import email.header as eh
unicode_data= u''.join(
str_data.decode(codec or 'ascii')
for str_data, codec
in eh.decode_header('=?ISO-8859-7?B?0OXm7/Dv8d/hIPP07+0gyuno4enx/u3h?='))
Here you should work with unicode_data. However, if you (it seems, you) need a UTF-8 encoded string, you can:
utf8data= unicode_data.encode('utf-8')
: .decode, , codec None (, eh.decode_header('plain text'))