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
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