traversing two lists simultaneously with multiple iterators

Question

I am trying to write a simple program that looks through two text files and reports any differences between the two files. For this function, I have two lists that contain a list of words for each line (so they're 2D lists). I would like to go through and compare each word and report an error if they are not the same, storing all of the errors in a list that is printed for the user. The i iterator would report the line of the error and then the two words are also reported.

def compare(lines1, lines2):
    for i, line1, line2 in izip(lines1, lines2):
        for word1, word2 in izip(line1, line2):
            if word1 != word2:
                report_error(i, word1, word2)

However, this is not working for me. I read on StackOverflow that I would need to use the zip() or izip() function to read two lists at once but it still isn't working for me. I am getting the following error.

  File "debugger.py", line 28, in compare
    for i, line1, line2 in izip(lines1, lines2):
ValueError: need more than 2 values to unpack

Any idea what I'm doing wrong? I can also provide the full file if that is helpful.

TigerhawkT3 · Accepted Answer · 2016-01-21 03:01:13Z

zip(), and similar functions, produce tuples of a length equal to the number of passed arguments. for word1, word2 in izip(line1, line2): would work, but for i, line1, line2 in izip(lines1, lines2): does not, as you're only zipping through two iterables, lines1 and lines2, so it can't unpack those two-element tuples into three references.

To fix this, use enumerate(), which adds an index. Use start=1 to start with a line number of 1 instead of the default of 0:

def compare(lines1, lines2):
    for i, (line1, line2) in enumerate(izip(lines1, lines2), start=1):
        for word1, word2 in izip(line1, line2):
            if word1 != word2:
                report_error(i, word1, word2)

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