[Python-Dev] Issue 2986: difflib.SequenceMatcher is partly broken

[Tim Peters]

[Nick Coghlan]
> You're right, I was misremembering how SequenceMatcher works.
>
> Terry's summary of the situation seems correct to me - adding a new
> flag to the constructor signature would mean we're taking a silent
> failure ("the heuristic makes my code give the wrong answer on 2.7.0")
> and making it a noisy failure ("my code needs to be able to turn the
> heuristic off to get the right answer, so it will fail noisily on
> 2.7.0").

Yup - exactly so.

> That's a far cry from the True/False mistake.

Which I'm sure refers to introducing True and False builtins in a
bugfix release.  That was _almost_ as bad as the sometimes-hated
heuristic I added to SequenceMatcher ;-)

Me, I like "fail noisily on 2.7.0" for this case - practicality beats
purity, especially when so few people and programs are likely to be
affected by the
backwards-incompatible-but-only-for-those-who-need-to-use-it
SequenceMatcher constructor signature change.