Question

I have a Python function that writes an output file to disk.

I want to write a unit test for it using Python’s unittest module.

How should I assert equality of files? I would like to get an error if the file content differs from the expected one + list of differences. As in the output of the Unix diff command.

Is there an official or recommended way of doing that?

Answer 1

The simplest thing is to write the output file, then read its contents, read the contents of the gold (expected) file, and compare them with simple string equality. If they are the same, delete the output file. If they are different, raise an assertion.

This way, when the tests are done, every failed test will be represented with an output file, and you can use a third-party tool to diff them against the gold files (Beyond Compare is wonderful for this).

If you really want to provide your own diff output, remember that the Python stdlib has the difflib module. The new unittest support in Python 3.1 includes an assertMultiLineEqual method that uses it to show diffs, similar to this:

    def assertMultiLineEqual(self, first, second, msg=None):
        """Assert that two multi-line strings are equal.

        If they aren't, show a nice diff.

        """
        self.assertTrue(isinstance(first, str),
                'First argument is not a string')
        self.assertTrue(isinstance(second, str),
                'Second argument is not a string')

        if first != second:
            message = ''.join(difflib.ndiff(first.splitlines(True),
                                                second.splitlines(True)))
            if msg:
                message += " : " + msg
            self.fail("Multi-line strings are unequal:\n" + message)