__lt__ and __gt__ should always be allowed to have return type bool #1101

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1. gvanrossum commented on Jan 8, 2016

   

   gvanrossum

   on Jan 8, 2016

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   Example:

   from typing import Any
   
   class Infinity:
       def __lt__(self, other: Any) -> bool:  # E: error: "Any" return type expected since argument to __lt__ has type "Any"
           return False
       def __gt__(self, other: Any) -> bool:  # E: error: "Any" return type expected since argument to __lt__ has type "Any"
           return True
   
2. JukkaL commented on Jan 8, 2016

   

   JukkaL

   on Jan 8, 2016

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   This guards against a weird corner case that could cause a wrong type to be inferred. The example in the comment near the linked code gives some context (though it's not a complete example). It originates from my PhD research where type soundness was a major thing. As the goals of mypy have shifted, this check doesn't make sense any more. I'll just remove it.
3. JukkaL

   added

   bugmypy got something wrongmypy got something wrong

   on Jan 8, 2016
4. gvanrossum

   added a commit that references this issue on Jan 12, 2016

   Merge pull request #1109 from gvanrossum/fix1101

   ...

   f121947
5. gvanrossum

   closed this as completedon Jan 12, 2016
6. kamahen commented on Mar 20, 2018

   

   kamahen

   on Mar 20, 2018

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   It seems that mypy doesn't allow __ne__ returning NotImplemented, (this is allowed for __eq__).
   And when I try explicitly adding NotImplemented as a possible return type, I get a run-time error:

     File ".../pod.py", line 65, in PlainOldData
       def __ne__(self, other: Any) -> Union[bool, NotImplemented]:
     File "/usr/lib/python3.6/typing.py", line 682, in inner
       return func(*args, **kwds)
     File "/usr/lib/python3.6/typing.py", line 800, in __getitem__
       parameters = tuple(_type_check(p, msg) for p in parameters)
     File "/usr/lib/python3.6/typing.py", line 800, in <genexpr>
       parameters = tuple(_type_check(p, msg) for p in parameters)
     File "/usr/lib/python3.6/typing.py", line 374, in _type_check
       raise TypeError(msg + " Got %.100r." % (arg,))
   TypeError: Union[arg, ...]: each arg must be a type. Got NotImplemented.
   

   If I change this to def __ne__(self, other: Any) -> Union[bool, type(NotImplemented)]:, then mypy complains:

   error: invalid type comment or annotation
   note: Suggestion: use type[...] instead of type(...)
   

   and when I try following the suggestion, I get a runtime error:

   TypeError: 'type' object is not subscriptable
   
7. gvanrossum commented on Mar 20, 2018

   

   gvanrossum

   on Mar 20, 2018

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   Please use a new issue for such feedback. The root cause seems to be that __ne__ is not included in the list BINARY_MAGIC_METHODS in mypy/sharedparse.py. You could submit a PR that fixes this (and adds a test).