How to iterate across lines in two files simultaneously?

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Python 2:

Use itertools.izip to join the two iterators.

from itertools import izip
for line_from_file_1, line_from_file_2 in izip(open(file_1), open(file_2)):

If the files are of unequal length, use izip_longest.

In Python 3, use zip and zip_longest instead. Also, use a with to open files, so that closing is handled automatically even in case of errors.

with open(file1name) as file1, open(file2name) as file2:
    for line1, line2 in zip(file1, file2):
        #do stuff

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