Mapping with Union for keys #8477

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1. RobertoPrevato commented on Mar 2, 2020

   

   RobertoPrevato

   on Mar 2, 2020

   Last edited by RobertoPrevato

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   Additionally:

   a: Union[Mapping[str, Union[str, int]],
            Mapping[int, Union[str, int]]]
   
   b: Union[str, int] = 20
   
   c = a[b]  < mypy reports two errors, written below

   Invalid index type "Union[str, int]" for "Union[Mapping[str, Union[str, int]], 
   Mapping[int, Union[str, int]]]"; expected type "str" mypy(error)
   
   Invalid index type "Union[str, int]" for "Union[Mapping[str, Union[str, int]], 
   Mapping[int, Union[str, int]]]"; expected type "int" mypy(error)
   
2. JelleZijlstra commented on Mar 3, 2020

   

   JelleZijlstra

   on Mar 3, 2020

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   This is generally because of https://mypy.readthedocs.io/en/stable/common_issues.html#invariance-vs-covariance. The key argument to Mapping is invariant. I don't remember exactly why this is, but I think we tried making it covariant at some point and it caused problems, so we had to revert.

   👍React with 👍1Reacted by Roberto Prevato
3. hauntsaninja commented on Mar 3, 2020

   

   hauntsaninja

   on Mar 3, 2020

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   It's because __getitem__ and get could do unsound things.
   python/typing#445
   #1114

   👍React with 👍2Reacted by Roberto Prevato and Jude
4. RobertoPrevato commented on Mar 3, 2020

   

   RobertoPrevato

   on Mar 3, 2020

   Last edited by RobertoPrevato

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   @JelleZijlstra, @hauntsaninja thank you for your replies! 🌞 I thought there was a good reason for this behavior.
   I am closing this issue, since I see it was already discussed.

   FWIW, I think I understood that Union is not the right tool to do the think I had in mind, described in my example above. The right tool is to define generic types for keys and values!

   from typing import *
   
   
   A = TypeVar("A", str, int)
   B = TypeVar("B", str, int)
   
   
   def example2(value: Mapping[A, B]):
       ...
   
   
   x: Mapping[str, int] = {
       "a": 1,
       "b": 2,
       "c": 3
   }
   
   example2(x)
   
   y: Mapping[str, str] = {
       "a": "1",
       "b": "2",
       "c": "3"
   }
   
   example2(y)

   And I understood this, because in C# it would be the same (Union doesn't exist in C#, you have to use interfaces to describe something like union). I often take C# as example because I know it, it's static, and clearly it is mature about generics.

   A basic example would be:

   using System;
   using System.Collections.Generic;
   using System.Linq;
   
   namespace netconsole
   {
       class Program
       {
           // a function that accepts any kind of dictionary as input and returns 
           // a new dictionary of only strings
           static Dictionary<string, string> Example<A, B>(Dictionary<A, B> dict)
               => dict.ToDictionary(key => key.ToString(), value => value.ToString());
   
           static void Main(string[] args)
           {
               var x = new Dictionary<string, int>() {
                   {"a", 1},
                   {"b", 2},
                   {"c", 3}
               };
   
               var a = Example(x);
           }
       }
   }
5. RobertoPrevato

   closed this as completedon Mar 3, 2020
6. RobertoPrevato commented on Mar 3, 2020

   

   RobertoPrevato

   on Mar 3, 2020

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   PS. I see the same was discussed here #6001