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Feature request: Catch operations on nothingness #12511

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LuminairPrime opened this issue Apr 2, 2022 · 4 comments
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Feature request: Catch operations on nothingness #12511

LuminairPrime opened this issue Apr 2, 2022 · 4 comments
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feature topic-reachability Detecting unreachable code

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@LuminairPrime
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LuminairPrime commented Apr 2, 2022 •
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Below are four examples of impossible code that operates on nothing. The latter two continue silently, throwing no errors. Cpython says it's out of the scope of the language, but mypy could catch it. (https://bugs.python.org/issue47202)

x =

for x in : print("This code is never reached")

while(None): print("This code is never reached")

emptylist: list[int] = []
for x in emptylist:
    if emptylist[x] == "This code is never reached":
        print("This code is never reached")
    else: print("This code is never reached")

I think the second two cases should throw a warning that they aren't doing anything and attached code will be thrown away. What do you think?
@JelleZijlstra JelleZijlstra added the topic-reachability Detecting unreachable code label Apr 2, 2022
@JelleZijlstra
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The while(None) case is already caught by the --warn-unreachable option.

Iterating over an empty list isn't. Implementing this check would require significant work, because mypy doesn't currently track whether a list is always empty. It seems doable at least in simple cases like your example, but I'm not sure how valuable it is, since I don't think it's a common problem in real-world code.

@AlexWaygood
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I think this is basically a duplicate of #8637

@AlexWaygood
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I think this is basically a duplicate of #8637

Closing as a duplicate of #8637 due to lack of response from OP.

@LuminairPrime
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I agree that checking for iteration over nothing would be a good feature in the future. I have nothing else to add because I can't build the feature ;)

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@JelleZijlstra @AlexWaygood @LuminairPrime