diff -r 4b64a049f451 Python/formatter_unicode.c
--- a/Python/formatter_unicode.c	Fri Aug 19 12:00:13 2016 +0300
+++ b/Python/formatter_unicode.c	Fri Aug 19 23:33:16 2016 +1000
@@ -32,11 +32,11 @@
 {
     if (presentation_type > 32 && presentation_type < 128)
         PyErr_Format(PyExc_ValueError,
-                     "Cannot specify ',' with '%c'.",
+                     "Cannot specify ',' or '_' with '%c'.",
                      (char)presentation_type);
     else
         PyErr_Format(PyExc_ValueError,
-                     "Cannot specify ',' with '\\x%x'.",
+                     "Cannot specify ',' or '_' with '\\x%x'.",
                      (unsigned int)presentation_type);
 }
 
@@ -105,6 +105,12 @@
     }
 }
 
+/* Locale type codes. LT_NO_LOCALE must be zero. */
+#define LT_NO_LOCALE 0
+#define LT_DEFAULT_LOCALE 1
+#define LT_UNDERSCORE_LOCALE 2
+#define LT_UNDER_FOUR_LOCALE 3
+#define LT_CURRENT_LOCALE 4
 
 typedef struct {
     Py_UCS4 fill_char;
@@ -218,7 +224,13 @@
 
     /* Comma signifies add thousands separators */
     if (end-pos && READ_spec(pos) == ',') {
-        format->thousands_separators = 1;
+        format->thousands_separators = LT_DEFAULT_LOCALE;
+        ++pos;
+    }
+
+    /* Underscore signifies add thousands separators */
+    if (end-pos && READ_spec(pos) == '_') {
+        format->thousands_separators = LT_UNDERSCORE_LOCALE;
         ++pos;
     }
 
@@ -270,6 +282,15 @@
         case '\0':
             /* These are allowed. See PEP 378.*/
             break;
+        case 'b':
+        case 'o':
+        case 'x':
+            /* Underscores are allowed in bin/oct/hex. See PEP 515. */
+            if (format->thousands_separators == LT_UNDERSCORE_LOCALE) {
+                /* Every four digits, not every three, in bin/oct/hex. */
+                format->thousands_separators = LT_UNDER_FOUR_LOCALE;
+                break;
+            }
         default:
             invalid_comma_type(format->type);
             return 0;
@@ -346,11 +367,6 @@
 /*********** common routines for numeric formatting *********************/
 /************************************************************************/
 
-/* Locale type codes. */
-#define LT_CURRENT_LOCALE 0
-#define LT_DEFAULT_LOCALE 1
-#define LT_NO_LOCALE 2
-
 /* Locale info needed for formatting integers and the part of floats
    before and including the decimal. Note that locales only support
    8-bit chars, not unicode. */
@@ -660,8 +676,8 @@
 
 /* Find the decimal point character(s?), thousands_separator(s?), and
    grouping description, either for the current locale if type is
-   LT_CURRENT_LOCALE, a hard-coded locale if LT_DEFAULT_LOCALE, or
-   none if LT_NO_LOCALE. */
+   LT_CURRENT_LOCALE, a hard-coded locale if LT_DEFAULT_LOCALE or
+   LT_UNDERSCORE_LOCALE/LT_UNDER_FOUR_LOCALE, or none if LT_NO_LOCALE. */
 static int
 get_locale_info(int type, LocaleInfo *locale_info)
 {
@@ -684,16 +700,22 @@
         break;
     }
     case LT_DEFAULT_LOCALE:
+    case LT_UNDERSCORE_LOCALE:
+    case LT_UNDER_FOUR_LOCALE:
         locale_info->decimal_point = PyUnicode_FromOrdinal('.');
-        locale_info->thousands_sep = PyUnicode_FromOrdinal(',');
+        locale_info->thousands_sep = PyUnicode_FromOrdinal(
+            type == LT_DEFAULT_LOCALE ? ',' : '_');
         if (!locale_info->decimal_point || !locale_info->thousands_sep) {
             Py_XDECREF(locale_info->decimal_point);
             Py_XDECREF(locale_info->thousands_sep);
             return -1;
         }
-        locale_info->grouping = "\3"; /* Group every 3 characters.  The
+        if (type != LT_UNDER_FOUR_LOCALE)
+            locale_info->grouping = "\3"; /* Group every 3 characters.  The
                                          (implicit) trailing 0 means repeat
                                          infinitely. */
+        else
+            locale_info->grouping = "\4"; /* Bin/oct/hex group every four. */
         break;
     case LT_NO_LOCALE:
         locale_info->decimal_point = PyUnicode_FromOrdinal('.');
@@ -945,9 +967,7 @@
 
     /* Determine the grouping, separator, and decimal point, if any. */
     if (get_locale_info(format->type == 'n' ? LT_CURRENT_LOCALE :
-                        (format->thousands_separators ?
-                         LT_DEFAULT_LOCALE :
-                         LT_NO_LOCALE),
+                        format->thousands_separators,
                         &locale) == -1)
         goto done;
 
@@ -1092,9 +1112,7 @@
 
     /* Determine the grouping, separator, and decimal point, if any. */
     if (get_locale_info(format->type == 'n' ? LT_CURRENT_LOCALE :
-                        (format->thousands_separators ?
-                         LT_DEFAULT_LOCALE :
-                         LT_NO_LOCALE),
+                        format->thousands_separators,
                         &locale) == -1)
         goto done;
 
@@ -1270,9 +1288,7 @@
 
     /* Determine the grouping, separator, and decimal point, if any. */
     if (get_locale_info(format->type == 'n' ? LT_CURRENT_LOCALE :
-                        (format->thousands_separators ?
-                         LT_DEFAULT_LOCALE :
-                         LT_NO_LOCALE),
+                        format->thousands_separators,
                         &locale) == -1)
         goto done;