Python double iteration

Question

What is the pythonic way of iterating simultaneously over two lists?

Suppose I want to compare two files line by line (compare each ith line in one file

Answer 1

I vote for using zip. The manual suggests "To loop over two or more sequences at the same time, the entries can be paired with the zip() function"

For example,

list_one = ['nachos', 'sandwich', 'name']
list_two = ['nachos', 'sandwich', 'the game']
for one, two in zip(list_one, list_two):
   if one != two:
      print "Difference found"

Answer 2

In lockstep (for Python ≥3):

for line1, line2 in zip(file1, file2):
   # etc.

As a "2D array":

for line1 in file1:
   for line2 in file2:
     # etc.
   # you may need to rewind file2 to the beginning.

Answer 3

In Python 2, you should import itertools and use its izip:

with open(file1) as f1:
  with open(file2) as f2:
    for line1, line2 in itertools.izip(f1, f2):
      if line1 != line2:
        print 'files are different'
        break

with the built-in zip, both files will be entirely read into memory at once at the start of the loop, which may not be what you want. In Python 3, the built-in zip works like itertools.izip does in Python 2 -- incrementally.