Fractions.from_float() no longer loses precision with large integer inputs.

Lib/fractions.py

@@ -111,7 +111,7 @@ class Fraction(Rational):
 
         """
         if isinstance(f, numbers.Integral):
-            f = float(f)
+            return cls(f)
         elif not isinstance(f, float):
             raise TypeError("%s.from_float() only takes floats, not %r (%s)" %
                             (cls.__name__, f, type(f).__name__))

Lib/test/test_fractions.py

@@ -139,6 +139,8 @@ class FractionTest(unittest.TestCase):
     def testFromFloat(self):
         self.assertRaises(TypeError, F.from_float, 3+4j)
         self.assertEquals((10, 1), _components(F.from_float(10)))
+        bigint = 1234567890123456789
+        self.assertEquals((bigint, 1), _components(F.from_float(bigint)))
         self.assertEquals((0, 1), _components(F.from_float(-0.0)))
         self.assertEquals((10, 1), _components(F.from_float(10.0)))
         self.assertEquals((-5, 2), _components(F.from_float(-2.5)))

Misc/NEWS

@@ -56,6 +56,9 @@ Core and Builtins
 Library
 -------
 
+- Fractions.from_float() no longer loses precision for integers to big to
+  cast as floats.
+
 - Issue 4790: The nsmallest() and nlargest() functions in the heapq module
   did unnecessary work in the common case where no key function was specified.