Secret Recipes of the Python Ninja

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When subclassing UserList, the subclass is expected to provide a constructor that can be called with either no arguments or one argument. If a list operation is expected to return a new sequence, it attempts to create an instance of the actual implementation class. Thus, it expects the constructor to provide the ability to be called with a single parameter, that is, the sequence object that serves as the source of data.

It is possible to create a class that doesn't follow this requirement, but all the special methods of the derived class must be overridden, as functionality using the default methods cannot be guaranteed.