Issue #5812: The two-argument form of the Fraction constructor

now accepts arbitrary Rational instances.

Lib/fractions.py

@@ -56,7 +56,7 @@ class Fraction(Rational):
     __slots__ = ('_numerator', '_denominator')
 
     # We're immutable, so use __new__ not __init__
-    def __new__(cls, numerator=0, denominator=1):
+    def __new__(cls, numerator=0, denominator=None):
         """Constructs a Fraction.
 
         Takes a string like '3/2' or '1.5', another Fraction, or a
@@ -65,8 +65,13 @@ class Fraction(Rational):
         """
         self = super(Fraction, cls).__new__(cls)
 
-        if type(numerator) not in (int, long) and denominator == 1:
-            if isinstance(numerator, basestring):
+        if denominator is None:
+            if isinstance(numerator, Rational):
+                self._numerator = numerator.numerator
+                self._denominator = numerator.denominator
+                return self
+
+            elif isinstance(numerator, basestring):
                 # Handle construction from strings.
                 m = _RATIONAL_FORMAT.match(numerator)
                 if m is None:
@@ -93,18 +98,22 @@ class Fraction(Rational):
                 if m.group('sign') == '-':
                     numerator = -numerator
 
-            elif isinstance(numerator, Rational):
-                # Handle copies from other rationals. Integrals get
-                # caught here too, but it doesn't matter because
-                # denominator is already 1.
-                other_rational = numerator
-                numerator = other_rational.numerator
-                denominator = other_rational.denominator
+            else:
+                raise TypeError("argument should be a string "
+                                "or a Rational instance")
+
+        elif (isinstance(numerator, Rational) and
+            isinstance(denominator, Rational)):
+            numerator, denominator = (
+                numerator.numerator * denominator.denominator,
+                denominator.numerator * numerator.denominator
+                )
+        else:
+            raise TypeError("both arguments should be "
+                            "Rational instances")
 
         if denominator == 0:
             raise ZeroDivisionError('Fraction(%s, 0)' % numerator)
-        numerator = operator.index(numerator)
-        denominator = operator.index(denominator)
         g = gcd(numerator, denominator)
         self._numerator = numerator // g
         self._denominator = denominator // g

Lib/test/test_fractions.py

@@ -60,13 +60,19 @@ class FractionTest(unittest.TestCase):
         self.assertEquals((7, 15), _components(F(7, 15)))
         self.assertEquals((10**23, 1), _components(F(10**23)))
 
+        self.assertEquals((3, 77), _components(F(F(3, 7), 11)))
+        self.assertEquals((-9, 5), _components(F(2, F(-10, 9))))
+        self.assertEquals((2486, 2485), _components(F(F(22, 7), F(355, 113))))
+
         self.assertRaisesMessage(ZeroDivisionError, "Fraction(12, 0)",
                                  F, 12, 0)
         self.assertRaises(TypeError, F, 1.5)
         self.assertRaises(TypeError, F, 1.5 + 3j)
 
-        self.assertRaises(TypeError, F, F(1, 2), 3)
         self.assertRaises(TypeError, F, "3/2", 3)
+        self.assertRaises(TypeError, F, 3, 0j)
+        self.assertRaises(TypeError, F, 3, 1j)
+
 
     def testFromString(self):
         self.assertEquals((5, 1), _components(F("5")))

Misc/NEWS

@@ -244,6 +244,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #5812: For the two-argument form of the Fraction constructor,
+  Fraction(m, n), m and n are permitted to be arbitrary Rational
+  instances.
+
 - Issue #5812: Fraction('1e6') is valid: more generally, any string
   that's valid for float() is now valid for Fraction(), with the
   exception of strings representing NaNs and infinities.