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[ty] Consider __len__ when determining the truthiness of an instance of a tuple class or a @final class (#21049)

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Alex Waygood 2025-10-24 10:29:55 +01:00 • committed by GitHub
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6 changed files with 349 additions and 65 deletions
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52
crates/ty_python_semantic/resources/mdtest/expression/boolean.md
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@ -78,7 +78,7 @@ python-version = "3.11"
```
```py
from typing import Literal
from typing import Literal, final
reveal_type(bool(1)) # revealed: Literal[True]
reveal_type(bool((0,))) # revealed: Literal[True]
@ -92,15 +92,11 @@ reveal_type(bool(foo)) # revealed: Literal[True]
class SingleElementTupleSubclass(tuple[int]): ...
reveal_type(bool(SingleElementTupleSubclass((0,)))) # revealed: Literal[True]
reveal_type(SingleElementTupleSubclass.__bool__) # revealed: (self: tuple[int], /) -> Literal[True]
reveal_type(SingleElementTupleSubclass((1,)).__bool__) # revealed: () -> Literal[True]
# Unknown length, but we know the length is guaranteed to be >=2
class MixedTupleSubclass(tuple[int, *tuple[str, ...], bytes]): ...
reveal_type(bool(MixedTupleSubclass((1, b"foo")))) # revealed: Literal[True]
reveal_type(MixedTupleSubclass.__bool__) # revealed: (self: tuple[int, *tuple[str, ...], bytes], /) -> Literal[True]
reveal_type(MixedTupleSubclass((1, b"foo")).__bool__) # revealed: () -> Literal[True]
# Unknown length with an overridden `__bool__`:
class VariadicTupleSubclassWithDunderBoolOverride(tuple[int, ...]):
@ -108,10 +104,6 @@ class VariadicTupleSubclassWithDunderBoolOverride(tuple[int, ...]):
return True
reveal_type(bool(VariadicTupleSubclassWithDunderBoolOverride((1,)))) # revealed: Literal[True]
reveal_type(VariadicTupleSubclassWithDunderBoolOverride.__bool__) # revealed: def __bool__(self) -> Literal[True]
# revealed: bound method VariadicTupleSubclassWithDunderBoolOverride.__bool__() -> Literal[True]
reveal_type(VariadicTupleSubclassWithDunderBoolOverride().__bool__)
# Same again but for a subclass of a fixed-length tuple:
class EmptyTupleSubclassWithDunderBoolOverride(tuple[()]):
@ -124,11 +116,28 @@ reveal_type(EmptyTupleSubclassWithDunderBoolOverride.__bool__) # revealed: def
# revealed: bound method EmptyTupleSubclassWithDunderBoolOverride.__bool__() -> Literal[True]
reveal_type(EmptyTupleSubclassWithDunderBoolOverride().__bool__)
@final
class FinalClassOverridingLenAndNotBool:
def __len__(self) -> Literal[42]:
return 42
reveal_type(bool(FinalClassOverridingLenAndNotBool())) # revealed: Literal[True]
@final
class FinalClassWithNoLenOrBool: ...
reveal_type(bool(FinalClassWithNoLenOrBool())) # revealed: Literal[True]
def f(x: SingleElementTupleSubclass | FinalClassOverridingLenAndNotBool | FinalClassWithNoLenOrBool):
reveal_type(bool(x)) # revealed: Literal[True]
```
## Falsy values
```py
from typing import final, Literal
reveal_type(bool(0)) # revealed: Literal[False]
reveal_type(bool(())) # revealed: Literal[False]
reveal_type(bool(None)) # revealed: Literal[False]
@ -139,13 +148,23 @@ reveal_type(bool()) # revealed: Literal[False]
class EmptyTupleSubclass(tuple[()]): ...
reveal_type(bool(EmptyTupleSubclass())) # revealed: Literal[False]
reveal_type(EmptyTupleSubclass.__bool__) # revealed: (self: tuple[()], /) -> Literal[False]
reveal_type(EmptyTupleSubclass().__bool__) # revealed: () -> Literal[False]
@final
class FinalClassOverridingLenAndNotBool:
def __len__(self) -> Literal[0]:
return 0
reveal_type(bool(FinalClassOverridingLenAndNotBool())) # revealed: Literal[False]
def f(x: EmptyTupleSubclass | FinalClassOverridingLenAndNotBool):
reveal_type(bool(x)) # revealed: Literal[False]
```
## Ambiguous values
```py
from typing import Literal
reveal_type(bool([])) # revealed: bool
reveal_type(bool({})) # revealed: bool
reveal_type(bool(set())) # revealed: bool
@ -154,8 +173,15 @@ class VariadicTupleSubclass(tuple[int, ...]): ...
def f(x: tuple[int, ...], y: VariadicTupleSubclass):
reveal_type(bool(x)) # revealed: bool
reveal_type(x.__bool__) # revealed: () -> bool
reveal_type(y.__bool__) # revealed: () -> bool
class NonFinalOverridingLenAndNotBool:
def __len__(self) -> Literal[42]:
return 42
# We cannot consider `__len__` for a non-`@final` type,
# because a subclass might override `__bool__`,
# and `__bool__` takes precedence over `__len__`
reveal_type(bool(NonFinalOverridingLenAndNotBool())) # revealed: bool
```
## `__bool__` returning `NoReturn`