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 with the ith line of the other file), I would want to do something like this:

file1 = csv.reader(open(filename1),...)
file2 = csv.reader(open(filename2),...)

for line1 in file1 and line2 in file2: #pseudo-code!
    if line1 != line2:
        print "files are not identical"
        break

What is the pythonic way of achieving this?

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Edit: I am not using a file handler but rather a CSV reader (csv.reader(open(file),...)), and zip() doesn’t seem to work with it…

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Final edit: like @Alex M. suggested, zip() loads the files to memory on first iteration, so on big files this is an issue. On Python 2, using itertools solves the issue.

Answer 1

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.