How to extend Python class init

Question

I have created a base class:

class Thing():
    def __init__(self, name):
        self.name = name

I want to extend the class and add to the init method so the that SubThing has both a name and a time property. How do I do it?

class SubThing(Thing):
    # something here to extend the init and add a "time" property

    def __repr__(self):
        return '<%s %s>' % (self.name, self.time)

Any help would be awesome.

Arum

Answer 1

You can just define __init__ in the subclass and call super to call the parents' __init__ methods appropriately:

class SubThing(Thing):
    def __init__(self, *args, **kwargs):
        super(SubThing, self).__init__(*args, **kwargs)
        self.time = datetime.now()

If you're still on Python 2.x, make sure the base class is a subclass of object, as super won't work with old-style classes:

class Thing(object):
    ...

Offal

Also instead of referencing SubThing in super, you can do super(self.__class__, self).__init__(*args, **kwargs). This also works with dynamically created classes using type(). – Fatma

I get this error with either of those solutions: TypeError: must be type, not classobj - the error is triggered by my super line. – Drip

@Drip look at what Jesse wrote: "Make sure to have your base class subclass from object though, as super won't work with old-style classes" – Spectacled

One note: I had to swap the order of self.time = ... and super(.... – Hemihydrate

Answer 2

You can eliminate the arguments in super:

class SubThing(Thing):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.time = datetime.now()

Claribel

Answer 3

You should write another __init__ method in SubThing and then call the constructor of the superclass to initialize its fields.

This Q&A should provide you some more examples.

Leoine