Interval measure property: length of a numeric interval

portion/interval.py

@@ -239,6 +239,23 @@ def enclosure(self):
         """
         return Interval.from_atomic(self.left, self.lower, self.upper, self.right)
 
+    @property
+    def measure(self):
+        """
+        Return a measure (~ length) of the interval.
+
+        Only valid for intervals with numeric bounds.
+        """
+        if self.empty:
+            return 0
+        if self.upper != inf and type(self.upper) not in (int, float):
+            raise TypeError("Interval bounds must be numeric")
+        if self.lower != -inf and type(self.lower) not in (int, float):
+            raise TypeError("Interval bounds must be numeric")
+        if self.upper == inf or self.lower == -inf:
+            return float("inf")
+        return sum([i.upper - i.lower for i in self._intervals])
+
     def replace(
         self, left=None, lower=None, upper=None, right=None, *, ignore_inf=True
     ):

tests/test_interval.py

@@ -139,6 +139,31 @@ def test_enclosure(self):
         assert P.closed(0, 4) == (P.closed(0, 1) | P.closed(3, 4)).enclosure
         assert P.openclosed(0, 4) == (P.open(0, 1) | P.closed(3, 4)).enclosure
 
+    def test_measure(self):
+        assert P.empty().measure == 0
+        assert P.singleton(1).measure == 0
+        assert (P.singleton(-1) | (P.singleton(0) | P.singleton(1))).measure == 0
+
+        assert P.open(0, 1).measure == 1
+        assert P.openclosed(0, 1).measure == 1
+        assert P.closedopen(0, 1).measure == 1
+        assert P.closed(0, 1).measure == 1
+
+        assert (P.open(0, 1) | P.open(3, 5)).measure == 3
+
+        assert (P.open(0, P.inf)).measure == float('inf')
+        assert (P.open(-P.inf, 0)).measure == float('inf')
+        assert (P.open(-P.inf, P.inf)).measure == float('inf')
+
+        with pytest.raises(TypeError):
+            P.open('a', 'b').measure
+
+        with pytest.raises(TypeError):
+            P.open('a', P.inf).measure
+
+        with pytest.raises(TypeError):
+            P.open(-P.inf, 'b').measure
+
 
 class TestIntervalReplace:
     def test_replace_bounds(self):