Validate Interval start/end

The start can't be bigger than the end and we raise a `ValueError` in
that case.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/frequenz/core/collections.py

@@ -27,6 +27,11 @@ class Interval(Generic[LessThanComparableOrNoneT]):
     The `start` and `end` are inclusive, meaning that the `start` and `end` limites are
     included in the range when checking if a value is contained by the interval.
 
+    If the `start` or `end` is `None`, it means that the interval is unbounded in that
+    direction.
+
+    If `start` is bigger than `end`, a `ValueError` is raised.
+
     The type stored in the interval must be comparable, meaning that it must implement
     the `__lt__` method to be able to compare values.
     """
@@ -37,6 +42,17 @@ class Interval(Generic[LessThanComparableOrNoneT]):
     end: LessThanComparableOrNoneT
     """The end of the interval."""
 
+    def __post_init__(self) -> None:
+        """Check if the start is less than or equal to the end."""
+        if self.start is None or self.end is None:
+            return
+        start = cast(LessThanComparable, self.start)
+        end = cast(LessThanComparable, self.end)
+        if start > end:
+            raise ValueError(
+                f"The start ({self.start}) can't be bigger than end ({self.end})"
+            )
+
     def __contains__(self, item: LessThanComparableOrNoneT) -> bool:
         """
         Check if the value is within the range of the container.

tests/test_collections.py

@@ -33,6 +33,22 @@ def __repr__(self) -> str:
         return str(self.value)
 
 
+@pytest.mark.parametrize(
+    "start, end",
+    [
+        (10.0, -100.0),
+        (CustomComparable(10), CustomComparable(-100)),
+    ],
+)
+def test_invalid_range(start: LessThanComparable, end: LessThanComparable) -> None:
+    """Test if the interval has an invalid range."""
+    with pytest.raises(
+        ValueError,
+        match=rf"The start \({start}\) can't be bigger than end \({end}\)",
+    ):
+        Interval(start, end)
+
+
 @pytest.mark.parametrize(
     "start, end, within, at_start, at_end, before_start, after_end",
     [