Compare two different files line by line in python

Solution 1:

This solution reads both files in one pass, excludes blank lines, and prints common lines regardless of their position in the file:

with open('some_file_1.txt', 'r') as file1:
    with open('some_file_2.txt', 'r') as file2:
        same = set(file1).intersection(file2)

same.discard('\n')

with open('some_output_file.txt', 'w') as file_out:
    for line in same:
        file_out.write(line)

Solution 2:

Yet another example...

from __future__ import print_function #Only for Python2

with open('file1.txt') as f1, open('file2.txt') as f2, open('outfile.txt', 'w') as outfile:
    for line1, line2 in zip(f1, f2):
        if line1 == line2:
            print(line1, end='', file=outfile)

And if you want to eliminate common blank lines, just change the if statement to:

if line1.strip() and line1 == line2:

.strip() removes all leading and trailing whitespace, so if that's all that's on a line, it will become an empty string "", which is considered false.

Solution 3:

If you are specifically looking for getting the difference between two files, then this might help:

with open('first_file', 'r') as file1:
    with open('second_file', 'r') as file2:
        difference = set(file1).difference(file2)

difference.discard('\n')

with open('diff.txt', 'w') as file_out:
    for line in difference:
        file_out.write(line)

Solution 4:

If order is preserved between files you might also prefer difflib. Although Robᵩ's result is the bona-fide standard for intersections you might actually be looking for a rough diff-like:

from difflib import Differ

with open('cfg1.txt') as f1, open('cfg2.txt') as f2:
    differ = Differ()

    for line in differ.compare(f1.readlines(), f2.readlines()):
        if line.startswith(" "):
            print(line[2:], end="")

That said, this has a different behaviour to what you asked for (order is important) even though in this instance the same output is produced.