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bpo-36027: Extend three-argument pow to negative second argument (GH-13266)

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Mark Dickinson 2019-06-02 10:24:06 +01:00 • committed by GitHub
parent 5ae299ac78
commit c52996785a
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6 changed files with 173 additions and 16 deletions
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26
Lib/test/test_pow.py
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@ -1,3 +1,4 @@
import math
import unittest
class PowTest(unittest.TestCase):
@ -119,5 +120,30 @@ class PowTest(unittest.TestCase):
eq(pow(a, -fiveto), expected)
eq(expected, 1.0) # else we didn't push fiveto to evenness
def test_negative_exponent(self):
for a in range(-50, 50):
for m in range(-50, 50):
with self.subTest(a=a, m=m):
if m != 0 and math.gcd(a, m) == 1:
# Exponent -1 should give an inverse, with the
# same sign as m.
inv = pow(a, -1, m)
self.assertEqual(inv, inv % m)
self.assertEqual((inv * a - 1) % m, 0)
# Larger exponents
self.assertEqual(pow(a, -2, m), pow(inv, 2, m))
self.assertEqual(pow(a, -3, m), pow(inv, 3, m))
self.assertEqual(pow(a, -1001, m), pow(inv, 1001, m))
else:
with self.assertRaises(ValueError):
pow(a, -1, m)
with self.assertRaises(ValueError):
pow(a, -2, m)
with self.assertRaises(ValueError):
pow(a, -1001, m)
if __name__ == "__main__":
unittest.main()