I understand that the SequenceMatcher's ratio method does not guarantee that SequenceMatcher(None, a, b).ratio() == SequenceMatcher(None, b, a).ratio(). Below is a counterexample:

    # Example from https://mail.python.org/pipermail/python-list/2010-November/593063.html
    >>> SequenceMatcher(None, 'BRADY', 'BYRD').ratio()
    0.6666666666666666
    >>> SequenceMatcher(None, 'BYRD', 'BRADY').ratio()
    0.4444444444444444

I was recently solving a problem that required a textual similarity ratio function and I wrongly assumed that SequenceMatcher treated both input strings symmetrically, which was an extremely difficult bug to find, especially because for many simple tests, the ratio IS symmetric:

    >>> SequenceMatcher(None, 'apple', 'banana').ratio()
    0.18181818181818182
    >>> SequenceMatcher(None, 'banana', 'apple').ratio()
    0.18181818181818182

I would like to see a clearer warning of this asymmetry in the documentation for the difflib module. Perhaps something like

      .. note::

         Caution: The result of a :meth:`ratio` call is *NOT* symmetric with 
         respect to the order of the arguments. For instance::
            
            >>> SequenceMatcher(None, 'brady', 'byrd').ratio()
            0.6666666666666666
            >>> SequenceMatcher(None, 'byrd', 'brady').ratio()
            0.4444444444444444

Without such a note near the ratio methods' documentations, it is far too easy to google for a Python stdlib functionality for computing text similarity, skip straight to the ratio method, look at the examples given, try some of your own simple examples, and accidentally convince oneself that this symmetry exists.

New changeset e9cbcd0018abd2a5f2348c45d5c9c4265c4f42dc by Miss Islington (bot) (sweeneyde) in branch 'master':
bpo-37004: Documented asymmetry of string arguments in difflib.SequenceMatcher for ratio method (GH-13482)
https://github.com/python/cpython/commit/e9cbcd0018abd2a5f2348c45d5c9c4265c4f42dc

New changeset 1a3a40c1cb582e436d568009fae2b06c0b1978ed by Terry Jan Reedy (Miss Islington (bot)) in branch '3.8':
bpo-37004: Documented asymmetry of string arguments in difflib.SequenceMatcher for ratio method (GH-13482) (#15157)
https://github.com/python/cpython/commit/1a3a40c1cb582e436d568009fae2b06c0b1978ed

New changeset 7dafbe81bd0afb8bd67bc3a4c851a6c728fd87fe by Terry Jan Reedy (Miss Islington (bot)) in branch '3.7':
bpo-37004: Documented asymmetry of string arguments in difflib.SequenceMatcher for ratio method (GH-13482) (#15158)
https://github.com/python/cpython/commit/7dafbe81bd0afb8bd67bc3a4c851a6c728fd87fe