Unit Testing File Modifications

Question

A common task in programs I\'ve been working on lately is modifying a text file in some way. (Hey, I\'m on Linux. Everything\'s a file. And I do large-scale system admin.)

Answer 1

For later readers who just want a way to test that code writing to files is working correctly, here is a "fake_open" that patches the open builtin of a module to use StringIO. fake_open returns a dict of opened files which can be examined in a unit test or doctest, all without needing a real file-system.

def fake_open(module):
    """Patch module's `open` builtin so that it returns StringIOs instead of
    creating real files, which is useful for testing. Returns a dict that maps
    opened file names to StringIO objects."""
    from contextlib import closing
    from StringIO import StringIO
    streams = {}
    def fakeopen(filename,mode):
        stream = StringIO()
        stream.close = lambda: None
        streams[filename] = stream
        return closing(stream)
    module.open = fakeopen
    return streams