join is not associative in the presence of unions #3339

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1. ilevkivskyi

   added

   bugmypy got something wrongmypy got something wrong

   topic-strict-optional

   on May 7, 2017
2. JukkaL commented on May 8, 2017

   

   JukkaL

   on May 8, 2017

   Collaborator

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   It looks like joins should only produce unions if one of the operands is a union with more than one non-None item. Examples:

   • join(Union[int, None], str) would be Union[object, None] which would get simplified into object.
   • join(Union[int, bytes, None], str) would be Union[int, bytes, str, None].
3. JelleZijlstra

   added

   topic-join-v-unionUsing join vs. using unionsUsing join vs. using unions

   on Mar 19, 2022
4. hauntsaninja commented on May 21, 2023

   

   hauntsaninja

   on May 21, 2023

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   Another example (which doesn't use None)

   def check(l_int: list[int], l_str: list[str], l_union: list[str] | list[bytes]):
       reveal_type([l_int, l_str])
       reveal_type([l_int, l_str, l_union])
       reveal_type([l_int, l_union, l_str])
   
5. hauntsaninja

   changed the title [-]join is not associative in strict optional mode[/-] [+]join is not associative in the presence of unions[/+] on May 21, 2023
6. ikonst commented on May 22, 2023

   

   ikonst

   on May 22, 2023

   Contributor

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   Maybe worth clarifying (in the original post) what's the desired outcome? I assume builtins.list[Union[builtins.str, builtins.int, builtins.None]] but wanted to make sure.