code: def authorize(client_key, client_secret, credfile=CONFIG_FILE, app='trlo.py',
                expiration='never', scope='read,write'):
      
      # 1. Obtain the request token.
      oauth = OAuth1Session(client_key, client_secret=client_secret)
      request_token = oauth.fetch_request_token(OAUTH_REQUEST_TOKEN_URL)
  
      # 2. Authorize the user (in the browser).
      authorization_url = oauth.authorization_url(OAUTH_BASE_AUTHORIZE_URL,
          name=app, expiration=expiration, scope=scope)
      print("Please visit the following URL to authorize this app:")
      print(authorization_url)
      print('')
      print("Once you've authorized, copy and paste the token Trello gives you below.")
      # Py3K backwards-compatibility shim
      global input
      try:
          input = raw_input
      except NameError:
          pass
      verifier = input("Trello's token: ").strip()
  
      # 3. Obtain the access token
      oauth = OAuth1Session(client_key, client_secret=client_secret,
          resource_owner_key=request_token['oauth_token'],
          resource_owner_secret=request_token['oauth_token_secret'],
          verifier=verifier)
      access_token = oauth.fetch_access_token(OAUTH_ACCESS_TOKEN_URL)
  
      # Save all our creds to ~/.trlo so we can get at 'em later.
      # The names are specially chosen so we can do OAuth1Session(**creds)
      creds = {
          'client_key': client_key,
          'client_secret': client_secret,
          'resource_owner_key': access_token['oauth_token'],
          'resource_owner_secret': access_token['oauth_token_secret'],
      }
      with open(credfile, 'w') as fp:
          json.dump(creds, fp)
docs: Authorize with Trello, saving creds to `credfile`.