Calling a derived class's constructor from a parent method

Chris Angelico rosuav at gmail.com
Wed Jan 14 17:56:46 CET 2015


On Thu, Jan 15, 2015 at 3:45 AM, jason <jasonsewall at gmail.com> wrote:
> If I have a class hierarchy like so:
>
>
> class A(object):
>       def __init__(self, s):
>             self.s = s
>       def foo(self, s):
>             return A(s)
>
> class B(A):
>       def __init__(self, s):
>             A.__init__(self, s)
>
> If I make a B:
>
> b = B(0)
>
> I'd like b.foo(1) to return an instance of B. Is there a way to do that besides implementing a construct(self, s) for each part of the hierarchy? I.e. is there a way for the base class to look at self and find out what type to create?
>
> I'm using Python 2.7.5, but I'm curious what the 3.x answer is too.

Assuming you require that every subclass be able to be constructed
with that single parameter, it's not too hard. You get a 'self'
parameter; its type is the type you want to construct.

    def foo(self, s):
        return type(self)(s)

That should work just fine on either 2.7 or 3.x.

I'm not sure what this function would do in context, but this basic
technique should work.

ChrisA



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