bpo-39350: Fix fractions for int subclasses (GH-18375)

Fix regression in fractions.Fraction if the numerator and/or the
denominator is an int subclass. The math.gcd() function is now
used to normalize the numerator and denominator. math.gcd() always
return a int type. Previously, the GCD type depended on numerator
and denominator.

2 files changed, 25 insertions, 7 deletions

diff --git a/Lib/fractions.py b/Lib/fractions.py
index f5a8544..de3e23b 100644
--- a/Lib/fractions.py
+++ b/Lib/fractions.py
@@ -155,13 +155,9 @@ class Fraction(numbers.Rational):
         if denominator == 0:
             raise ZeroDivisionError('Fraction(%s, 0)' % numerator)
         if _normalize:
-            if type(numerator) is int is type(denominator):
-                # *very* normal case
-                g = math.gcd(numerator, denominator)
-                if denominator < 0:
-                    g = -g
-            else:
-                g = _gcd(numerator, denominator)
+            g = math.gcd(numerator, denominator)
+            if denominator < 0:
+                g = -g
             numerator //= g
             denominator //= g
         self._numerator = numerator
diff --git a/Lib/test/test_fractions.py b/Lib/test/test_fractions.py
index 4649a34..c748533 100644
--- a/Lib/test/test_fractions.py
+++ b/Lib/test/test_fractions.py
@@ -703,6 +703,28 @@ class FractionTest(unittest.TestCase):
         r = F(13, 7)
         self.assertRaises(AttributeError, setattr, r, 'a', 10)
 
+    def test_int_subclass(self):
+        class myint(int):
+            def __mul__(self, other):
+                return type(self)(int(self) * int(other))
+            def __floordiv__(self, other):
+                return type(self)(int(self) // int(other))
+            def __mod__(self, other):
+                x = type(self)(int(self) % int(other))
+                return x
+            @property
+            def numerator(self):
+                return type(self)(int(self))
+            @property
+            def denominator(self):
+                return type(self)(1)
+
+        f = fractions.Fraction(myint(1 * 3), myint(2 * 3))
+        self.assertEqual(f.numerator, 1)
+        self.assertEqual(f.denominator, 2)
+        self.assertEqual(type(f.numerator), myint)
+        self.assertEqual(type(f.denominator), myint)
+
 
 if __name__ == '__main__':
     unittest.main()