8000 Error when assigning a ternary with an empty collection to a union type variable · Issue #7780 · python/mypy · GitHub
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raphj opened this issue Oct 23, 2019 · 4 comments · Fixed by #7892
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Error when assigning a ternary with an empty collection to a union type variable #7780

raphj opened this issue Oct 23, 2019 · 4 comments · Fixed by #7892
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good-first-issue priority-1-normal topic-usability

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@raphj
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raphj commented Oct 23, 2019

Hello,

the following code:

from typing import List, Union
import sys
p : Union[int, List[int]] = 1 if len(sys.argv) else []

incorrectly generates the following error:

t.py:3: error: Incompatible types in assignment (expression has type "Union[int, List[<nothing>]]", variable has type "Union[int, List[int]]")

Adding an integer in the list makes the error disappear. I first observed this issue with a dictionary:

from typing import Dict, Union
import sys
p : Union[int, Dict[int, int]] = 1 if len(sys.argv) else {}

Error:

t.py:3: error: Incompatible types in assignment (expression has type "Union[int, Dict[<nothing>, <nothing>]]", variable has type "Union[int, Dict[int, int]]")

It seems <nothing> should be matched with any type in type parameters in unions.
@raphj raphj changed the title Error when assigning a ternary to a union type variable Error when assigning a ternary with an empty collection to a union type variable Oct 23, 2019
@nautics889
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Please, take a look at this topic. Perhaps mypy should consider an empty list like an object that fits List[int], but unfortunately it doesn't.
I think the best solution for you would be use Any type:

p : Union[int, List[Any]] = 1 if len(sys.argv) else []

@raphj
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raphj commented Oct 24, 2019

I worked around this issue by getting rid of the ternary condition and using a regular if statement instead.

@ilevkivskyi
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Another possible workaround could be using a covariant collection like Sequence. I however think mypy can do better here. We can probably pass through the type context in case of the ternary expressions.

@ilevkivskyi
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This actually may be not hard to fix, one just needs to update the logic in mypy/checkexpr.py in visit_conditional_expr() to try the original context ctx for the else expression.

@TH3CHARLie TH3CHARLie mentioned this issue Nov 6, 2019
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ilevkivskyi pushed a commit that referenced this issue Nov 7, 2019
@TH3CHARLie @ilevkivskyi
Fix error when assigning a ternary with an empty collection to a unio…
9ee7b8a 
…n type variable (#7892)

resolves #7780
@TH3CHARLie TH3CHARLie mentioned this issue Nov 15, 2019
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@vnavascues vnavascues mentioned this issue Dec 29, 2020
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Fix error when assigning a ternary with an empty collection to a union type variable
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@raphj @ilevkivskyi @nautics889
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