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Prohibit use of names in annotations that are undefined in runtime #3088

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pkch opened this issue Mar 30, 2017 · 2 comments
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Prohibit use of names in annotations that are undefined in runtime #3088

pkch opened this issue Mar 30, 2017 · 2 comments
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feature priority-0-high topic-usability

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@pkch
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pkch commented Mar 30, 2017

This type checks, but crashes in runtime with NameError:

class A:
    def f(x: A) -> None:
        pass

I think it's better if mypy reported an error Name "A" should be a forward reference.
@gvanrossum
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Oh, that's weird. I didn't even know mypy accepts that! Agreed it would be better to flag as error. (Outside stubs, and not in type comments of course.)

Michael0x2a added a commit to Michael0x2a/mypy that referenced this issue Jul 5, 2017
@Michael0x2a
Prohibit referring to class within its definition
8279c20 
This partially, but not completely, resolves
python#3088

Previously, code of the following form was accepted by mypy:

    class A:
        def foo(self) -> A:
            ...

This results in an error at runtime because 'A' is not yet defined.
This pull request modifies the parsing process to report an error when
a type is being incorrectly used within its class definition.

However, this commit does *not* attempt to handle other cases where
the user is using a type that is undefined at runtime. For example, mypy
will continue to accept the following code without an error:

    def f() -> A:
        ...

    class A:
        ...

I decided it would probably be best to address this in a future pull
request.
@Michael0x2a Michael0x2a mentioned this issue Jul 5, 2017
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@JukkaL JukkaL changed the title Prohibit use of names undefined in runtime Prohibit use of names in annotations that are undefined in runtime Sep 18, 2017
@ilevkivskyi ilevkivskyi mentioned this issue Sep 29, 2017
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@ethanhs
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ethanhs commented Nov 4, 2017

Closing as a duplicate in favor of #948, which has a bit more information.

@ethanhs ethanhs closed this as completed Nov 4, 2017
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@JukkaL @pkch @gvanrossum @ethanhs @ilevkivskyi