Require SupportsBool instead of bool for comparisons. (#14375)

Author: randolf-scholz

stdlib/@tests/test_cases/builtins/check_min.py

@@ -0,0 +1,94 @@
+from typing_extensions import assert_type
+
+
+def test_min_builtin() -> None:
+    # legal comparisons that succeed at runtime
+    b1, b2 = bool(True), bool(False)
+    i1, i2 = int(1), int(2)
+    s1, s2 = str("a"), str("b")
+    f1, f2 = float(0.5), float(2.3)
+    l1, l2 = list[int]([1, 2]), list[int]([3, 4])
+    t1, t2 = tuple[str, str](("A", "B")), tuple[str, str](("C", "D"))
+    tN = tuple[str, ...](["A", "B", "C"])
+
+    assert_type(min(b1, b2), bool)
+    assert_type(min(i1, i2), int)
+    assert_type(min(s1, s2), str)
+    assert_type(min(f1, f2), float)
+
+    # mixed numerical types (note: float = int or float)
+    assert_type(min(b1, i1), int)
+    assert_type(min(i1, b1), int)
+
+    assert_type(min(b1, f1), float)
+    assert_type(min(f1, b1), float)
+
+    assert_type(min(i1, f1), float)
+    assert_type(min(f1, i1), float)
+
+    # comparisons with lists and tuples
+    assert_type(min(l1, l2), list[int])
+    assert_type(min(t1, t2), tuple[str, str])
+    assert_type(min(tN, t2), tuple[str, ...])
+
+
+def test_min_bad_builtin() -> None:
+    # illegal comparisons that fail at runtime
+    i1 = int(1)
+    s1 = str("a")
+    f1 = float(1.0)
+    c1, c2 = complex(1.0, 2.0), complex(3.0, 4.0)
+    list_str = list[str](["A", "B"])
+    list_int = list[int]([2, 3])
+    tup_str = tuple[str, str](("A", "B"))
+    tup_int = tuple[int, int]((2, 3))
+
+    # True negatives.
+    min(c1, c2)  # type: ignore
+
+    # FIXME: False negatives.
+    min(i1, s1)
+    min(s1, f1)
+    min(f1, list_str)
+    min(list_str, list_int)
+    min(tup_str, tup_int)
+
+
+def test_min_custom_comparison() -> None:
+    class BoolScalar:
+        def __bool__(self) -> bool: ...
+
+    class FloatScalar:
+        def __float__(self) -> float: ...
+        def __ge__(self, other: "FloatScalar") -> BoolScalar: ...
+        def __gt__(self, other: "FloatScalar") -> BoolScalar: ...
+        def __lt__(self, other: "FloatScalar") -> BoolScalar: ...
+        def __le__(self, other: "FloatScalar") -> BoolScalar: ...
+
+    f1 = FloatScalar()
+    f2 = FloatScalar()
+
+    assert_type(min(f1, f2), FloatScalar)
+
+
+def test_min_bad_custom_type() -> None:
+    class FloatScalar:
+        def __float__(self) -> float: ...
+        def __ge__(self, other: "FloatScalar") -> object:
+            return object()
+
+        def __gt__(self, other: "FloatScalar") -> object:
+            return object()
+
+        def __lt__(self, other: "FloatScalar") -> object:
+            return object()
+
+        def __le__(self, other: "FloatScalar") -> object:
+            return object()
+
+    f1 = FloatScalar()
+    f2 = FloatScalar()
+
+    # Note: min(f1, f2) works at runtime, but always returns the second argument.
+    #   therefore, we require returning a boolean-like type for comparisons.
+    min(f1, f2)  # type: ignore

stdlib/_typeshed/__init__.pyi

@@ -82,19 +82,21 @@ class SupportsNext(Protocol[_T_co]):
 class SupportsAnext(Protocol[_T_co]):
     def __anext__(self) -> Awaitable[_T_co]: ...
 
-# Comparison protocols
+class SupportsBool(Protocol):
+    def __bool__(self) -> bool: ...
 
+# Comparison protocols
 class SupportsDunderLT(Protocol[_T_contra]):
-    def __lt__(self, other: _T_contra, /) -> bool: ...
+    def __lt__(self, other: _T_contra, /) -> SupportsBool: ...
 
 class SupportsDunderGT(Protocol[_T_contra]):
-    def __gt__(self, other: _T_contra, /) -> bool: ...
+    def __gt__(self, other: _T_contra, /) -> SupportsBool: ...
 
 class SupportsDunderLE(Protocol[_T_contra]):
-    def __le__(self, other: _T_contra, /) -> bool: ...
+    def __le__(self, other: _T_contra, /) -> SupportsBool: ...
 
 class SupportsDunderGE(Protocol[_T_contra]):
-    def __ge__(self, other: _T_contra, /) -> bool: ...
+    def __ge__(self, other: _T_contra, /) -> SupportsBool: ...
 
 class SupportsAllComparisons(
     SupportsDunderLT[Any], SupportsDunderGT[Any], SupportsDunderLE[Any], SupportsDunderGE[Any], Protocol