How to do unit testing of functions writing files using Python's 'unittest'

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

I prefer to have output functions explicitly accept a file handle (or file-like object), rather than accept a file name and opening the file themselves. This way, I can pass a StringIO object to the output function in my unit test, then .read() the contents back from that StringIO object (after a .seek(0) call) and compare with my expected output.

For example, we would transition code like this

##File:lamb.py import sys   def write_lamb(outfile_path):     with open(outfile_path, 'w') as outfile:         outfile.write("Mary had a little lamb.\n")   if __name__ == '__main__':     write_lamb(sys.argv[1])    ##File test_lamb.py import unittest import tempfile  import lamb   class LambTests(unittest.TestCase):     def test_lamb_output(self):         outfile_path = tempfile.mkstemp()[1]         try:             lamb.write_lamb(outfile_path)             contents = open(tempfile_path).read()         finally:             # NOTE: To retain the tempfile if the test fails, remove             # the try-finally clauses             os.remove(outfile_path)         self.assertEqual(contents, "Mary had a little lamb.\n") 

to code like this

##File:lamb.py import sys   def write_lamb(outfile):     outfile.write("Mary had a little lamb.\n")   if __name__ == '__main__':     with open(sys.argv[1], 'w') as outfile:         write_lamb(outfile)    ##File test_lamb.py import unittest from io import StringIO  import lamb   class LambTests(unittest.TestCase):     def test_lamb_output(self):         outfile = StringIO()         # NOTE: Alternatively, for Python 2.6+, you can use         # tempfile.SpooledTemporaryFile, e.g.,         #outfile = tempfile.SpooledTemporaryFile(10 ** 9)         lamb.write_lamb(outfile)         outfile.seek(0)         content = outfile.read()         self.assertEqual(content, "Mary had a little lamb.\n") 

This approach has the added benefit of making your output function more flexible if, for instance, you decide you don't want to write to a file, but some other buffer, since it will accept all file-like objects.

Note that using StringIO assumes the contents of the test output can fit into main memory. For very large output, you can use a temporary file approach (e.g., tempfile.SpooledTemporaryFile).

Answer 2

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)