code: def retrieve_token(self):
          
  
          if self.authorization_code:
              request_param = {
                  "client_id": self.client_id,
                  "client_secret": self.client_secret,
                  "redirect_uri": self.redirect_uri,
                  "code": self.authorization_code
                  }
  
              if self._extra_token_params:
                  request_param.update(self._extra_token_params)
  
              content_length = len(urlencode(request_param))
              headers = {
                  'Content-Length': str(content_length),
                  'Content-Type': 'application/x-www-form-urlencoded'
                  }
          
              r = requests.post(self.token_uri, data=request_param,
                                headers=headers)
              jsondata = json.loads(r.text)
              self.access_token = jsondata
              return self.access_token
  
          else:
              print("authorization code is required before getting accesss token")
              print("Please call retrieve_authorization_code() beforehand")
docs: retrieve access token with code fetched via 
          retrieve_authorization_code method.