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None inferred as variable type from UninhabitedType #4104

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JukkaL opened this issue Oct 12, 2017 · 3 comments
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None inferred as variable type from UninhabitedType #4104

JukkaL opened this issue Oct 12, 2017 · 3 comments
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bug mypy got something wrong priority-1-normal topic-type-variables

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@JukkaL
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JukkaL commented Oct 12, 2017

The inferred type for x is unexpected (especially when using --strict-optional):

from typing import TypeVar

T = TypeVar('T')

def f() -> T: pass

x = f()
reveal_type(x)  # None

It would be better to require an annotation for x.
@gvanrossum
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Why is the definition of f() not considered an error?

@ilevkivskyi
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@gvanrossum

Why is the definition of f() not considered an error?

Because it is not unsafe. As discussed in #2885 we can't prohibit all possible useless annotations. Moreover now we have #4066, so that I think the way forward is to prohibit empty bodies (so that one can't make a body for such annotations).

@JukkaL JukkaL mentioned this issue Oct 13, 2017
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@gvanrossum
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It may not be unsafe, but I'm pretty sure that whenever you see that, the user misunderstands what type variables are on a fundamental level (e.g. they confuse them with type aliases).

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@JukkaL @gvanrossum @ilevkivskyi