itertools.ifilter Vs. filter Vs. list comprehensions

Question

I am trying to become more familiar with the itertools module and have found a function called ifilter.

From what I understand, it filters

Answer 1

The example below includes a number generator that prints a message immediately before yielding the number, shows up how filter() first builds the list, then runs through that and filters it. Whereas itertools.ifilter filters as it goes, never building a list. If you're filtering 500,000 significant things, you want ifilter, so you're not building a list.

import itertools

def number_generator():
    for i in range(0, 3):
        print "yield", i
        yield i
    print "stopping"

function = lambda x: x > 0

numbers = number_generator()
print "itertools.ifilter:"
for n in itertools.ifilter(function, numbers):
    print n

print "\nfilter:"
numbers = number_generator()
for n in filter(function, numbers):
    print n

Output:

itertools.ifilter:
yield 0
yield 1
1
yield 2
2
stopping

filter:
yield 0
yield 1
yield 2
stopping
1
2