How can I create a sub class when given a base class in Python3?

Question

The typical way to do subclassing in Python is this

class Base:
    def __init__(self):
        self.base = 1

class Sub(Base):
    def __init__(self):
        self.sub = 2
        super().__init__()

And this allows me to create an instance of type Sub that can access both base and sub properties.

However, I'm using an api that returns a list of Base instances, and I need to convert each instance of Base into an instance of Sub. Because this is an API that is subject to change, I don't want to specifically unpack each property and reassign them by hand.

How can this be done in Python3?

I'm not sure if I understand correctly but why don't you use *args and **kwargs? — sahinakkaya
– sahinakkaya, Commented May 21, 2020 at 20:07
Does this answer your question? How to convert (inherit) parent to child class? — PM 77-1
– PM 77-1, Commented May 21, 2020 at 20:09
@Asocia Sounds like you have a solution, feel free to answer the question. — Raven
– Raven, Commented May 21, 2020 at 20:09
You'll need to write a function that knows how to turn a Base object into a Sub object. Perhaps this can be done in Sub.__init__ or perhaps you write a class method in Sub that knows how. Or even just a stand-alone function. Alternately, perhaps Sub shouldn't inherit from Base at all and just do composition by keeping a Base member variable. Lots of choices! — tdelaney
– tdelaney, Commented May 21, 2020 at 20:24

sahinakkaya · Accepted Answer · 2020-05-21 20:21:05Z

1

You can use *args and **kwargs if you don't want to hard code the name of the arguments.

class Base:
    def __init__(self, a, b, c="keyword_arg"):
        self.a = a
        self.b = b
        self.c = c


class Sub(Base):
    def __init__(self, *args, **kwargs):
        self.sub = 2
        super().__init__(*args, **kwargs)


bases = [Base(1, 2, "test") for _ in range(10)]
subs = [Sub(**b.__dict__) for b in bases]

Note that this assumes that all the properties of Base is given as arguments to the Base.

answered May 21, 2020 at 20:21

sahinakkaya

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