def opt_arg(spec, property, default_value=None):
    """Get an optional property, possibly with a default value.
    @param spec: the JSON-like filter spec (a dict)(
    @param property: the property name to retrieve
    @param default_value: the default value to return if not found
    @returns: the value if found, otherwise default_value/None
    """
    value = spec.get(property)
    if value is None:
        return default_value
    else:
        return value
