Nathan Fellman Nathan Fellman - 3 months ago 22
Python Question

How can I iterate through two lists in parallel in Python?

I have two iterables in Python, and I want to go over them in pairs:

foo = (1, 2, 3)
bar = (4, 5, 6)

for (f, b) in some_iterator(foo, bar):
print "f: ", f, "; b: ", b

It should result in:

f: 1; b: 4
f: 2; b: 5
f: 3; b: 6

One way to do it is to iterate over the indices:

for i in xrange(len(foo)):
print "f: ", foo[i], "; b: ", b[i]

But that seems somewhat unpythonic to me. Is there a better way to do it?

for f, b in zip(foo, bar):
    print(f, b)

zip stops when the shorter of foo or bar stops.

In Python 2, zip returns a list of tuples. This is fine when foo and bar are not massive. If they are both massive then forming zip(foo,bar) is an unnecessarily massive temporary variable, and should be replaced by itertools.izip or itertools.izip_longest, which returns an iterator instead of a list.

import itertools
for f,b in itertools.izip(foo,bar):
for f,b in itertools.izip_longest(foo,bar):

izip stops when either foo or bar is exhausted. izip_longest stops when both foo and bar are exhausted. When the shorter iterator(s) are exhausted, izip_longest yields a tuple with None in the position corresponding to that iterator. You can also set a different fillvalue besides None if you wish. See here for the full story.

In Python 3, zip returns an iterator of tuples, like itertools.izip in Python2. To get a list of tuples, use list(zip(foo, bar)). And to zip until both iterators are exhausted, you would use itertools.zip_longest.