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za3k opened this issue Aug 11, 2016 · 4 comments
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set.union not working correctly #2013

za3k opened this issue Aug 11, 2016 · 4 comments
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false-positive mypy gave an error on correct code feature priority-2-low

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@za3k
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za3k commented Aug 11, 2016 •
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Code:
a = set().union(*[[1],[2],[3]]) # type: Set[int]

Expected:This should work fine
Result:

error: Argument 1 to "union" of "set" has incompatible type *List[List[int]]; expected Iterable[None]
error: Incompatible types in assignment (expression has type Set[None], variable has type Set[int])

I'm fairly new to mypy, so please let me know if this is user error.
@za3k
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za3k commented Aug 11, 2016

Worth noting that this works:

base_set = set()  # type: Set[int]
a = base_set.union(*[[1],[2],[3]])```

@gnprice
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gnprice commented Aug 12, 2016

Thanks for the report! I think this is a case where our type inference within a statement/expression is a little too dependent on specific rules and consequently only able to reason through N layers of indirection but not N+1. We've talked about this before (/cc especially @ddfisher), but I'm not sure we have an issue squarely directed at it -- #1055 appears to be closest.

I'd certainly like to fix this kind of situation, preferably in a way that involves reducing the number of special cases, but I don't think we yet know precisely how to do that. For now, the workaround in your follow-up comment is the recommended approach -- fortunately it's usually not particularly cumbersome.

@gnprice gnprice added this to the Future milestone Aug 12, 2016
@gvanrossum gvanrossum removed this from the Future milestone Mar 29, 2017
@goodmami goodmami mentioned this issue Jun 30, 2020
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@JukkaL JukkaL added false-positive mypy gave an error on correct code feature labels Jul 3, 2020
@djrochford
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djrochford commented Sep 20, 2020 •
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Note that this:

a = set.union(*[{1},{2},{3}])

works fine. This may be a nice workaround in some situations.

@tomaszkot1 tomaszkot1 mentioned this issue Jan 23, 2022
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@AlexWaygood
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Can't reproduce on 0.940

https://mypy-play.net/?mypy=0.940&python=3.10&flags=strict&gist=40e9a86b91e4e4727bdf743b544847dd

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