Question

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

foo = [1, 2, 3]
bar = [4, 5, 6]

for (f, b) in iterate_together(foo, bar):
    print("f:", f, " |  b:", b)

That 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 range(len(foo)):
    print("f:", foo[i], " |  b:", bar[i])

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

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<sub>Related tasks:
* How to merge lists into a list of tuples? – given the above foo and bar, create the list [(1, 4), (2, 5), (3, 6)].
* How can I make a dictionary (dict) from separate lists of keys and values? – create the dict {1: 4, 2: 5, 3: 6}.
* Create a dictionary with comprehension – constructing dict using zip in a dict comprehension.</sub>

Answer 1

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 list
of tuples, use list(zip(foo, bar)). And to zip until both iterators are
exhausted, you would use
itertools.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 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):
    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.

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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 manchego
2 blue stilton
3 green brie