difflib.SequenceMatcher.find_longest_match default arguments

[lrjball]

BPO	40394
Nosy	@tim-one, @lrjball
PRs	bpo-40394 - difflib.SequenceMatched.find_longest_match default args #19742

^{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 = 'https://github.com/tim-one'
closed_at = <Date 2020-04-30.03:45:43.891>
created_at = <Date 2020-04-26.14:15:58.546>
labels = ['type-feature', 'library', '3.9']
title = 'difflib.SequenceMatcher.find_longest_match default arguments'
updated_at = <Date 2020-05-02.00:12:06.898>
user = 'https://github.com/lrjball'

bugs.python.org fields:

activity = <Date 2020-05-02.00:12:06.898>
actor = 'Lewis Ball'
assignee = 'tim.peters'
closed = True
closed_date = <Date 2020-04-30.03:45:43.891>
closer = 'tim.peters'
components = ['Library (Lib)']
creation = <Date 2020-04-26.14:15:58.546>
creator = 'Lewis Ball'
dependencies = []
files = []
hgrepos = []
issue_num = 40394
keywords = ['patch']
message_count = 9.0
messages = ['367306', '367380', '367456', '367459', '367461', '367464', '367731', '367732', '367902']
nosy_count = 2.0
nosy_names = ['tim.peters', 'Lewis Ball']
pr_nums = ['19742']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue40394'
versions = ['Python 3.9']

[lrjball]

The usage of difflib.SequenceMatcher.find_longest_match could be simplified for the most common use case (finding the longest match between the entirety of the two strings) by taking default args.

At the moment you have to do:

>>> from difflib import SequenceMatcher
>>> a, b = 'foo bar', 'foo baz'
>>> s = SequenceMatcher(a=a, b=b)
>>> s.find_longest_match(0, len(a), 0, len(b))
Match(a=0, b=0, size=6)

but with default args the final line could be simplified to just:

>>> s.find_longest_match()
Match(a=0, b=0, size=6)

which seems to be much cleaned and more readable.

I'd suggest updating the code so that the function signature becomes:

find_longest_match(alo=None, ahi=None, blo=None, bhi=None)

which is consistent with the current docstring of "Find longest matching block in a[alo:ahi] and b[blo:bhi]." as a[None:None] is the whole of a.

I think this would only be a minor code change, and if it is something that would be useful I'd be happy to have a go at a PR.

[tim-one]

Sounds good to me, Lewis - thanks! Note, though, that alo and blo should default to 0. None is best reserved for cases where the default value needs to be computed at runtime. But alo == blo == 0 apply to all possible instances.

[lrjball]

Okay, that makes sense. I will raise a PR

[lrjball]

Adding a test for this and noticed I can add one more test case to get the method to full coverage. Can I add that to this PR or should I raise a separate one?

[tim-one]

I'm not clear on exactly what it is you're asking, but it's better to ask for forgiveness than permission ;-) That is, it's unlikely anyone will object to adding a test in a feature PR.

[lrjball]

Oh okay, well I was just saying I have added a test which is unrelated to the feature I have added, but it does test a different part of the same function. Anyway, I have raised a PR for this now (19742) and can separate it out if needed.

[tim-one]

New changeset 3209cbd by lrjball in branch 'master':
bpo-40394 - difflib.SequenceMatched.find_longest_match default args (GH-19742)
3209cbd

[tim-one]

All done. Thank you, Lewis! You're now an official Python contributor, and are entitled to all the fame, fortune, and power that follows. Use your new powers only for good :-)

[lrjball]

Thanks Tim. Cheers for your support with this :)