Comparing 2 .txt files using difflib in Python

Question

I am trying to compare 2 text files and output the first string in the comparison file that does not match but am having difficulty since I am very new to python. Can anybody please give me a sample way to use this module.

When I try something like:

result = difflib.SequenceMatcher(None, testFile, comparisonFile)

I get an error saying object of type 'file' has no len.

Answer 1

Are you sure both files exist ?

Just tested it and i get a perfect result.

To get the results i use something like:

import difflib

diff=difflib.ndiff(open(testFile).readlines(), open(comparisonFile).readlines())

try:
    while 1:
        print diff.next(),
except:
    pass

the first character of each line indicates if they are different: eg.: '+' means the following line has been added, etc.

Answer 2

For starters, you need to pass strings to difflib.SequenceMatcher, not files:

# Like so
difflib.SequenceMatcher(None, str1, str2)

# Or just read the files in
difflib.SequenceMatcher(None, file1.read(), file2.read())

That'll fix your error anyway. To get the first non-matching string, I'll direct you to the wonderful world of difflib documentation.

Answer 3

It sounds like you may not need difflib at all. If you're comparing line by line, try something like this:

test_lines = open("test.txt").readlines()
correct_lines = open("correct.txt").readlines()

for test, correct in zip(test_lines, correct_lines):
    if test != correct:
        print "Oh no! Expected %r; got %r." % (correct, test)
        break
else:
    len_diff = len(test_lines) - len(correct_lines)
    if len_diff > 0:
        print "Test file had too much data."
    elif len_diff < 0:
        print "Test file had too little data."
    else:
        print "Everything was correct!"