Note that PEP 378 also applies to int.

Doc/whatsnew/2.7.rst

@@ -218,6 +218,11 @@ width and the precision::
    >>> '{:20,.2f}'.format(18446744073709551616.0)
    '18,446,744,073,709,551,616.00'
 
+When formatting an integer, include the comma after the width:
+
+   >>> '{:20,d}'.format(18446744073709551616)
+   '18,446,744,073,709,551,616'
+
 This mechanism is not adaptable at all; commas are always used as the
 separator and the grouping is always into three-digit groups.  The
 comma-formatting mechanism isn't as general as the :mod:`locale`