Example:

>>> '\u0100'
'Ā'
>>> '\u0100\U00010000'
'\u0100\U00010000'
>>> print('\u0100')
Ā
>>> print('\u0100\U00010000')
Traceback (most recent call last):
  File "<pyshell#33>", line 1, in <module>
    print('\u0100\U00010000')
UnicodeEncodeError: 'UCS-2' codec can't encode characters in position 1-1: Non-BMP character not supported in Tk

But I think that it is too specific problem and too specific solution. It would be better if IDLE itself escapes the string in the most appropriate way.

def utf8bmp_encode(s):
    return ''.join(c if ord(c) <= 0xffff else '\\U%08x' % ord(c) for c in s).encode('utf-8')

or

def utf8bmp_encode(s):
    return re.sub('[^\x00-\uffff]', lambda m: '\\U%08x' % ord(m.group()), s).encode('utf-8')