[issue44658] No ValueError for duplicate key value in mapping patern when lengths do not match

[Jack DeVries]

New submission from Jack DeVries <jdevries3133 at gmail.com>:

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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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue44658>
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