@staticmethod
def _get_value(obj, ds_key, transformer):
    """
        Get value from source object, transforming if specified

        :param obj: the input object we are translating to STIX
        :param ds_key: the property from the input object
        :param transformer: the transform to apply to the property value (can be None)
        :return: the resulting STIX value
        """
    if ds_key not in obj:
        print('{} not found in object'.format(ds_key))
        return None
    ret_val = obj[ds_key]
    if ret_val and transformer is not None:
        return transformer.transform(ret_val)
    return ret_val
