bpo-40780: Fix failure of _Py_dg_dtoa to remove trailing zeros (GH-20435) (GH-20514)

miss-islington · mdickinson · web-flow · commit ad088ca5c6ad · 2020-05-29T14:46:54.000+01:00
* 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 (cherry picked from commit 895c9c1) Co-authored-by: Mark Dickinson <mdickinson@enthought.com>
diff --git a/Lib/test/test_format.py b/Lib/test/test_format.py
@@ -484,6 +484,17 @@ def test_precision_c_limits(self):
         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()
diff --git a/Misc/NEWS.d/next/Core and Builtins/2020-05-26-17-43-58.bpo-40780.3Ckdgm.rst b/Misc/NEWS.d/next/Core and Builtins/2020-05-26-17-43-58.bpo-40780.3Ckdgm.rst
@@ -0,0 +1,2 @@
+Fix a corner case where g-style string formatting of a float failed to
+remove trailing zeros.
diff --git a/Python/dtoa.c b/Python/dtoa.c
@@ -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;
             }
         }

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	`+Fix a corner case where g-style string formatting of a float failed to`
	`2`	`+remove trailing zeros.`
Original file line number	Diff line number	Diff line change
`@@ -64,6 +64,9 @@`
`64`	`64`	`* 7. _Py_dg_strtod has been modified so that it doesn't accept strings with`
`65`	`65`	`* leading whitespace.`
`66`	`66`	`*`
	`67`	`+ * 8. A corner case where _Py_dg_dtoa didn't strip trailing zeros has been`
	`68`	`+ * fixed. (bugs.python.org/issue40780)`
	`69`	`+ *`
`67`	`70`	`***************************************************************/`
`68`	`71`
`69`	`72`	`/* 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,`
`2563`	`2566`	`}`
`2564`	`2567`	`++*s++;`
`2565`	`2568`	`}`
	`2569`	`+ else {`
	`2570`	`+ /* Strip trailing zeros. This branch was missing from the`
	`2571`	`+ original dtoa.c, leading to surplus trailing zeros in`
	`2572`	`+ some cases. See bugs.python.org/issue40780. */`
	`2573`	`+ while (s > s0 && s[-1] == '0') {`
	`2574`	`+ --s;`
	`2575`	`+ }`
	`2576`	`+ }`
`2566`	`2577`	`break;`
`2567`	`2578`	`}`
`2568`	`2579`	`}`