Brandt Bucher <brandtbuc...@gmail.com> added the comment:
Evaluating every key in every mapping pattern and checking them for duplicates every time a match block is entered would be prohibitively expensive, for negligible gain. The length check exists to avoid this step, replacing it with a much cheaper O(1) trapdoor. Imagine the overhead of validating this match statement, for example: https://github.com/brandtbucher/patmaperformance/blob/2fe8b79691d4f11b18d65b957ed36a85cb4761a4/bm_patma_holdem.py#L54-L95 > 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. That's okay with me; it's not the VM's job to check the runtime validity of branches that aren't taken. > Another option if this problem is isolated to mapping patterns, we could > introduce a new op-code: BUILD_MATCH_MAP, which is essentially a wrapper > around BUILD_MAP that disallows duplicate key values. I'm not sure what you mean by this. We don't call BUILD_MAP when evaluating mapping patterns... in fact, we don't build any dictionaries at all (except for collecting remaining items using **rest). Unless others disagree, I'm probably going to close this as "not a bug". ---------- assignee: -> brandtbucher _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue44658> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
