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Fix Any inference when unpacking iterators that don't directly inherit from typing.Iterator #14821

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Mar 3, 2023
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Fix Any inference when unpacking iterators that don't directly inherit from typing.Iterator #14821

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bf7a76f
Fix `Any` inference when unpacking iterators that return `self` and d…
AlexWaygood Mar 2, 2023
6e93052
Update test-data/unit/check-inference.test
AlexWaygood Mar 3, 2023
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9 changes: 1 addition & 8 deletions mypy/checker.py
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Original file line number Diff line number Diff line change
Expand Up @@ -6395,14 +6395,7 @@ def iterable_item_type(self, instance: Instance) -> Type:
# in case there is no explicit base class.
return item_type
# Try also structural typing.
ret_type, _ = self.expr_checker.check_method_call_by_name(
"__iter__", instance, [], [], instance
)
ret_type = get_proper_type(ret_type)
if isinstance(ret_type, Instance):
iterator = map_instance_to_supertype(ret_type, self.lookup_typeinfo("typing.Iterator"))
item_type = iterator.args[0]
return item_type
return self.analyze_iterable_item_type_without_expression(instance, instance)[1]

def function_type(self, func: FuncBase) -> FunctionLike:
return function_type(func, self.named_type("builtins.function"))
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34 changes: 34 additions & 0 deletions test-data/unit/check-inference.test
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Expand Up @@ -380,6 +380,8 @@ class Nums(Iterable[int]):
def __iter__(self): pass
def __next__(self): pass
a, b = Nums()
reveal_type(a) # N: Revealed type is "builtins.int"
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The other option here is to infer Any, because of def __iter__(self): pass

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Yup, but as discussed in the issue, two long-standing tests fail if I make that change, so I'd rather not make that behaviour change in this PR 👍

reveal_type(b) # N: Revealed type is "builtins.int"
if int():
a = b = 1
if int():
Expand All @@ -388,6 +390,38 @@ if int():
b = '' # E: Incompatible types in assignment (expression has type "str", variable has type "int")
[builtins fixtures/for.pyi]

[case testInferringTypesFromIterableStructuralSubtyping1]
from typing import Iterator
class Nums:
def __iter__(self) -> Iterator[int]: pass
def __next__(self) -> int: pass
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a, b = Nums()
reveal_type(a) # N: Revealed type is "builtins.int"
reveal_type(b) # N: Revealed type is "builtins.int"
if int():
a = b = 1
if int():
a = '' # E: Incompatible types in assignment (expression has type "str", variable has type "int")
if int():
b = '' # E: Incompatible types in assignment (expression has type "str", variable has type "int")
[builtins fixtures/for.pyi]

[case testInferringTypesFromIterableStructuralSubtyping2]
from typing import Self
class Nums:
def __iter__(self) -> Self: pass
def __next__(self) -> int: pass
a, b = Nums()
reveal_type(a) # N: Revealed type is "builtins.int"
reveal_type(b) # N: Revealed type is "builtins.int"
if int():
a = b = 1
if int():
a = '' # E: Incompatible types in assignment (expression has type "str", variable has type "int")
if int():
b = '' # E: Incompatible types in assignment (expression has type "str", variable has type "int")
[builtins fixtures/tuple.pyi]


-- Type variable inference for generic functions
-- ---------------------------------------------
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