How can I add to the initial definition of a python class inheriting from another class?

Question

I'm trying to define self.data inside a class inheriting from a class

class Object():
    def __init__(self):
        self.data="1234"

class New_Object(Object):
    # Code changing self.data here

But I ran into an issue.

class Object():
    def __init__(self):
        self.data="1234"

So I have the beginning class here, which is imported from elsewhere, and let's say that the class is a universal one so I can't modify the original at all.

In the original, the instance is referred to as "self" inside the class, and it is defined as self inside the definition __init__.

class New_Object(Object):
    # Code changing self.data here

So if I wanted to inherit from the class Object, but define self.data inside New_Object, I thought I would have to define __init__ in New_Object, but this overrides the __init__ from New_Object

Is there any way I could do this without copypasting the __init__ from Object?

Any edits would be useful, I'm terrible at terminology and phrasing what I mean. — That One Random Scrub, Sep 23 '16 at 15:30
Note that `self` is an *instance* of the class, not the class itself. — MisterMiyagi, Sep 23 '16 at 15:35

Daniel Roseman · Accepted Answer · 2016-09-23T15:34:09.480

3

You use super to call the original implementation.

class New_Object(Object):
    def __init__(self):
        super(NewObject, self).__init__()
        self.info = 'whatever'

edited Sep 23 '16 at 15:34

answered Sep 23 '16 at 15:32

Daniel Roseman

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score 2 · Answer 2 · answered Sep 23 '16 at 15:32

2

That's what super is for:

class NewObject(Object):

    def __init__(self):
        super(NewObject, self).__init__()
        # self.data exists now, and you can modify it if necessary

answered Sep 23 '16 at 15:32

wim

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score 1 · Answer 3 · answered Sep 23 '16 at 15:36

You can use super().__init__() to call Object.__init__() from New_Object.__init__().

What you would do:

class Object:
    def __init__(self):
        print("Object init")
        self.data = "1234"

class New_Object(Object):
    def __init__(self):
        print("calling super")
        super().__init__()
        print("data is now", self.data)
        self.data = self.data.split("3")

o = New_Object()

# calling super
# Object init
# data is now 1234

Note that you do not have to give any arguments to super(), as long as you are using Python 3.

score 1 · Answer 4 · edited May 23 '17 at 12:09

The answer is that you call the superclass's __init__ explicitly during the subclass's __init__. This can be done either of two ways:

Object.__init__(self)   # requires you to name the superclass explicitly

or

super(NewObject, self).__init__()   # requires you to name the subclass explicitly

The latter also requires you to ensure that you're using "new-style" classes: in Python 3 that's always the case, but in Python 2 you must be sure to inherit from the builtin object class. In Python 3 it can actually be expressed even more simply:

super().__init__()

Personally, in most of my code the "disadvantage" of having to name the superclass explicitly is no disadvantage at all, and Object.__init__() lends transparency since it makes it absolutely clear what is being called. This is because most of my code is single-inheritance only. The super route comes into its own when you have multiple inheritance. See What does 'super' do in Python?

Python 2 example:

class Object(object):
    def __init__(self):
        self.data = "1234"

class NewObject:
    def __init__(self):
        # subclass-specific stuff
        super(NewObject, self).__init__()
        # more subclass-specific stuff

How can I add to the initial definition of a python class inheriting from another class?

4 Answers4