[ty] Extend tuple __len__ and __bool__ special casing to also cover tuple subclasses (astral-sh#19289)

Co-authored-by: Brent Westbrook

Author: AlexWaygood

crates/ty_python_semantic/resources/mdtest/expression/boolean.md

@@ -72,7 +72,14 @@ reveal_type(my_bool(0))  # revealed: bool
 
 ## Truthy values
 
+```toml
+[environment]
+python-version = "3.11"
+```
+
 ```py
+from typing import Literal
+
 reveal_type(bool(1))  # revealed: Literal[True]
 reveal_type(bool((0,)))  # revealed: Literal[True]
 reveal_type(bool("NON EMPTY"))  # revealed: Literal[True]
@@ -81,6 +88,42 @@ reveal_type(bool(True))  # revealed: Literal[True]
 def foo(): ...
 
 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().__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().__bool__)  # revealed: () -> Literal[True]
+
+# Unknown length with an overridden `__bool__`:
+class VariadicTupleSubclassWithDunderBoolOverride(tuple[int, ...]):
+    def __bool__(self) -> Literal[True]:
+        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[()]):
+    # TODO: we should reject this override as a Liskov violation:
+    def __bool__(self) -> Literal[True]:
+        return True
+
+reveal_type(bool(EmptyTupleSubclassWithDunderBoolOverride(())))  # revealed: Literal[True]
+reveal_type(EmptyTupleSubclassWithDunderBoolOverride.__bool__)  # revealed: def __bool__(self) -> Literal[True]
+
+# revealed: bound method EmptyTupleSubclassWithDunderBoolOverride.__bool__() -> Literal[True]
+reveal_type(EmptyTupleSubclassWithDunderBoolOverride().__bool__)
 ```
 
 ## Falsy values
@@ -92,6 +135,12 @@ reveal_type(bool(None))  # revealed: Literal[False]
 reveal_type(bool(""))  # revealed: Literal[False]
 reveal_type(bool(False))  # revealed: Literal[False]
 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]
 ```
 
 ## Ambiguous values
@@ -100,6 +149,13 @@ reveal_type(bool())  # revealed: Literal[False]
 reveal_type(bool([]))  # revealed: bool
 reveal_type(bool({}))  # revealed: bool
 reveal_type(bool(set()))  # revealed: bool
+
+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
 ```
 
 ## `__bool__` returning `NoReturn`

crates/ty_python_semantic/resources/mdtest/expression/len.md

@@ -65,6 +65,51 @@ reveal_type(len((*[], 1, 2)))  # revealed: Literal[3]
 reveal_type(len((*[], *{})))  # revealed: Literal[2]
 ```
 
+Tuple subclasses:
+
+```py
+class EmptyTupleSubclass(tuple[()]): ...
+class Length1TupleSubclass(tuple[int]): ...
+class Length2TupleSubclass(tuple[int, str]): ...
+class UnknownLengthTupleSubclass(tuple[int, ...]): ...
+
+reveal_type(len(EmptyTupleSubclass()))  # revealed: Literal[0]
+reveal_type(len(Length1TupleSubclass((1,))))  # revealed: Literal[1]
+reveal_type(len(Length2TupleSubclass((1, "foo"))))  # revealed: Literal[2]
+reveal_type(len(UnknownLengthTupleSubclass((1, 2, 3))))  # revealed: int
+
+reveal_type(tuple[int, int].__len__)  # revealed: (self: tuple[int, int], /) -> Literal[2]
+reveal_type(tuple[int, ...].__len__)  # revealed: (self: tuple[int, ...], /) -> int
+
+def f(x: tuple[int, int], y: tuple[int, ...]):
+    reveal_type(x.__len__)  # revealed: () -> Literal[2]
+    reveal_type(y.__len__)  # revealed: () -> int
+
+reveal_type(EmptyTupleSubclass.__len__)  # revealed: (self: tuple[()], /) -> Literal[0]
+reveal_type(EmptyTupleSubclass().__len__)  # revealed: () -> Literal[0]
+reveal_type(UnknownLengthTupleSubclass.__len__)  # revealed: (self: tuple[int, ...], /) -> int
+reveal_type(UnknownLengthTupleSubclass().__len__)  # revealed: () -> int
+```
+
+If `__len__` is overridden, we use the overridden return type:
+
+```py
+from typing import Literal
+
+class UnknownLengthSubclassWithDunderLenOverridden(tuple[int, ...]):
+    def __len__(self) -> Literal[42]:
+        return 42
+
+reveal_type(len(UnknownLengthSubclassWithDunderLenOverridden()))  # revealed: Literal[42]
+
+class FixedLengthSubclassWithDunderLenOverridden(tuple[int]):
+    # TODO: we should complain about this as a Liskov violation (incompatible override)
+    def __len__(self) -> Literal[42]:
+        return 42
+
+reveal_type(len(FixedLengthSubclassWithDunderLenOverridden((1,))))  # revealed: Literal[42]
+```
+
 ### Lists, sets and dictionaries
 
