Python函数只需要2个参数(给定1个)

我正在尝试解析Gmail电子邮件.来自http://yuji.wordpress.com/2011/06/22/python-imaplib-imap-example-with-gmail/我有以下内容:

def get_first_text_block(self, email_message_instance):
    maintype = email_message_instance.get_content_maintype()
    if maintype == 'multipart':
        for part in email_message_instance.get_payload():
            if part.get_content_maintype() == 'text':
                return part.get_payload()
    elif maintype == 'text':
        return email_message_instance.get_payload()    

def hello(request):
    ....   

    first = qs[0]
    one = first.get_email_object()
    print one['To']    

    out = get_first_text_block(one)

我越来越:

get_first_text_block() takes exactly 2 arguments (1 given)

我究竟做错了什么？