def get_attribute(element_node, attribute_name, default_value=None):
    """Extract an attribute value from an element node.
    Does not fail if the attribute isn't present.
    @param element_node: the DOM element node containing the attribute.
    @param attribute_name: the name of the attribute.
    @param default_value: the value to return if the attribute is unspecified.
    @returns: the attribute value, or default_value (None) if it is unspecified.
    """
    node = element_node.attributes.get(attribute_name)
    if node:
        return node.value
    else:
        return default_value
