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bpo-44954: Fix wrong result in float.fromhex corner case #27834

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Merged
mdickinson merged 5 commits into from
Aug 20, 2021
Merged

bpo-44954: Fix wrong result in float.fromhex corner case #27834

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617180e
bpo-44954: Fix wrong result in float.fromhex corner case
mdickinson Aug 19, 2021
dfb3fef
A few more test variants
mdickinson Aug 20, 2021
45cd7dd
Fix spacing consistency
mdickinson Aug 20, 2021
2fed72c
Fix a bad test
mdickinson Aug 20, 2021
e6e6a24
Merge remote-tracking branch 'mdickinson/fix-44954' into fix-44954
mdickinson Aug 20, 2021
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14 changes: 14 additions & 0 deletions Lib/test/test_float.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1468,6 +1468,20 @@ def test_from_hex(self):
self.identical(fromHex('0X1.0000000000001fp0'), 1.0+2*EPS)
self.identical(fromHex('0x1.00000000000020p0'), 1.0+2*EPS)

# Regression test for a corner-case bug reported in b.p.o. 44954
self.identical(fromHex('0x.8p-1074'), 0.0)
self.identical(fromHex('0x.80p-1074'), 0.0)
self.identical(fromHex('0x.81p-1074'), TINY)
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@serhiy-storchaka serhiy-storchaka Aug 20, 2021

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I tried to find what is the minimal example for non-zero result, but seems it works for arbitrary number of zeroes between 8 and 1: 0x.800000000000000000000...0000000000000000001p-1074.

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@mdickinson mdickinson Aug 20, 2021

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Thanks, that's reassuring, and as it should be: 0x.8p-1074 is the exact threshold for rounding, so if it's larger than 0x.8p-1074 by even the tiniest amount, it should round up.

self.identical(fromHex('0x8p-1078'), 0.0)
self.identical(fromHex('0x8.0p-1078'), 0.0)
self.identical(fromHex('0x8.1p-1078'), TINY)
self.identical(fromHex('0x80p-1082'), 0.0)
self.identical(fromHex('0x81p-1082'), TINY)
self.identical(fromHex('.8p-1074'), 0.0)
self.identical(fromHex('8p-1078'), 0.0)
self.identical(fromHex('-.8p-1074'), -0.0)
self.identical(fromHex('+8p-1078'), 0.0)

def test_roundtrip(self):
def roundtrip(x):
return fromHex(toHex(x))
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core and Builtins/2021-08-19-14-43-24.bpo-44954.dLn3lg.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fixed a corner case bug where the result of ``float.fromhex('0x.8p-1074')``
was rounded the wrong way.
4 changes: 2 additions & 2 deletions Objects/floatobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1463,8 +1463,8 @@ float_fromhex(PyTypeObject *type, PyObject *string)
bits lsb, lsb-2, lsb-3, lsb-4, ... is 1. */
if ((digit & half_eps) != 0) {
round_up = 0;
if ((digit & (3*half_eps-1)) != 0 ||
(half_eps == 8 && (HEX_DIGIT(key_digit+1) & 1) != 0))
if ((digit & (3*half_eps-1)) != 0 || (half_eps == 8 &&
key_digit+1 < ndigits && (HEX_DIGIT(key_digit+1) & 1) != 0))
round_up = 1;
else
for (i = key_digit-1; i >= 0; i--)
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