def _build_projection_expression(clean_table_keys):
    """Return a projection expression for the DynamoDB lookup.

    Args:
        clean_table_keys (Dict[str, Any]): Keys without the data types attached.

    Returns:
        str: A projection expression for the DynamoDB lookup.

    """
    projection_expression = ''
    for key in clean_table_keys[:-1]:
        projection_expression += ('{},').format(key)
    projection_expression += clean_table_keys[-1]
    return projection_expression