 ```py

crates/ty_python_semantic/resources/mdtest/type_properties/is_subtype_of.md

@@ -551,6 +551,11 @@ static_assert(is_subtype_of(Never, AlwaysFalsy))
 
 ### `AlwaysTruthy` and `AlwaysFalsy`
 
+```toml
+[environment]
+python-version = "3.11"
+```
+
 ```py
 from ty_extensions import AlwaysTruthy, AlwaysFalsy, Intersection, Not, is_subtype_of, static_assert
 from typing_extensions import Literal, LiteralString
@@ -588,6 +593,30 @@ static_assert(is_subtype_of(Intersection[LiteralString, Not[Literal["", "a"]]],
 static_assert(is_subtype_of(Intersection[LiteralString, Not[Literal[""]]], Not[AlwaysFalsy]))
 # error: [static-assert-error]
 static_assert(is_subtype_of(Intersection[LiteralString, Not[Literal["", "a"]]], Not[AlwaysFalsy]))
+
+class Length2TupleSubclass(tuple[int, str]): ...
+
+static_assert(is_subtype_of(Length2TupleSubclass, AlwaysTruthy))
+
+class EmptyTupleSubclass(tuple[()]): ...
+
+static_assert(is_subtype_of(EmptyTupleSubclass, AlwaysFalsy))
+
+class TupleSubclassWithAtLeastLength2(tuple[int, *tuple[str, ...], bytes]): ...
+
+static_assert(is_subtype_of(TupleSubclassWithAtLeastLength2, AlwaysTruthy))
+
+class UnknownLength(tuple[int, ...]): ...
+
+static_assert(not is_subtype_of(UnknownLength, AlwaysTruthy))
+static_assert(not is_subtype_of(UnknownLength, AlwaysFalsy))
+
+class Invalid(tuple[int, str]):
+    # TODO: we should emit an error here (Liskov violation)
+    def __bool__(self) -> Literal[False]:
+        return False
+
+static_assert(is_subtype_of(Invalid, AlwaysFalsy))
 ```
 
 ### `TypeGuard` and `TypeIs`

crates/ty_python_semantic/src/types.rs

@@ -56,7 +56,7 @@ use crate::types::infer::infer_unpack_types;
 use crate::types::mro::{Mro, MroError, MroIterator};
 pub(crate) use crate::types::narrow::infer_narrowing_constraint;
 use crate::types::signatures::{Parameter, ParameterForm, Parameters, walk_signature};
-use crate::types::tuple::{TupleSpec, TupleType};
+use crate::types::tuple::TupleType;
 pub use crate::util::diagnostics::add_inferred_python_version_hint_to_diagnostic;
 use crate::{Db, FxOrderSet, Module, Program};
 pub(crate) use class::{ClassLiteral, ClassType, GenericAlias, KnownClass};
@@ -3508,14 +3508,7 @@ impl<'db> Type<'db> {
             Type::BooleanLiteral(bool) => Truthiness::from(*bool),
             Type::StringLiteral(str) => Truthiness::from(!str.value(db).is_empty()),
             Type::BytesLiteral(bytes) => Truthiness::from(!bytes.value(db).is_empty()),
-            Type::Tuple(tuple) => match tuple.tuple(db).len().size_hint() {
-                // The tuple type is AlwaysFalse if it contains only the empty tuple
-                (_, Some(0)) => Truthiness::AlwaysFalse,
-                // The tuple type is AlwaysTrue if its inhabitants must always have length >=1
-                (minimum, _) if minimum > 0 => Truthiness::AlwaysTrue,
-                // The tuple type is Ambiguous if its inhabitants could be of any length
-                _ => Truthiness::Ambiguous,
-            },
+            Type::Tuple(tuple) => tuple.truthiness(db),
         };
 
