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pythontype-hintingmypy

Why Union[List[List[int]], List[int]] is casted to List[Union[List[List[int]], List[int]]]? (Correct use of type hints)

I have following code:

from typing import Union,List,Any
v: Union[list[list[int]],list[int]] = [-1,3,1,6,-5] # Create a list of inst
if not isinstance(v[0],list):
    v =[v]
v =[v] # Cast list of ints to list of list of ints
print(v)

mypy complains about it in the following way:

functions.py:5: error: Incompatible types in assignment 
(expression has type "List[Union[List[List[int]], List[int]]]", 
variable has type "Union[List[List[int]], List[int]]")  [assignment]
Found 1 error in 1 file (checked 1 source file)

I probably do not understand how type hints work but I really can not understand why assigning the new v variable this way makes mypy complain, because the end object is actually a list[list[int]]. Can anybody explain?

PS

I want to add a note: I encountered this issue while adding type hinting to some code that I do not own on github, so I can not really touch the "logic" of it.

Solution

  • Apparently the Issue is solved by changing v = [v] to v=[[*v]]. Not clear why...