docs: def identify(user_id, previous_id=None, group_id=None, group_attributes=None,
              first_name=None, last_name=None, email=None,
              phone_number=None, apns_tokens=None, gcm_tokens=None,
              attributes=None, on_error=None, on_success=None):
      """ Identifying a user creates a record of your user in Outbound. Identify
      calls should be made prior to sending any track events for a user.
  
      :param str | number user_id: the id you use to identify a user. this should
      be static for the lifetime of a user.
  
      :param str | number previous_id: OPTIONAL the id you previously used to identify the user.
  
      :param str | number group_id: OPTIONAL  the id that identifies a group of users the current user
      belongs to.
  
      :param dict group_attributes: OPTIONAL An optional dictionary of attributes that are shared
      among the group this user belongs to.
  
      :param str first_name: OPTIONAL the user's first name.
  
      :param str last_name: OPTIONAL the user's last name.
  
      :param str email: OPTIONAL the user's email address.
  
      :param str phone_number: OPTIONAL the user's phone number.
  
      :param str | list apns_tokens: OPTIONAL the device tokens for the user's iOS
      devices. If a single string is given it is put into a list.
  
      :param str | list gcm_tokens: OPTIONAL the device tokens for the user's Android
      devices. If a single string is given it is put into a list.
  
      :param dict attributes: An optional dictionary with any additional freeform
      attributes describing the user.
  
      :param func on_error: An optional function to call in the event of an error.
      on_error callback should take 2 parameters: `code` and `error`. `code` will be
      one of outbound.ERROR_XXXXXX. `error` will be the corresponding message.
  
      :param func on_success: An optional function to call if/when the API call succeeds.
      on_success callback takes no parameters.
      """
  
      on_error = on_error or __on_error
      on_success = on_success or __on_success
  
      if not __is_init():
          on_error(ERROR_INIT, __error_message(ERROR_INIT))
          return
  
      if not isinstance(user_id, six.string_types + (Number,)):
          on_error(ERROR_USER_ID, __error_message(ERROR_USER_ID))
          return
  
      data = __user(
          first_name,
          last_name,
          email,
          phone_number,
          apns_tokens,
          gcm_tokens,
          attributes,
          previous_id,
          group_id,
          group_attributes,)
      data['user_id'] = user_id
  
      try:
          resp = requests.post(
              "%s/identify" % __BASE_URL,
              data=json.dumps(data),
              headers=__HEADERS,
          )
  
          if resp.status_code >= 200 and resp.status_code < 400:
              on_success()
          else:
              on_error(ERROR_UNKNOWN, resp.text)
      except requests.exceptions.ConnectionError:
          on_error(ERROR_CONNECTION, __error_message(ERROR_CONNECTION))