Fix issue 3411: default float format spec fails on negative numbers.

Lib/test/test_types.py

@@ -570,6 +570,12 @@ class TypesTests(unittest.TestCase):
         test(0.01, '', '0.01')
         test(0.01, 'g', '0.01')
 
+        # test for issue 3411
+        test(1.23, '1', '1.23')
+        test(-1.23, '1', '-1.23')
+        test(1.23, '1g', '1.23')
+        test(-1.23, '1g', '-1.23')
+
         test( 1.0, ' g', ' 1')
         test(-1.0, ' g', '-1')
         test( 1.0, '+g', '+1')

Python/pystrtod.c

@@ -302,6 +302,10 @@ ensure_decimal_point(char* buffer, size_t buf_size)
 
 	/* search for the first non-digit character */
 	char *p = buffer;
+	if (*p == '-' || *p == '+')
+		/* Skip leading sign, if present.  I think this could only
+		   ever be '-', but it can't hurt to check for both. */
+		++p;
 	while (*p && isdigit(Py_CHARMASK(*p)))
 		++p;