Alex Rivera | Logout

Length of generator output

Asked 2008-12-25T18:50:18.977
196

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.

python generator iterable
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20
def count(iter):
    return sum(1 for _ in iter)

Or better yet:

def count(iter):
    try:
        return len(iter)
    except TypeError:
        return sum(1 for _ in iter)

If it's not iterable, it will throw a TypeError.

Or, if you want to count something specific in the generator:

def count(iter, key=None):
    if key:
        if callable(key):
            return sum(bool(key(x)) for x in iter)
        return sum(x == key for x in iter)
    try:
        return len(iter)
    except TypeError:
        return sum(1 for _ in iter)
answered 2012-07-13T04:22:31.997

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