On 11/14/20 10:17 PM, Guido van Rossum wrote:
It’s a usability issue; mappings are used quite differently than
sequences. Compare to class patterns rather than sequence patterns.
I just found the following explanation from the superceded PEP 622 as to
why extra keys are ignored:
> 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.
I suppose this makes sense when using "match" to work with a dictionary
used as a lightweight object, which I expect would be relatively common.
The examples I originally presented assume use of "match" for parsing,
and parsing tends to default to stricter matching. :)
--
David Foster | Seattle, WA, USA
Contributor to TypedDict support for mypy
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