New submission from Jack DeVries <[email protected]>:
Consider the following code:
class A:
a = 'a'
# runs without error
match {'a': 1}:
case {'a': 1, A.a: 1}:
pass
# raises ValueError
match {'a': 1, 'b': 1}:
case {'a': 1, A.a: 1}:
pass
In both cases, the mapping pattern is the same (and both are not valid due to
duplicate key values). However, the pattern is only evaluated in the second
case. This is because a key-length optimization provides a shortcut around
pattern evaluation. The docs gives users a hint that things like this might
happen, which is a good thing:
> Users should generally never rely on a pattern being evaluated. Depending on
> > implementation, the interpreter may cache values or use other optimizations
> > which skip repeated evaluations.
> https://docs.python.org/3.10/reference/compound_stmts.html#overview
However, I can't help but think that these ergonomics are strange. Consider if
some other code is mutating the value of `A.a`. This could create some very
strange and flaky bugs where the state of `A.a` can change and make the pattern
invalid, but nonetheless not cause an exception until much later, or not at all.
There is mapping pattern validation code in the `match_keys` function in
ceval.c. I haven't looked, but I assume there is some other runtime validation
for other match case types. I propose factoring Exception-raising validation
into a separate procedure that is called before any optimization jumps occur.
This trades speed for consistent behavior, and I'm interested to hear what
others think!
----------
components: Interpreter Core
messages: 397662
nosy: jack__d
priority: normal
severity: normal
status: open
title: No ValueError for duplicate key value in mapping patern when lengths do
not match
type: behavior
versions: Python 3.10, Python 3.11
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<https://bugs.python.org/issue44658>
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