dropbox api身份验证（错误：[400]'invalid_client'）

Question

以下是dropbox教程中的代码

from dropbox.client import DropboxOAuth2FlowNoRedirect, DropboxClient
from dropbox import rest as dbrest

    auth_flow = DropboxOAuth2FlowNoRedirect('r********', 'r***********')

    authorize_url = auth_flow.start()
    print ('1. Go to: ' + authorize_url)
    print ('2. Click \'Allow\' (you might have to log in first).')
    print ('3. Copy the authorization code.')
    auth_code = input('Enter the authorization code here: ')

    try:
        access_token, user_id = auth_flow.finish(auth_code)
    except (dbrest.ErrorResponse) as e:
        print('Error: %s' % (e,))

    c = DropboxClient(access_token)

我得到以下回复

  

错误：[400]'invalid_client'回溯（最近一次呼叫最后）：
  文件“test2.py”，第17行，in       c = DropboxClient（access_token）NameError：未定义名称“access_token”