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Retain None (unreachable) when typemap is None with type(x) is Foo check #18486

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merged 2 commits into from
Jan 27, 2025
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Retain None (unreachable) when typemap is None with type(x) is Foo check #18486

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dc581d7
Retain all-None typemaps when merging in ``type(x) == SomeType`` narr…
sterliakov Jan 18, 2025
0d549ad
Add more explicit tests
sterliakov Jan 18, 2025
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2 changes: 2 additions & 0 deletions mypy/checker.py
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Original file line number Diff line number Diff line change
Expand Up @@ -5914,6 +5914,8 @@ def is_type_call(expr: CallExpr) -> bool:

def combine_maps(list_maps: list[TypeMap]) -> TypeMap:
"""Combine all typemaps in list_maps into one typemap"""
if all(m is None for m in list_maps):
return None
result_map = {}
for d in list_maps:
if d is not None:
Comment on lines +5917 to 5921
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Suggested change
if all(m is None for m in list_maps):
return None
result_map = {}
for d in list_maps:
if d is not None:
result_map = None
for d in list_maps:
if d is not None:
if result_map is None:
result_map = {}

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I'd keep my version if you don't mind: this part isn't hot (only reachable when we actually have some type(x) <op> Something checks), and I find it more explicit/slightly easier to follow. This shouldn't have any measurable performance impact.

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27 changes: 27 additions & 0 deletions test-data/unit/check-isinstance.test
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Expand Up @@ -2661,6 +2661,33 @@ y: Any
if type(y) is int:
reveal_type(y) # N: Revealed type is "builtins.int"

[case testTypeEqualsCheckUsingIsNonOverlapping]
# flags: --warn-unreachable
from typing import Union

y: str
if type(y) is int: # E: Subclass of "str" and "int" cannot exist: would have incompatible method signatures
y # E: Statement is unreachable
else:
reveal_type(y) # N: Revealed type is "builtins.str"
[builtins fixtures/isinstance.pyi]

[case testTypeEqualsCheckUsingIsNonOverlappingChild-xfail]
# flags: --warn-unreachable
from typing import Union

class A: ...
class B: ...
class C(A): ...
x: Union[B, C]
# C instance cannot be exactly its parent A, we need reversed subtyping relationship
# here (type(parent) is Child).
if type(x) is A:
reveal_type(x) # E: Statement is unreachable
else:
reveal_type(x) # N: Revealed type is "Union[__main__.B, __main__.C]"
[builtins fixtures/isinstance.pyi]

[case testTypeEqualsNarrowingUnionWithElse]
from typing import Union

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13 changes: 13 additions & 0 deletions test-data/unit/check-typevar-values.test
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Expand Up @@ -732,3 +732,16 @@ def foo3(x: NT) -> None:
def foo4(x: NT) -> None:
p, q = 1, 2.0 # type: (int, float)
[builtins fixtures/tuple.pyi]

[case testTypeVarValuesNarrowing]
from typing import TypeVar

W = TypeVar("W", int, str)

def fn(w: W) -> W:
if type(w) is str:
reveal_type(w) # N: Revealed type is "builtins.str"
elif type(w) is int:
reveal_type(w) # N: Revealed type is "builtins.int"
return w
[builtins fixtures/isinstance.pyi]
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