python: get number of items from list(sequence) with certain condition

Question

Assuming that I have a list with huge number of items.

l = [ 1, 4, 6, 30, 2, ... ]

I want to get the number of items from that list, where

Answer 1

You want a generator comprehension rather than a list here.

For example,

l = [1, 4, 6, 7, 30, 2]

def my_condition(x):
    return x > 5 and x < 20

print sum(1 for x in l if my_condition(x))
# -> 2
print sum(1 for x in range(1000000) if my_condition(x))
# -> 14

Or use itertools.imap (though I think the explicit list and generator expressions look somewhat more Pythonic).

Note that, though it's not obvious from the sum example, you can compose generator comprehensions nicely. For example,

inputs = xrange(1000000)      # In Python 3 and above, use range instead of xrange
odds = (x for x in inputs if x % 2)  # Pick odd numbers
sq_inc = (x**2 + 1 for x in odds)    # Square and add one
print sum(x/2 for x in sq_inc)       # Actually evaluate each one
# -> 83333333333500000

The cool thing about this technique is that you can specify conceptually separate steps in code without forcing evaluation and storage in memory until the final result is evaluated.