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Discard partials remaining after inference failure #20126

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Merged
ilevkivskyi merged 3 commits into from
Oct 28, 2025
Merged

Discard partials remaining after inference failure #20126

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665e091
Discard partials remaining after inference failure
sterliakov Oct 26, 2025
e9a35ad
Use the same fallback strategy as when `[] -> [something]` inference …
sterliakov Oct 27, 2025
3cc8725
Push this assignment through binder to retain `T | None` as var type
sterliakov Oct 27, 2025
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16 changes: 13 additions & 3 deletions mypy/checker.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3294,9 +3294,19 @@ def check_assignment(
del partial_types[var]
lvalue_type = var.type
else:
# Try to infer a partial type. No need to check the return value, as
# an error will be reported elsewhere.
self.infer_partial_type(lvalue_type.var, lvalue, rvalue_type)
# Try to infer a partial type.
if not self.infer_partial_type(var, lvalue, rvalue_type):
# If that also failed, give up and let the caller know that we
# cannot read their mind. The definition site will be reported later.
# Calling .put() directly because the newly inferred type is
# not a subtype of None - we are not looking for narrowing
fallback = self.inference_error_fallback_type(rvalue_type)
self.binder.put(lvalue, fallback)
# Same as self.set_inference_error_fallback_type but inlined
# to avoid computing fallback twice.
# We are replacing partial<None> now, so the variable type
# should remain optional.
self.set_inferred_type(var, lvalue, make_optional_type(fallback))
elif (
is_literal_none(rvalue)
and isinstance(lvalue, NameExpr)
Expand Down
77 changes: 71 additions & 6 deletions test-data/unit/check-inference.test
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Original file line number Diff line number Diff line change
Expand Up @@ -2522,7 +2522,6 @@ class C:
def __init__(self) -> None:
self.x = [] # E: Need type annotation for "x" (hint: "x: list[<type>] = ...")
[builtins fixtures/list.pyi]
[out]

[case testNoCrashOnPartialVariable]
from typing import Tuple, TypeVar
Expand All @@ -2534,7 +2533,6 @@ x = None
(x,) = f('')
reveal_type(x) # N: Revealed type is "builtins.str"
[builtins fixtures/tuple.pyi]
[out]

[case testNoCrashOnPartialVariable2]
# flags: --no-local-partial-types
Expand All @@ -2543,11 +2541,10 @@ T = TypeVar('T', bound=str)

def f() -> Tuple[T]:
...
x = None
x = None # E: Need type annotation for "x"
if int():
(x,) = f()
[builtins fixtures/tuple.pyi]
[out]

[case testNoCrashOnPartialVariable3]
from typing import Tuple, TypeVar
Expand All @@ -2559,7 +2556,76 @@ x = None
(x, x) = f('')
reveal_type(x) # N: Revealed type is "builtins.str"
[builtins fixtures/tuple.pyi]
[out]

[case testRejectsPartialWithUninhabited]
from typing import Generic, TypeVar
T = TypeVar('T')

class Foo(Generic[T]): ...

def check() -> None:
x = None # E: Need type annotation for "x"
if int():
x = Foo()
reveal_type(x) # N: Revealed type is "__main__.Foo[Any]"
reveal_type(x) # N: Revealed type is "Union[__main__.Foo[Any], None]"

[case testRejectsPartialWithUninhabited2]
from typing import Generic, TypeVar
T = TypeVar('T')

class Foo(Generic[T]): ...

x = None # E: Need type annotation for "x"

def check() -> None:
global x
x = Foo()
reveal_type(x) # N: Revealed type is "__main__.Foo[Any]"

reveal_type(x) # N: Revealed type is "Union[__main__.Foo[Any], None]"

[case testRejectsPartialWithUninhabited3]
# Without force-rejecting Partial<None>, this crashes:
# https://github.com/python/mypy/issues/16573
from typing import Generic, TypeVar
T = TypeVar('T')

class Foo(Generic[T]): ...

def check() -> None:
client = None # E: Need type annotation for "client"

if client := Foo():
reveal_type(client) # N: Revealed type is "__main__.Foo[Any]"

reveal_type(client) # N: Revealed type is "Union[__main__.Foo[Any], None]"

client = 0 # E: Incompatible types in assignment (expression has type "int", variable has type "Optional[Foo[Any]]")
reveal_type(client) # N: Revealed type is "Union[__main__.Foo[Any], None]"

[case testRejectsPartialWithUninhabitedIndependently]
from typing import Generic, TypeVar
T = TypeVar('T')

class Foo(Generic[T]): ...

client = None # E: Need type annotation for "client"

def bad() -> None:
global client
client = Foo()
reveal_type(client) # N: Revealed type is "__main__.Foo[Any]"

def good() -> None:
global client
client = 1 # E: Incompatible types in assignment (expression has type "int", variable has type "Optional[Foo[Any]]")
reveal_type(client) # N: Revealed type is "Union[__main__.Foo[Any], None]"

def bad2() -> None:
global client
client = Foo()
reveal_type(client) # N: Revealed type is "__main__.Foo[Any]"

[case testInferenceNestedTuplesFromGenericIterable]
from typing import Tuple, TypeVar
Expand All @@ -2574,7 +2640,6 @@ def main() -> None:
reveal_type(a) # N: Revealed type is "builtins.int"
reveal_type(b) # N: Revealed type is "builtins.int"
[builtins fixtures/tuple.pyi]
[out]

[case testDontMarkUnreachableAfterInferenceUninhabited]
from typing import TypeVar
Expand Down