         Ok(truthiness)
@@ -3542,10 +3535,12 @@ impl<'db> Type<'db> {
         let usize_len = match self {
             Type::BytesLiteral(bytes) => Some(bytes.python_len(db)),
             Type::StringLiteral(string) => Some(string.python_len(db)),
-            Type::Tuple(tuple) => match tuple.tuple(db) {
-                TupleSpec::Fixed(tuple) => Some(tuple.len()),
-                TupleSpec::Variable(_) => None,
-            },
+
+            // N.B. This is strictly-speaking redundant, since the `__len__` method on tuples
+            // is special-cased in `ClassType::own_class_member`. However, it's probably more
+            // efficient to short-circuit here and check against the tuple spec directly,
+            // rather than going through the `__len__` method.
+            Type::Tuple(tuple) => tuple.tuple(db).len().into_fixed_length(),
 
             _ => None,
         };

crates/ty_python_semantic/src/types/class.rs

@@ -574,9 +574,39 @@ impl<'db> ClassType<'db> {
     /// traverse through the MRO until it finds the member.
     pub(super) fn own_class_member(self, db: &'db dyn Db, name: &str) -> PlaceAndQualifiers<'db> {
         let (class_literal, specialization) = self.class_literal(db);
-        class_literal
-            .own_class_member(db, specialization, name)
-            .map_type(|ty| ty.apply_optional_specialization(db, specialization))
+
+        let synthesize_tuple_method = |return_type| {
+            let parameters =
+                Parameters::new([Parameter::positional_only(Some(Name::new_static("self")))
+                    .with_annotated_type(Type::instance(db, self))]);
+
+            let synthesized_dunder_method =
+                CallableType::function_like(db, Signature::new(parameters, Some(return_type)));
+
+            Place::bound(synthesized_dunder_method).into()
+        };
+
+        match name {
+            "__len__" if class_literal.is_known(db, KnownClass::Tuple) => {
+                let return_type = specialization
+                    .and_then(|spec| spec.tuple(db).len().into_fixed_length())
+                    .and_then(|len| i64::try_from(len).ok())
+                    .map(Type::IntLiteral)
+                    .unwrap_or_else(|| KnownClass::Int.to_instance(db));
+
+                synthesize_tuple_method(return_type)
+            }
+            "__bool__" if class_literal.is_known(db, KnownClass::Tuple) => {
+                let return_type = specialization
+                    .map(|spec| spec.tuple(db).truthiness().into_type(db))
+                    .unwrap_or_else(|| KnownClass::Bool.to_instance(db));
+
+                synthesize_tuple_method(return_type)
+            }
+            _ => class_literal
+                .own_class_member(db, specialization, name)
+                .map_type(|ty| ty.apply_optional_specialization(db, specialization)),
+        }
     }
 
     /// Look up an instance attribute (available in `__dict__`) of the given name.

crates/ty_python_semantic/src/types/tuple.rs

@@ -22,6 +22,7 @@ use std::hash::Hash;
 
 use itertools::{Either, EitherOrBoth, Itertools};
 
+use crate::types::Truthiness;
 use crate::types::class::{ClassType, KnownClass};
 use crate::types::{
     Type, TypeMapping, TypeRelation, TypeTransformer, TypeVarInstance, TypeVarVariance,
@@ -76,6 +77,13 @@ impl TupleLength {
             None => "unlimited".to_string(),
         }
     }
+
+    pub(crate) fn into_fixed_length(self) -> Option<usize> {
+        match self {
+            TupleLength::Fixed(len) => Some(len),
+            TupleLength::Variable(_, _) => None,
+        }
+    }
 }
 
 /// # Ordering
@@ -240,6 +248,10 @@ impl<'db> TupleType<'db> {
     pub(crate) fn is_single_valued(self, db: &'db dyn Db) -> bool {
         self.tuple(db).is_single_valued(db)
     }
+
+    pub(crate) fn truthiness(self, db: &'db dyn Db) -> Truthiness {
+        self.tuple(db).truthiness()
+    }
 }
 
 /// A tuple spec describes the contents of a tuple type, which might be fixed- or variable-length.
@@ -967,6 +979,17 @@ impl<T> Tuple<T> {
         }
     }
 
+    pub(crate) fn truthiness(&self) -> Truthiness {
+        match self.len().size_hint() {
+            // The tuple type is AlwaysFalse if it contains only the empty tuple
+            (_, Some(0)) => Truthiness::AlwaysFalse,
+            // The tuple type is AlwaysTrue if its inhabitants must always have length >=1
+            (minimum, _) if minimum > 0 => Truthiness::AlwaysTrue,
+            // The tuple type is Ambiguous if its inhabitants could be of any length
+            _ => Truthiness::Ambiguous,
+        }
+    }
+
     pub(crate) fn is_empty(&self) -> bool {
         match self {
             Tuple::Fixed(tuple) => tuple.is_empty(),