How to iterate through two lists in parallel?
Python 3
for f, b in zip(foo, bar): print(f, b)
zip
stops when the shorter of foo
or bar
stops.
In Python 3, zip
returns an iterator of tuples, like itertools.izip
in Python2. To get a listof tuples, use list(zip(foo, bar))
. And to zip until both iterators areexhausted, you would useitertools.zip_longest.
Python 2
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 massivetemporary variable, and should be replaced by itertools.izip
oritertools.izip_longest
, which returns an iterator instead of a list.
import itertoolsfor f,b in itertools.izip(foo,bar): print(f,b)for f,b in itertools.izip_longest(foo,bar): print(f,b)
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.
Note also that zip
and its zip
-like brethen can accept an arbitrary number of iterables as arguments. For example,
for num, cheese, color in zip([1,2,3], ['manchego', 'stilton', 'brie'], ['red', 'blue', 'green']): print('{} {} {}'.format(num, color, cheese))
prints
1 red manchego2 blue stilton3 green brie