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Narrow types after 'in' operator #4072

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Oct 13, 2017
Merged

Narrow types after 'in' operator #4072

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9b89f03
Narrow types after 'in' operator
ilevkivskyi Oct 8, 2017
4ece088
Be safe: no Any or promotions
ilevkivskyi Oct 8, 2017
59e69d6
Limit narrowing after in only to optional types
ilevkivskyi Oct 13, 2017
a23ad9d
Fix strict-optional
ilevkivskyi Oct 13, 2017
ff33241
Address CR
ilevkivskyi Oct 13, 2017
42d7cc7
Add one more test
ilevkivskyi Oct 13, 2017
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14 changes: 13 additions & 1 deletion mypy/checker.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2875,7 +2875,19 @@ def remove_optional(typ: Type) -> Type:


def builtin_item_type(tp: Type) -> Optional[Type]:
# This is only OK for built-in containers, where we know the behavior of __contains__.
"""Get the item type of a builtin container.

If 'tp' is not one of the built containers (these includes NamedTuple and TypedDict)
or if the container is not parameterized (like List or List[Any])
return None. This function is used to narrow optional types in situations like this:

x: Optional[int]
if x in (1, 2, 3):
x + 42 # OK

Note: this is only OK for built-in containers, where we know the behavior
of __contains__.
"""
if isinstance(tp, Instance):
if tp.type.fullname() in ['builtins.list', 'builtins.tuple', 'builtins.dict',
'builtins.set', 'builtins.frozenset']:
Expand Down
16 changes: 15 additions & 1 deletion test-data/unit/check-isinstance.test
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Expand Up @@ -1788,6 +1788,20 @@ else:
[builtins fixtures/list.pyi]
[out]

[case testNarrowTypeAfterInListOfOptional]
# flags: --strict-optional
from typing import List, Optional

x: List[Optional[int]]
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Here's another special case: What if the container is a nested one, say List[List[x]], where x might be Any? How do we deal with various item types, such as List[int] and List[Any]?

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a) If the container is List[Any], then we do nothing (there is a test for this).
b) If the container is List[List[Any]] we narrow the type (provided there is an overlap, this is consistent with how == currently treated), for example:

x: Optional[int]
lst: Optional[List[int]]
nested: List[List[Any]]
if lst in nested:
    reveal_type(lst) # List[int]
if x in nested:
    reveal_type(x) # Optional[int]

There is already a test for non-overlapping items int vs str. I will add one more test for other (nested) types?

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OK, added one more test.

y: Optional[int]

if y not in x:
reveal_type(y) # E: Revealed type is 'Union[builtins.int, builtins.None]'
else:
reveal_type(y) # E: Revealed type is 'Union[builtins.int, builtins.None]'
[builtins fixtures/list.pyi]
[out]

[case testNarrowTypeAfterInListNonOverlapping]
# flags: --strict-optional
from typing import List, Optional
Expand Down Expand Up @@ -1836,7 +1850,7 @@ else:
[case testNarrowTypeAfterInDict]
# flags: --strict-optional
from typing import Dict, Optional
x: Dict[str, str]
x: Dict[str, int]
y: Optional[str]

if y in x:
Expand Down