Fix RationalFunction subtraction operator dispatch

blueprint/src/python/functions.py

@@ -785,7 +785,7 @@ def __add__(self, other):
         return self.add(other)
 
     def __sub__(self, other):
-        return self.sub(other)
+        return self.subtract(other)
 
     def __mul__(self, other):
         return self.mul(other)

blueprint/src/python/tests/test_all.py

@@ -36,6 +36,7 @@
 
 import test_rationalfunction
 import test_rational_roots_regression
+import test_rational_subtraction
 
 print("All RationalFunction test cases passed.")
 

blueprint/src/python/tests/test_rational_subtraction.py

@@ -0,0 +1,17 @@
+from fractions import Fraction
+
+from functions import RationalFunction
+
+
+def test_subtraction_operator():
+    f = RationalFunction([1, 1], [1, 2])
+    g = RationalFunction([2, 1], [1, 3])
+    for other in (g, 2, Fraction(1, 3)):
+        result = f - other
+        for x in (Fraction(0), Fraction(1), Fraction(3, 2)):
+            expected = f.at(x) - (other.at(x) if isinstance(other, RationalFunction) else other)
+            assert result.at(x) == expected
+    assert (f - f).at(Fraction(1)) == 0
+
+
+test_subtraction_operator()