bpo-32968: Make modulo and floor division involving Fraction and float consistent with other operations (#5956)

Make mixed-type `%` and `//` operations involving `Fraction` and `float` objects behave like all other mixed-type arithmetic operations: first the `Fraction` object is converted to a `float`, then the `float` operation is performed as normal. This fixes some surprising corner cases, like `Fraction('1/3') % inf` giving a NaN.

Thanks Elias Zamaria for the patch.

Lib/fractions.py

@@ -427,23 +427,18 @@ class Fraction(numbers.Rational):
 
     __truediv__, __rtruediv__ = _operator_fallbacks(_div, operator.truediv)
 
-    def __floordiv__(a, b):
+    def _floordiv(a, b):
         """a // b"""
         return math.floor(a / b)
 
-    def __rfloordiv__(b, a):
-        """a // b"""
-        return math.floor(a / b)
+    __floordiv__, __rfloordiv__ = _operator_fallbacks(_floordiv, operator.floordiv)
 
-    def __mod__(a, b):
+    def _mod(a, b):
         """a % b"""
         div = a // b
         return a - b * div
 
-    def __rmod__(b, a):
-        """a % b"""
-        div = a // b
-        return a - b * div
+    __mod__, __rmod__ = _operator_fallbacks(_mod, operator.mod)
 
     def __pow__(a, b):
         """a ** b

Lib/test/test_fractions.py

@@ -401,15 +401,19 @@ class FractionTest(unittest.TestCase):
         self.assertTypedEquals(10.0 + 0j, (1.0 + 0j) / F(1, 10))
 
         self.assertTypedEquals(0, F(1, 10) // 1)
-        self.assertTypedEquals(0, F(1, 10) // 1.0)
+        self.assertTypedEquals(0.0, F(1, 10) // 1.0)
         self.assertTypedEquals(10, 1 // F(1, 10))
         self.assertTypedEquals(10**23, 10**22 // F(1, 10))
-        self.assertTypedEquals(10, 1.0 // F(1, 10))
+        self.assertTypedEquals(1.0 // 0.1, 1.0 // F(1, 10))
 
         self.assertTypedEquals(F(1, 10), F(1, 10) % 1)
         self.assertTypedEquals(0.1, F(1, 10) % 1.0)
         self.assertTypedEquals(F(0, 1), 1 % F(1, 10))
-        self.assertTypedEquals(0.0, 1.0 % F(1, 10))
+        self.assertTypedEquals(1.0 % 0.1, 1.0 % F(1, 10))
+        self.assertTypedEquals(0.1, F(1, 10) % float('inf'))
+        self.assertTypedEquals(float('-inf'), F(1, 10) % float('-inf'))
+        self.assertTypedEquals(float('inf'), F(-1, 10) % float('inf'))
+        self.assertTypedEquals(-0.1, F(-1, 10) % float('-inf'))
 
         # No need for divmod since we don't override it.
 

Misc/ACKS

@@ -1814,6 +1814,7 @@ Masazumi Yoshikawa
 Arnaud Ysmal
 Bernard Yue
 Moshe Zadka
+Elias Zamaria
 Milan Zamazal
 Artur Zaprzala
 Mike Zarnstorff

Misc/NEWS.d/next/Library/2018-03-18-15-57-32.bpo-32968.E4G7BO.rst

@@ -0,0 +1 @@
+Modulo and floor division involving Fraction and float should return float.