SequenceMatcher.ratio() noncommutativity not well-documented

[sweeneyde]

BPO	37004
Nosy	@tim-one, @terryjreedy, @miss-islington, @sweeneyde
PRs	bpo-37004: Documented asymmetry of string arguments in difflib.SequenceMatcher for ratio method #13482 [3.8] bpo-37004: Documented asymmetry of string arguments in difflib.SequenceMatcher for ratio method (GH-13482) #15157 [3.7] bpo-37004: Documented asymmetry of string arguments in difflib.SequenceMatcher for ratio method (GH-13482) #15158 [3.6] bpo-37004: Documented asymmetry of string arguments in difflib.SequenceMatcher for ratio method (GH-13482) #15159

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2019-08-07.16:13:49.300>
created_at = <Date 2019-05-22.02:09:15.379>
labels = ['3.7', '3.8', '3.9']
title = 'SequenceMatcher.ratio() noncommutativity not well-documented'
updated_at = <Date 2019-08-07.16:13:49.293>
user = 'https://github.com/sweeneyde'

bugs.python.org fields:

activity = <Date 2019-08-07.16:13:49.293>
actor = 'tim.peters'
assignee = 'none'
closed = True
closed_date = <Date 2019-08-07.16:13:49.300>
closer = 'tim.peters'
components = []
creation = <Date 2019-05-22.02:09:15.379>
creator = 'Dennis Sweeney'
dependencies = []
files = []
hgrepos = []
issue_num = 37004
keywords = ['patch']
message_count = 4.0
messages = ['343138', '349151', '349169', '349170']
nosy_count = 5.0
nosy_names = ['tim.peters', 'terry.reedy', 'docs@python', 'miss-islington', 'Dennis Sweeney']
pr_nums = ['13482', '15157', '15158', '15159']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue37004'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']

[sweeneyde]

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.

[miss-islington]

New changeset e9cbcd0 by Miss Islington (bot) (sweeneyde) in branch 'master':
bpo-37004: Documented asymmetry of string arguments in difflib.SequenceMatcher for ratio method (GH-13482)
e9cbcd0

[terryjreedy]

New changeset 1a3a40c 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) (bpo-15157)
1a3a40c

[terryjreedy]

New changeset 7dafbe8 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) (bpo-15158)
7dafbe8