Incompatibles types in assignment #5765

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1. gvanrossum commented on Oct 10, 2018

   

   gvanrossum

   on Oct 10, 2018

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   This is not a bug in mypy. Your issue is that List[Union[int, str]] is not a subtype of List[int]. To understand this, read up on Liskov substitution and variance: https://mypy.readthedocs.io/en/latest/generics.html#variance-of-generic-types

   👍React with 👍1Reacted by Martins, F.
2. gvanrossum

   closed this as completedon Oct 10, 2018
3. ilevkivskyi commented on Oct 11, 2018

   

   ilevkivskyi

   on Oct 11, 2018

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   Same misunderstanding appeared previously in #4186 (and in other issues). I am raising priority of that issue to high (also there is a simple solution, add the same note with the link to invariance that we add for bad argument type).
4. remdragon commented on Oct 12, 2018

   

   remdragon

   on Oct 12, 2018

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   gvanrossum / ilevkivskyi, thank you for your attention on this.

   I researched the suggested topics worked up a couple more examples and at first I couldn't see the problem. I decided to document some of my own thought process on this in case it helps others.

   In my example, the problem is that GetParams() could save a reference to the List it returns for later manipulation. It would assume (rightly) that the List could ever only contain Variant objects, but params has a Union and so could violate the type of the saved copy of the list inside GetParams().

   Presumably then, the fix in my case would be to make a copy of the List returned, but this still fails (see code example below).

   So I did even more research and now I think I understand the problem. While this code is technically correct in it's current state, it fails the "Liskov substitution code smell test" and that boils down to "there's probably a cleaner way to implement this".

   Right after I posted this issue, in the interest of moving past it, I did end up finding a cleaner solution which essentially involved params below no longer needing to expect to have to contain a VALUE and so the type definition of params became List[Variant] and mypy is now happy and I now don't have to do instanceof checks on it's values.

   tldr; you guys are awesome, thanks!!!!

   from __future__ import print_function
   
   import builtins
   from datetime import date, datetime, time, timedelta
   from decimal import Decimal
   from six import text_type as unicode # pip install six
   
   if not getattr ( builtins, 'long', None ):
   	long = int
   
   MYPY = False
   if MYPY:
   	from typing import List, Optional, Text, Union
   	VALUE = Union[int,long,float,Decimal,unicode,datetime,date,time,timedelta]
   
   class Variant ( object ):
   	name = None # type: Text
   	value = None # type: Optional[VALUE]
   	
   	def __init__ ( self, name, value ):
   		# type: ( Text, Optional[VALUE] ) -> None
   		self.name = name
   		self.value = value
   
   def GetParams():
   	# type: () -> List[Variant]
   	return [ Variant ( u'Foo', u'Bar' ) ]
   
   params = [] # type: List[Union[Variant,VALUE]]
   
   params = GetParams()[:] # make a copy but this still fails
   
   for param in params:
   	if isinstance ( param, Variant ):
   		print ( param.value )
   	else:
   		print ( repr ( param ) )
   
5. ilevkivskyi commented on Oct 12, 2018

   

   ilevkivskyi

   on Oct 12, 2018

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   Yes, mypy isn't smart enough to figure out that making a copy is safe. This can probably be solved by giving list.copy() and list.__getitem__() a better signature in typeshed in the same way as python/typeshed#2404 does. cc @JelleZijlstra