of memory.

P.P.S.: re-checked... Disregard the P.S., seems I made a mistake while trying that, it works fine. Sorry for the trouble.

Answer

Matt Dunham picture Matt Dunham · Aug 28, 2011

The easiest way is probably just sum(1 for _ in gen) where gen is your generator.

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