Inherit variable from superclass

Question

Let's assume I have two classes:

#!/usr/bin/env python

class First():

        glob = 'Global data'

        def __init__(self, arg1, arg2):
                self.arg1 = arg1
                self.arg2 = arg2
                print (arg1, arg2)

class Second(First):
        def __init__(self):
                print (self.glob)

And they are called from script as:

#!/usr/bin/env python

import classes

data1 = 'First data part'
data2 = 'Second data part'

inst1 = classes.First(data1, data2)
inst2 = classes.Second()

This works, OK:

$ ./script.py
('First data part', 'Second data part')
Global data

I want to make arg1 and arg2 in First class something 'global', and use then it in Second:

class Second(First):
        def __init__(self):
                print (self.glob, self.arg1, self.arg2)

How I can achieve it?

You are confusing instance data with class data. The attributes you set in the __init__ of First are per-instance attributes; instances of another class (even a subclass) won't see those. What values would instances of Second ever have for those attributes? — Martijn Pieters
– Martijn Pieters, Commented Sep 11, 2014 at 13:56
You will have to call First.__init__ yourself (since you are overriding it in the child class), possibly via the super function. After this super call, you will have access to those attributes. — wim
– wim, Commented Sep 11, 2014 at 13:58

Anshul Goyal · Accepted Answer · 2014-09-11 14:44:49Z

1

class First(object):

    glob = 'Global data'

    def __init__(self, arg1, arg2):
        self.arg1 = arg1
        self.arg2 = arg2
        #print (arg1, arg2)

class Second(First):
    def __init__(self, *args):
        super(Second, self).__init__(*args)
        print (self.glob, self.arg1, self.arg2)

data1 = 'First data part'
data2 = 'Second data part'

inst1 = First(data1, data2)
inst2 = Second(data1, data2)

edited Sep 11, 2014 at 14:44

Anshul Goyal

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answered Sep 11, 2014 at 13:58

Andrey

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5 Comments

wim Over a year ago

I think the OP question was python3 (just a guess)

setevoy Over a year ago

super - seems what I need... Thanks, don't know about it.

Andrey Over a year ago

@setevoy if you want to use fields of the parent class you need to call parent __init__

setevoy Over a year ago

@Andrey yep, I understood... Tried do same with direct call First(data1, data2) inside __init__ of Second :-)

setevoy Over a year ago

Btw - it also works: First.__init__(self, *args); but seems this is incorrect way...

Agate · Accepted Answer · 2014-09-11 14:04:36Z

You can use unpacking in your First class __init__() method:

class First:

    def __init__(self, *args):
        for data in args:
            print(data)

# in your child class, you need to call First.__init__(), via super()

class Second(First):

    def __init__(self, *args):
        super(Second, self).__init__(*args)  # pass args to First.__init__()

Usage:

import classes

i = Second("data", "data1", "data2", "data3")

Using unpacking allow your class to accept any number of arguments and will make writing subclasses faster and easier. Depending on your use case, it will sometimes better to use dictionnary unpacking, especially if you want named arguments:

class First:

    def __init__(self, **kwargs): # note the double wildcards here
        print(kwargs.get('data1'))
        print(kwargs.get('data2'))

class Second(First):

    def __init__(self, **kwargs):
        super(Second, self).__init__(**kwargs) 

# usage

i = Second(data1="something", data2="something else", another_arg="Yahoo")

Anshul Goyal · Accepted Answer · 2014-09-11 14:45:28Z

1

The init of First is overloaded by Second, call the one of your parent class from your Second class

class Second(First):
    def __init__(self):
        super(Second, self).__init__("argument1", "argument2")
        print (self.glob, self.arg1, self.arg2)

In python3 it is like this:

class Second(First):
    def __init__(self):
        super().__init__("argument1", "argument2")
        print (self.glob, self.arg1, self.arg2)

edited Sep 11, 2014 at 14:45

Anshul Goyal

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answered Sep 11, 2014 at 14:01

Bart Vanherck

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