The rough edges of filecmp

• Posted 6 October 2019
• Tagged with python

I’ve been cleaning up some old files recently, and as part of that I’m using the filecmp module to find duplicates. It exposes one function I’m interested in: filecmp.cmp(f1, f2), which tells you if f1 and f2 are the same. It’s a really simple API, and I’ve been using it this way for years.

As I was running my scripts, I happened to spot something that looked weird. The module was telling me two files were the same, even though they contained different data:

>>> path1 = "Semaphore_Golf.svg"
>>> path2 = "Semaphore_Lima.svg"

>>> filecmp.cmp(path1, path2)
True

>>> open(path1, "rb").read() == open(path2, "rb").read()
False

I went to re-read the docs, and it turns out I’d forgotten a detail – by default, filecmp.cmp only compares the os.stat() signature of the two files. If that matches, it tells you the files are the same, regardless of the contents. If you want an actual byte-for-byte comparison, you need to pass shallow=False:

>>> filecmp.cmp(path1, path2, shallow=False)
False

I’ve been making this mistake for years, because the API is so simple I’d never needed read the docs in detail. Nobody’s ever spotted it in a code review, and I’ve seen it in plenty of other code examples that don’t pass an explicit shallow flag.

Initially this struck me as a dangerous default – if you use filecmp without paying close attention, you might not realise it’s not doing a byte-for-byte comparison. If you get a positive result and delete the second file as a duplicate, you’ve just lost data. Oops.

But the more I read about it, the more I think it’s a bad API design. I went to look in the Python bug tracker, and I found multiple bugs from people complaining that the behaviour of the module was confusing or misleading. The problem is that “the same file” could mean at least three different things:

• Do the files have the same os.stat() signature?

  In filecmp, this signature is the size, modified time, and the mode.

• Do the files have the same byte-for-byte contents?

  This is the comparison you get by passing shallow=True, and what I was originally expecting. It’s also what the cmp command-line tool will check.

• Are the files the same underlying file on disk, modulo hard links? (h/t @kapellosaur)

  This is a stronger version of the previous two conditions – the files aren’t just equal, they’re the same. Apparently this is handled by the os.path.samefile function, although I’ve never used it myself.

You want different notions of “same” in different contexts, and having a binary flag to choose the behaviour is pretty unclear. Even if you do pass an explicit value for shallow, it’s not so clear which comparison you’re using.

I’d prefer an API that’s explicit about the comparison:

def cmp_path_contents(path1, path2):
    """
    Returns True if the files at paths ``path1`` and ``path2``
    have the same contents.
    """


def cmp_stat(path1, path2):
    """
    Returns True if the os.stat() signature of ``path1`` and ``path2``
    are the same.
    """


def cmp_same_file(path1, path2):
    """
    Returns True if ``path1`` and ``path2`` point to the same file on disk.
    """

With these names, you can see which version of sameness you’re using, and because they each return a boolean, you can combine conditions with logical operators.

I’ve created a tiny module which implements this API, which I’ve called filecmp2. Install it from PyPI or copy it directly into your project, and enjoy more explicit file comparisons. I’ll be using it the next time I need to compare files, so hopefully I won’t get caught out by this confusion again.