New submission from Dennis Sweeney <sweeney.dennis...@gmail.com>:
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.
----------
messages: 343138
nosy: Dennis Sweeney, docs@python
priority: normal
severity: normal
status: open
title: SequenceMatcher.ratio() noncommutativity not well-documented
_______________________________________
Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue37004>
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