constructing an object from another instance of the same class

[Christoph Groth]

Dear all,

sometimes it is handy to have a function which can take as argument
anything which can be converted into something, e.g.

def foo(arg):
    arg = float(arg)
    # ...

I would like to mimic this behavior of float for a user-defined type,
e.g. 

def bar(arg):
    arg = My_type(arg)
    # ...

Now I wonder what is the most pythonic way to write the __init__ method
of My_type?  The following comes to my mind:

class My_type:
    def __init__(self, other):
        if isinstance(other, type(self)):
            self.a = other.a
            self.b = other.b
            return
        # initialize self in some other way

It seems to me that in this way I might get problems when I pass an
instance of Derived_from_my_type to bar, as it will become an instance
of My_type.

What is a good way to express this?  In C++ (which I know better than
python) I would make bar accept a const reference to My_type.  Then I
could use it directly with instances of My_type, Derived_from_my_type
and other types which can be converted into My_type.

thanks
Christoph