bpo-40780: Fix failure of _Py_dg_dtoa to remove trailing zeros

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, "1.5e+03")
+        self.assertEqual("%#.3g" % 1505, "1.50e+03")
+
+        self.assertEqual(format(1505, ".3g"), "1.5e+03")
+        self.assertEqual(format(1505, "#.3g"), "1.50e+03")
+
+        self.assertEqual(format(12300050, ".6g"), "1.23e+07")
+        self.assertEqual(format(12300050, "#.6g"), "1.23000e+07")
+
 
 if __name__ == "__main__":
     unittest.main()

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.

Python/dtoa.c

@@ -2563,6 +2563,18 @@ _Py_dg_dtoa(double dd, int mode, int ndigits,
                         }
                     ++*s++;
                 }
+                /* This branch was missing from the original dtoa.c, leading
+                   to surplus trailing zeros in some cases.
+                   See bugs.python.org/issue40780. */
+                else {
+                    /* At the beginning of the for loop we have 10**k <= d; it
+                       follows that on the first iteration, ds <= dval(&u), and
+                       so the first digit written to s is nonzero and it's safe
+                       to strip trailing zeros. */
+                    assert(k_check == 0);
+                    while(*--s == '0');
+                    s++;
+                }
                 break;
             }
         }