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bpo-40780: Fix failure of _Py_dg_dtoa to remove trailing zeros (GH-20435) 

* Fix failure of _Py_dg_dtoa to remove trailing zeros

* Add regression test and news entry

* Add explanation about why it's safe to strip trailing zeros

* Make code safer, clean up comments, add change note at top of file

* Nitpick: avoid implicit int-to-float conversion in tests
This commit is contained in:
Mark Dickinson 2020-05-29 14:23:57 +01:00 committed by GitHub
parent 2831642212
commit 895c9c1d43
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3 changed files with 24 additions and 0 deletions
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11
Lib/test/test_format.py
View File

@ -484,6 +484,17 @@ class FormatTest(unittest.TestCase):
with self.assertRaises(ValueError) as cm:
format(c, ".%sf" % (INT_MAX + 1))
def test_g_format_has_no_trailing_zeros(self):
# regression test for bugs.python.org/issue40780
self.assertEqual("%.3g" % 1505.0, "1.5e+03")
self.assertEqual("%#.3g" % 1505.0, "1.50e+03")
self.assertEqual(format(1505.0, ".3g"), "1.5e+03")
self.assertEqual(format(1505.0, "#.3g"), "1.50e+03")
self.assertEqual(format(12300050.0, ".6g"), "1.23e+07")
self.assertEqual(format(12300050.0, "#.6g"), "1.23000e+07")
if __name__ == "__main__":
unittest.main()

2
Misc/NEWS.d/next/Core and Builtins/2020-05-26-17-43-58.bpo-40780.3Ckdgm.rst Normal file
View File

@ -0,0 +1,2 @@
Fix a corner case where g-style string formatting of a float failed to
remove trailing zeros.

11
Python/dtoa.c
View File

@ -64,6 +64,9 @@
* 7. _Py_dg_strtod has been modified so that it doesn't accept strings with
* leading whitespace.
*
* 8. A corner case where _Py_dg_dtoa didn't strip trailing zeros has been
* fixed. (bugs.python.org/issue40780)
*
***************************************************************/
/* Please send bug reports for the original dtoa.c code to David M. Gay (dmg
@ -2563,6 +2566,14 @@ _Py_dg_dtoa(double dd, int mode, int ndigits,
}
++*s++;
}
else {
/* Strip trailing zeros. This branch was missing from the
original dtoa.c, leading to surplus trailing zeros in
some cases. See bugs.python.org/issue40780. */
while (s > s0 && s[-1] == '0') {
--s;
}
}
break;
}
}