Index: Python/pystrtod.c
===================================================================
--- Python/pystrtod.c	(revision 71669)
+++ Python/pystrtod.c	(working copy)
@@ -384,6 +384,50 @@
 	}
 }
 
+/* Remove trailing zeros after the decimal point from a numeric string; also
+   remove the decimal point if all digits following it are zero.  The numeric
+   string must end in '\0', and should not have any leading or trailing
+   whitespace.  Assumes that the decimal point is '.'. */
+Py_LOCAL_INLINE(void)
+remove_trailing_zeros(char *buffer)
+{
+	char *old_fraction_end, *new_fraction_end, *end, *p;
+
+	p = buffer;
+	if (*p == '-' || *p == '+')
+		/* Skip leading sign, if present */
+		++p;
+	while (ISDIGIT(Py_CHARMASK(*p)))
+		++p;
+
+	/* if there's no decimal point there's nothing to do */
+	if (*p++ != '.')
+		return;
+
+	/* scan any digits after the point */
+	while (ISDIGIT(Py_CHARMASK(*p)))
+		++p;
+	old_fraction_end = p;
+
+	/* scan up to ending '\0' */
+	while (*p != '\0')
+		p++;
+	/* +1 to make sure that we move the null byte as well */
+	end = p+1;
+
+	/* scan back from fraction_end, looking for removable zeros */
+	p = old_fraction_end;
+	while (*(p-1) == '0')
+		--p;
+	/* and remove point if we've got that far */
+	if (*(p-1) == '.')
+		--p;
+	new_fraction_end = p;
+
+	memmove(new_fraction_end, old_fraction_end, end-old_fraction_end);
+}
+
+
 /* see FORMATBUFLEN in unicodeobject.c */
 #define FLOAT_FORMATBUFLEN 120
 
@@ -400,8 +444,8 @@
  * a printf()-style format string. Allowed conversion
  * specifiers are 'e', 'E', 'f', 'F', 'g', 'G', and 'Z'.
  * 
- * 'Z' is the same as 'g', except it always has a decimal and
- *     at least one digit after the decimal.
+ * 'Z' is the same as 'g', except it always has a decimal point and
+ *     at least one digit after the decimal point.
  *
  * Return value: The pointer to the buffer with the converted string.
  **/
@@ -498,6 +542,7 @@
 	char *p;
 	int t;
 	int upper = 0;
+	int strip_trailing_zeros = 0;
 
 	/* Validate format_code, and map upper and lower case */
 	switch (format_code) {
@@ -523,8 +568,16 @@
 			PyErr_BadInternalCall();
 			return NULL;
 		}
-		precision = 17;
-		format_code = 'g';
+		/* switch to exponential notation at 1e16 */
+		if (fabs(val) >= 1e16) {
+			precision = 16;
+			format_code = 'e';
+			strip_trailing_zeros = 1;
+		}
+		else {
+			precision = 17;
+			format_code = 'g';
+		}
 		break;
 	case 's':          /* str format */
 		/* Supplied precision is unused, must be 0. */
@@ -532,8 +585,17 @@
 			PyErr_BadInternalCall();
 			return NULL;
 		}
-		precision = 12;
-		format_code = 'g';
+		/* switch to exponential notation at 1e11, or 1e12 if we're
+		   not adding a .0 */
+		if (fabs(val) >= (flags & Py_DTSF_ADD_DOT_0 ? 1e11 : 1e12)) {
+			precision = 11;
+			format_code = 'e';
+			strip_trailing_zeros = 1;
+		}
+		else {
+			precision = 12;
+			format_code = 'g';
+		}
 		break;
 	default:
 		PyErr_BadInternalCall();
@@ -552,13 +614,14 @@
 		t = Py_DTST_INFINITE;
 	} else {
 		t = Py_DTST_FINITE;
-
-
-		if (flags & Py_DTSF_ADD_DOT_0)
+		if ((flags & Py_DTSF_ADD_DOT_0) && (format_code != 'e'))
 			format_code = 'Z';
 
 		PyOS_snprintf(format, 32, "%%%s.%i%c", (flags & Py_DTSF_ALT ? "#" : ""), precision, format_code);
 		PyOS_ascii_formatd(buf, sizeof(buf), format, val);
+		/* remove trailing zeros if necessary */
+		if (strip_trailing_zeros)
+			remove_trailing_zeros(buf);
 	}
 
 	len = strlen(buf);