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Remove support for nth root of negative numbers with odd powers.

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Although nth roots of negative numbers are real for odd n, the
statistics module doesn't make use of this. Remove support for
negative roots from the private _nth_root function, which
simplifies the test suite.
This commit is contained in:
Steven D'Aprano 2016-08-24 12:48:12 +10:00
parent e5803d9d2f
commit d6ea3011c5
2 changed files with 8 additions and 28 deletions
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7
Lib/statistics.py
View file

@ -335,10 +335,7 @@ def float_nroot(x, n):
"""Handle nth root of Reals, treated as a float."""
assert isinstance(n, int) and n > 1
if x < 0:
if n%2 == 0:
raise ValueError('domain error: even root of negative number')
else:
return -_nroot_NS.nroot(-x, n)
raise ValueError('domain error: root of negative number')
elif x == 0:
return math.copysign(0.0, x)
elif x > 0:
@ -433,6 +430,8 @@ def decimal_nroot(x, n):
else:
# Preserve the input NAN.
return x
if x < 0:
raise ValueError('domain error: root of negative number')
if x.is_infinite():
return x
# FIXME this hasn't had the extensive testing of the float

29
Lib/test/test_statistics.py
View file

@ -1095,13 +1095,6 @@ def test_inf(self):
with self.subTest(n=n, inf=INF):
self.assertEqual(self.nroot(INF, n), INF)
def testNInf(self):
# Test that the root of -inf is -inf for odd n.
for NINF in (float('-inf'), decimal.Decimal('-inf')):
for n in range(3, 11, 2):
with self.subTest(n=n, inf=NINF):
self.assertEqual(self.nroot(NINF, n), NINF)
# FIXME: need to check Decimal zeroes too.
def test_zero(self):
# Test that the root of +0.0 is +0.0.
@ -1157,13 +1150,15 @@ def testBadTypes(self):
with self.subTest(x=x):
self.assertRaises(TypeError, self.nroot, x, 3)
def testNegativeEvenPower(self):
# Test negative x with even n raises correctly.
def testNegativeError(self):
# Test negative x raises correctly.
x = random.uniform(-20.0, -0.1)
assert x < 0
for n in range(2, 9, 2):
for n in range(3, 7):
with self.subTest(x=x, n=n):
self.assertRaises(ValueError, self.nroot, x, n)
# And Decimal.
self.assertRaises(ValueError, self.nroot, Decimal(-27), 3)
# --- Test that nroot is never worse than calling math.pow() ---
@ -1216,25 +1211,11 @@ def testExactPowers(self):
x = i**n
self.assertEqual(self.nroot(x, n), i)
def testExactPowersNegatives(self):
# Test that small negative integer powers are calculated exactly.
for i in range(-1, -51, -1):
for n in range(3, 16, 2):
if (i, n) == (-35, 13):
# See testExpectedFailure35p13
continue
with self.subTest(i=i, n=n):
x = i**n
assert sign(x) == -1
self.assertEqual(self.nroot(x, n), i)
def testExpectedFailure35p13(self):
# Test the expected failure 35**13 is almost exact.
x = 35**13
err = abs(self.nroot(x, 13) - 35)
self.assertLessEqual(err, 0.000000001)
err = abs(self.nroot(-x, 13) + 35)
self.assertLessEqual(err, 0.000000001)
def testOne(self):
# Test that the root of 1.0 is 1.0.