From: Mark Dickinson <dickinsm@gmail.com>
Date: Mon, 22 Aug 2016 09:50:53 +0000 (+0100)
Subject: Issue #27539: Fix unnormalised Fraction.__pow__ result for negative exponent and... 
X-Git-Tag: v3.6.0b1~608^2
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=844796530a21f2a8689f2b9e01035d4a64a95275;p=thirdparty%2FPython%2Fcpython.git

Issue #27539: Fix unnormalised Fraction.__pow__ result for negative exponent and base. Thanks Vedran Čačić.
---

diff --git a/Lib/fractions.py b/Lib/fractions.py
index 60b072880703..9aabab3e3ba9 100644
--- a/Lib/fractions.py
+++ b/Lib/fractions.py
@@ -484,10 +484,14 @@ class Fraction(numbers.Rational):
                     return Fraction(a._numerator ** power,
                                     a._denominator ** power,
                                     _normalize=False)
-                else:
+                elif a._numerator >= 0:
                     return Fraction(a._denominator ** -power,
                                     a._numerator ** -power,
                                     _normalize=False)
+                else:
+                    return Fraction((-a._denominator) ** -power,
+                                    (-a._numerator) ** -power,
+                                    _normalize=False)
             else:
                 # A fractional power will generally produce an
                 # irrational number.
diff --git a/Lib/test/test_fractions.py b/Lib/test/test_fractions.py
index 16998522160b..9df4a54fcbde 100644
--- a/Lib/test/test_fractions.py
+++ b/Lib/test/test_fractions.py
@@ -356,6 +356,19 @@ class FractionTest(unittest.TestCase):
         z = pow(F(-1), F(1, 2))
         self.assertAlmostEqual(z.real, 0)
         self.assertEqual(z.imag, 1)
+        # Regression test for #27539.
+        p = F(-1, 2) ** 0
+        self.assertEqual(p, F(1, 1))
+        self.assertEqual(p.numerator, 1)
+        self.assertEqual(p.denominator, 1)
+        p = F(-1, 2) ** -1
+        self.assertEqual(p, F(-2, 1))
+        self.assertEqual(p.numerator, -2)
+        self.assertEqual(p.denominator, 1)
+        p = F(-1, 2) ** -2
+        self.assertEqual(p, F(4, 1))
+        self.assertEqual(p.numerator, 4)
+        self.assertEqual(p.denominator, 1)
 
     def testMixedArithmetic(self):
         self.assertTypedEquals(F(11, 10), F(1, 10) + 1)
diff --git a/Misc/ACKS b/Misc/ACKS
index 51c8d101d02d..1c9363aab671 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -217,6 +217,7 @@ Katherine Busch
 Ralph Butler
 Laurent De Buyst
 Zach Byrne
+Vedran Čačić
 Nicolas Cadou
 Jp Calderone
 Arnaud Calmettes
diff --git a/Misc/NEWS b/Misc/NEWS
index 54cae1da9fb5..7530624751b6 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -46,6 +46,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #27539: Fix unnormalised ``Fraction.__pow__`` result in the case
+  of negative exponent and negative base.
+
 - Issue #21718: cursor.description is now available for queries using CTEs.
 
 - Issue #2466: posixpath.ismount now correctly recognizes mount points which