fix: Make Interval subclasses properly inherit bounds (#266)

Author: antonagestam

src/phantom/interval.py

@@ -110,6 +110,24 @@ def _get_scalar_float_bounds(
     return low, high
 
 
+def _resolve_bound(
+    cls: type,
+    name: str,
+    argument: Comparable | None,
+    default: Comparable,
+) -> None:
+    inherited = getattr(cls, name, None)
+
+    if argument is not None:
+        resolved = argument
+    elif inherited is not None:
+        resolved = inherited
+    else:
+        resolved = default
+
+    setattr(cls, name, resolved)
+
+
 class Interval(Phantom[Comparable], bound=Comparable, abstract=True):
     """
     Base class for all interval types, providing the following class arguments:
@@ -128,12 +146,12 @@ class Interval(Phantom[Comparable], bound=Comparable, abstract=True):
     def __init_subclass__(
         cls,
         check: IntervalCheck | None = None,
-        low: Comparable = neg_inf,
-        high: Comparable = inf,
+        low: Comparable | None = None,
+        high: Comparable | None = None,
         **kwargs: Any,
     ) -> None:
-        resolve_class_attr(cls, "__low__", low)
-        resolve_class_attr(cls, "__high__", high)
+        _resolve_bound(cls, "__low__", low, neg_inf)
+        _resolve_bound(cls, "__high__", high, inf)
         resolve_class_attr(cls, "__check__", check)
         if getattr(cls, "__check__", None) is None:
             raise TypeError(f"{cls.__qualname__} must define an interval check")

tests/test_interval.py

@@ -23,9 +23,9 @@
 
 class TestInterval:
     def test_subclassing_without_check_raises(self):
-        with pytest.raises(TypeError, match="I must define an interval check$"):
+        with pytest.raises(TypeError, match="A must define an interval check$"):
 
-            class I(Interval, abstract=False):  # noqa: E742
+            class A(Interval, abstract=False):
                 ...
 
     def test_parse_coerces_str(self):
@@ -35,7 +35,7 @@ class Great(int, Inclusive, low=10):
         assert Great.parse("10") == 10
 
     def test_allows_decimal_bound(self):
-        class I(  # noqa: E742
+        class A(
             Decimal,
             Interval,
             check=interval.exclusive,
@@ -44,9 +44,40 @@ class I(  # noqa: E742
         ):
             ...
 
-        assert not isinstance(2, I)
-        assert not isinstance(1.98, I)
-        assert isinstance(Decimal("1.98"), I)
+        assert not isinstance(2, A)
+        assert not isinstance(1.98, A)
+        assert isinstance(Decimal("1.98"), A)
+
+    def test_subclass_inherits_bounds(self):
+        class A(int, Inclusive, low=-10, high=10):
+            ...
+
+        class B(A):
+            ...
+
+        assert B.__check__ is A.__check__
+        assert isinstance(-10, B)
+        assert isinstance(10, B)
+        assert not isinstance(-11, B)
+        assert not isinstance(11, B)
+
+        class C(A, low=0):
+            ...
+
+        assert C.__check__ is A.__check__
+        assert isinstance(0, C)
+        assert isinstance(10, C)
+        assert not isinstance(-1, C)
+        assert not isinstance(11, C)
+
+        class D(A, high=0):
+            ...
+
+        assert D.__check__ is A.__check__
+        assert isinstance(-10, D)
+        assert isinstance(0, D)
+        assert not isinstance(-11, D)
+        assert not isinstance(1, D)
 
 
 parametrize_negative_ints = pytest.mark.parametrize("i", (-10, -1, -0, +0))