code: def obtain_access_token(self):
          
          if self.oauth_version != 2:
              raise TwythonError('This method can only be called when your \
                                 OAuth version is 2.0.')
  
          data = {'grant_type': 'client_credentials'}
          basic_auth = HTTPBasicAuth(self.app_key, self.app_secret)
          try:
              response = self.client.post(self.request_token_url,
                                          data=data, auth=basic_auth)
              content = response.content.decode('utf-8')
              try:
                  content = content.json()
              except AttributeError:
                  content = json.loads(content)
                  access_token = content['access_token']
          except (KeyError, ValueError, requests.exceptions.RequestException):
              raise TwythonAuthError('Unable to obtain OAuth 2 access token.')
          else:
              return access_token
docs: Returns an OAuth 2 access token to make OAuth 2 authenticated
          read-only calls.
  
          :rtype: string