There is a call to the overriding method, and potentially the overridden method, with arguments that are not legal for the overridden method. This will cause an error if the overridden method is called and is a violation of the Liskov substitution principle.

Ensure that the overridden method accepts all the parameters that are legal for overriding method(s).

In this example there is a mismatch between the legal parameters for the base class method (self, source, filename) and the extension method (self, source). Since there is a call that uses the signature of the extension method then it can be inferred that the base signature is erroneous and should be updated to match that of the extension method.

The base method should be updated to either remove the filename parameters, or add a default value for it.

Wikipedia: Liskov Substitution Principle, Method overriding.