def expander(property_name):
    """Decorator adding a function to the ``EXPANDERS``."""
    def expander_decorator(function):
        """Add ``function`` to the ``EXPANDERS``."""
        assert property_name not in EXPANDERS, property_name
        EXPANDERS[property_name] = function
        return function
    return expander_decorator
Decorator adding a function to the ``EXPANDERS``.
weasyprint/css/validation/expanders.py