Question

When inherit from a base class, that implements __deepcopy__ and the inheriting class changes the arguments in __init__, how can the __deepcopy__ from the base class be reused in the inheriting class?

Here an example:

class A(object):
    def __init__(self, arg1, arg2):
        self.arg1 = arg1
        self.arg2 = arg2

    def __deepcopy__(self, memo):
        newone = type(self)(self.arg1, self.arg2)
        ...

class B(A):
    def __init__(self, arg1):
        A.__init__(self, arg1, None)

    def __deepcopy__(self, memo):
        newone = A.__deepcopy__(self, memo) # fails, because __deepcopy__ of
                                            # A tries to create an instance of
                                            # B with to many arguments
        ...

Answer 1

You could work around that by accepting an arbitrary number of arguments, ignoring them in your own constructor:

class B(A):
    def __init__(self, arg1, *ignored):
        # `ignored` is.. ignored
        A.__init__(self, arg1, None)

Since you call A.__init__() with None as the second positional argument anyway, it is safe to ignore that same argument when __deepcopy__ passes it back to the instance initializer again.