Ask a yes/no question.

def ask_yesno(query, default=None):
    """Ask a yes/no question."""
    if default is None:
        default_q = ' [y/n]'
    elif default is True:
        default_q = ' [Y/n]'
    elif default is False:
        default_q = ' [y/N]'
    if sys.version_info[0] == 3:
        inp = raw_input("{query}{default_q} ".format(
            query=query, default_q=default_q)).strip()
    else:
        inp = raw_input("{query}{default_q} ".format(
            query=query, default_q=default_q).encode('utf-8')).strip()
    if inp:
        return inp.lower().startswith('y')
    elif default is not None:
        return default
    else:
        # Loop if no answer and no default.
        return ask_yesno(query, default)