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Issue #3166: Make long -> float (and int -> float) conversions

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correctly rounded, using round-half-to-even.  This ensures that the
value of float(n) doesn't depend on whether we're using 15-bit digits
or 30-bit digits for Python longs.
This commit is contained in:
Mark Dickinson 2009-04-20 21:13:33 +00:00
parent cccfc825e4
commit 6736cf8d20
5 changed files with 318 additions and 29 deletions
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34
Lib/test/test_int.py
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@ -275,6 +275,40 @@ class IntTestCases(unittest.TestCase):
self.assertEqual((a+1).bit_length(), i+1)
self.assertEqual((-a-1).bit_length(), i+1)
@unittest.skipUnless(float.__getformat__("double").startswith("IEEE"),
"test requires IEEE 754 doubles")
def test_float_conversion(self):
# values exactly representable as floats
exact_values = [-2, -1, 0, 1, 2, 2**52, 2**53-1, 2**53, 2**53+2,
2**53+4, 2**54-4, 2**54-2, 2**63, -2**63, 2**64,
-2**64, 10**20, 10**21, 10**22]
for value in exact_values:
self.assertEqual(int(float(int(value))), value)
# test round-half-to-even
self.assertEqual(int(float(2**53+1)), 2**53)
self.assertEqual(int(float(2**53+2)), 2**53+2)
self.assertEqual(int(float(2**53+3)), 2**53+4)
self.assertEqual(int(float(2**53+5)), 2**53+4)
self.assertEqual(int(float(2**53+6)), 2**53+6)
self.assertEqual(int(float(2**53+7)), 2**53+8)
self.assertEqual(int(float(-2**53-1)), -2**53)
self.assertEqual(int(float(-2**53-2)), -2**53-2)
self.assertEqual(int(float(-2**53-3)), -2**53-4)
self.assertEqual(int(float(-2**53-5)), -2**53-4)
self.assertEqual(int(float(-2**53-6)), -2**53-6)
self.assertEqual(int(float(-2**53-7)), -2**53-8)
self.assertEqual(int(float(2**54-2)), 2**54-2)
self.assertEqual(int(float(2**54-1)), 2**54)
self.assertEqual(int(float(2**54+2)), 2**54)
self.assertEqual(int(float(2**54+3)), 2**54+4)
self.assertEqual(int(float(2**54+5)), 2**54+4)
self.assertEqual(int(float(2**54+6)), 2**54+8)
self.assertEqual(int(float(2**54+10)), 2**54+8)
self.assertEqual(int(float(2**54+11)), 2**54+12)
def test_intconversion(self):
# Test __int__()
class ClassicMissingMethods: