def empty_if_none(_string, _source=None):
    """If _source if None, return an empty string, otherwise return string. 
    This is useful when you build strings and want it to be empty if you don't have any data.
    For example when building a comma-separated string with a dynamic number of parameters: 
    .. code-block:: python
    
    command = (_id +";" +  +  empty_if_none(";"+str(_comment), _comment))
    """
    if _source is None:
        return ''
    else:
        return str(_string)
