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[red-knot] Empty tuple is always-falsy #17213

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Merged
sharkdp merged 2 commits into from
Apr 4, 2025
Merged

[red-knot] Empty tuple is always-falsy #17213

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c7e1c64
[red-knot] Empty tuple is always-falsy
sharkdp Apr 4, 2025
e45c862
Add TODO comment
sharkdp Apr 4, 2025
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10 changes: 9 additions & 1 deletion ...s/red_knot_python_semantic/resources/mdtest/type_properties/is_assignable_to.md
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Original file line number Diff line number Diff line change
Expand Up @@ -183,7 +183,7 @@ static_assert(is_assignable_to(Meta, type[Unknown]))
## Tuple types

```py
from knot_extensions import static_assert, is_assignable_to
from knot_extensions import static_assert, is_assignable_to, AlwaysTruthy, AlwaysFalsy
from typing import Literal, Any

static_assert(is_assignable_to(tuple[()], tuple[()]))
Expand All @@ -198,6 +198,14 @@ static_assert(is_assignable_to(tuple[()], tuple))
static_assert(is_assignable_to(tuple[int, str], tuple))
static_assert(is_assignable_to(tuple[Any], tuple))

# TODO: It is not yet clear if we want the following two assertions to hold.
# See https://github.com/astral-sh/ruff/issues/15528 for more details. The
# short version is: We either need to special-case enforcement of the Liskov
# substitution principle on `__bool__` and `__len__` for tuple subclasses,
# or we need to negate these assertions.
static_assert(is_assignable_to(tuple[()], AlwaysFalsy))
static_assert(is_assignable_to(tuple[int], AlwaysTruthy))

static_assert(not is_assignable_to(tuple[()], tuple[int]))
static_assert(not is_assignable_to(tuple[int], tuple[str]))
static_assert(not is_assignable_to(tuple[int], tuple[int, str]))
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10 changes: 7 additions & 3 deletions crates/red_knot_python_semantic/src/types.rs
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Expand Up @@ -1071,9 +1071,13 @@ impl<'db> Type<'db> {
// This special case is required because the left-hand side tuple might be a
// gradual type, so we can not rely on subtyping. This allows us to assign e.g.
// `tuple[Any, int]` to `tuple`.
(Type::Tuple(_), _) => KnownClass::Tuple
.to_instance(db)
.is_assignable_to(db, target),
(Type::Tuple(_), _)
if KnownClass::Tuple
.to_instance(db)
.is_assignable_to(db, target) =>
{
true
}
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@sharkdp sharkdp Apr 4, 2025

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The difference is that we now allow a fall-through to the catch-all "self is subtype of target" branch.


// `type[Any]` is assignable to any `type[...]` type, because `type[Any]` can
// materialize to any `type[...]` type.
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