add narrowing for lists via index isinstance check #9382
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Allows for narrowing a union of list types by indexing into the list
and checking the type of an element.
mypy already narrows the specific index expression.
Since mypy already narrows the index expression's type, we use the type
to compare against the type arguments of the lists and generate a new
type that is narrower than the original.
```
from typing import Union, List
a: Union[List[List[int]], List[int]]
# before
if isinstance(a[0], list):
reveal_type(a) # N: Revealed type is 'Union[builtins.list[builtins.list[builtins.int]], builtins.list[builtins.int]]'
else:
reveal_type(a) # N: Revealed type is 'Union[builtins.list[builtins.list[builtins.int]], builtins.list[builtins.int]]'
# after
if isinstance(a[0], list):
reveal_type(a) # N: Revealed type is 'builtins.list[builtins.list[builtins.int]]'
else:
reveal_type(a) # N: Revealed type is 'builtins.list[builtins.int]'
```
fixes: python#9362
sbdchd
commented
Aug 30, 2020
| t = if_map[expr] | ||
| for x in arg_type.items: | ||
| x = get_proper_type(x) | ||
| if not isinstance(x, Instance) or not x.args: |
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I wasn't sure how to restrict this to only list or sequence like types, so ended up doing this which is probably incorrect.
sbdchd
commented
Aug 30, 2020
mypy/checker.py
Outdated
| x = get_proper_type(x) | ||
| if not isinstance(x, Instance) or not x.args: | ||
| return if_map, else_map | ||
| if is_overlapping_types(x.args[0], t) is not covers_at_runtime(x.args[0], t, False): |
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wasn't sure about this, essentially needed this special case to get object to work as expected in the test cases
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Allows for narrowing a union of list types by indexing into the list
and checking the type of an element.
Since mypy already narrows the index expression's type, we use this
expression's type to compare against the type arguments of the lists
in the union and generate a new type that is narrower than the original.
fixes: #9362