Thanks very much for the survey (which actually surprised me somewhat).
Regarding the suggested update for the PEP, I'll make a PR to change that
-- I agree it's worth saying it.
On Sat, Nov 21, 2020 at 5:00 PM David Foster
On 11/19/20 10:08 PM, David Foster wrote:
I've completed my survey of how other languages use pattern matching to match Mapping-like and dict-like types, especially focusing on whether they ignore (𝔸) or disallow (𝔹) extra keys by default. [...]
To close the loop on this thread:
* Based on (1) the explanation from PEP 622 and Guido RE that "mappings [...] have natural structural sub-typing behavior, i.e., passing a dictionary with extra keys somewhere will likely just work" and (2) the survey results, I'm now personally fine (+0) with keys being ignored by default when matching against mappings.
* I do think it might be illustrative to copy the following explanatory sentences from the "Mapping Patterns" section of the older PEP 622 to the same section of PEP 635 (Structural Pattern Matching: Motivation and Rationale):
Extra keys in the subject are ignored even if **rest is not present. This is different from sequence pattern, where extra items will cause a match to fail. But mappings are actually different from sequences: they have natural structural sub-typing behavior, i.e., passing a dictionary with extra keys somewhere will likely just work.
Specifically the above might replace the following sentence in PEP 635, which doesn't really give a rationale:
Moreover, the mapping pattern does not check for the presence of additional keys.
* I still have an interest in strictly matching dictionaries that are destined to become TypedDicts, but I can take that conversation to a different thread.
-- David Foster | Seattle, WA, USA Contributor to TypedDict support for mypy _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-leave@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/K5BUKM... Code of Conduct: http://python.org/psf/codeofconduct/
-- --Guido van Rossum (python.org/~guido) *Pronouns: he/him **(why is my pronoun here?)* http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-c...