Now, we want to be able to parse an event from several data sources, in different formats (XML and JSON, for instance). Following good practice, we decide to target an interface as our dependency instead of a concrete class, and something like the following is devised:

In order to create this as an interface in Python, we would use an abstract base class and define the methods (from_xml() and from_json()) as abstract, to force derived classes to implement them. Events that derive from this abstract base class and implement these methods would be able to work with their corresponding types.

But what if a particular class does not need the XML method, and can only be constructed from a JSON? It would still carry the from_xml() method from the interface, and since it does not need it, it will have to pass. This is not very flexible as it creates coupling and forces clients of the interface to work with methods that they do not need.