Inferred Union subtype changed inconistently after a failed operation #3122

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1. pkch

   changed the title [-]Inferred Union subtype changed incorrectly after a failed operation[/-] [+]Inferred Union subtype changed inconistently after a failed operation[/+] on Apr 3, 2017
2. JukkaL commented on Apr 4, 2017

   

   JukkaL

   on Apr 4, 2017

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   This boils down to the inferred types of these expressions:

   reveal_type('' + 1)  # int
   reveal_type(1 + '')  # int
   

   This seems inconsistent.
3. JukkaL

   added

   bugmypy got something wrongmypy got something wrong

   on Apr 4, 2017
4. AlexWaygood

   added

   topic-union-types

   on Mar 24, 2022
5. AlexWaygood commented on Mar 24, 2022

   

   AlexWaygood

   on Mar 24, 2022

   Last edited by AlexWaygood

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   The output on mypy 0.941 is slightly different:

   from typing import Union
   
   x: Union[int, str]
   
   if isinstance(x, str):
       reveal_type(x)  # Revealed type is 'builtins.str'
       x = x + 1   # E: Unsupported operand types for + ("str" and "int")
       reveal_type(x)  # Revealed type is 'Union[builtins.int, builtins.str]'  - CHANGED
   
   if isinstance(x, int):
       reveal_type(x)  # Revealed type is 'builtins.int'
       x = x + 'a'   # E: Unsupported operand types for + ("str" and "int")
       reveal_type(x)  # Revealed type is 'builtins.int'  - DIDN'T CHANGE

   And for Jukka's example:

   reveal_type('' + 1)  # Any
   reveal_type(1 + '')  # int
6. JelleZijlstra commented on Mar 24, 2022

   

   JelleZijlstra

   on Mar 24, 2022

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   Possibly related: int.__radd__ exists in typeshed, but str.__radd__ doesn't. This matches reality:

   >>> str.__radd__
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   AttributeError: type object 'str' has no attribute '__radd__'. Did you mean: '__add__'?
   >>> int.__radd__
   <slot wrapper '__radd__' of 'int' objects>