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1. gvanrossum commented on Sep 7, 2017

   

   gvanrossum

   on Sep 7, 2017

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   Please include full self-contained examples of code showing the error you are experiencing. It makes helping you so much easier

   I suppose you were writing something like

   x = [A(), A()]
   y = [B(), B()]
   z = x + y  # error here
   

   right? Your problem here is that the type of x is List[A], and the type of y is List[B]. Because the List type is invariant this is rejected. You can fix it by adding # type: List[A] to the assignment to y.
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3. JukkaL commented on Sep 7, 2017

   

   JukkaL

   on Sep 7, 2017

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   Actually x + y would be safe if the inferred type of the result was List[A]. There currently is no way to represent this in the type system, though. We could probably write a type system plugin to support this if this seems to be a common enough issue.
4. henryJack commented on Sep 7, 2017

   

   henryJack

   on Sep 7, 2017

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   Yes, sorry. I'll try to be more explicit:

   If I declare the following classes:

   Class A:
   
   Class B(A):
   
   Class C(A):
   
   Class D(B):
   

   Then I prepare some pure lists:

   w = [A(), A()]
   x = [B(), B()]
   y = [C(), C()]
   z = [D(), D()]
   

   Now, if I add any combination of different lists I will get the mypy error error: Unsupported operand types for + (List[X] and List[Y]) :

   new_list = w + x  # should be type: List[A]
   new_list = w + y  # should be type: List[A]
   new_list = w + z  # should be type: List[A]
   new_list = x + y  # should be type: List[A]
   new_list = x + z  # should be type: List[B]
   new_list = y + z  # should be type: List[A]
   

   i.e., The highest common Parent class should become the type of the new list
5. henryJack commented on Sep 7, 2017

   

   henryJack

   on Sep 7, 2017

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   Then of course,

   new_list = x + z + w  # should be type: List[A]
   

   Because the highest common parent class is now A
6. JukkaL

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7. gvanrossum commented on Sep 7, 2017

   

   gvanrossum

   on Sep 7, 2017

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   @henryJack Without writing a mypy plugin your only type-safe way to get this to work is to add explicit declarations so that all lists have type List[A].
9. mokulus commented on Aug 7, 2024

   

   mokulus

   on Aug 7, 2024

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   This is still a problem:

   mix: list[int | str] = [1, "foo"]
   strings: list[str] = ["bar"]
   
   # mix = mix + strings  # doesn't work
   mix = [*mix, *strings]  # works