Access changed base class instance attribute in derived class

Question

I have trouble understanding the logic of instance variables in inheritance. This is my (simplified) code with comments explaining how I understand its behaviour:

class Main(object):

    def __init__(self):
        self.p = Parent()  # self.parameter = []
        self.c = Child()  # self.parameter = []

    def run(self):
        self.p.setting()  # assigning value to self.parameter
        self.c.getting()


class Parent(object):

    def __init__(self):
        self.parameter = []

    def setting(self):
        self.parameter = [1, 2, 3]


class Child(Parent):
    # not redefining __init__, so Parent __init__ is called

    def getting(self):
        # value was assigned to self.parameter in setting method,
        # called before getting
        print self.parameter


Main().run()

getting prints [], instead of [1, 2, 3] which I expected. Why is it so? Since Child shares __init__ with Parent, at the beginning self.parameter = [] for both but why is it still [] when it was assigned a value long after Child().__init__ was called? What should I do to get changed self.parameter value in getting?

Python is not Java; not everything has to be a class. Get rid of Main. — chepner
– chepner, Commented Oct 10, 2017 at 13:53

Mateusz · Accepted Answer · 2017-10-12 13:24:19Z

2

Let's go through execution of your code.

main instance is created with Main().

At this moment we have main.p = [] and main.s = [], as defined in Main.__init__

main.run() is called, and it calls main.p.setting() and main.c.getting()

So now, main.p.setting() changes the parameter value of main.p to [1,2,3], and main.c.setting() just prints its own parameter value, which is still [], as it was never modified.

If you want to modify main.c.parameter value, simply call main.c.setting()

main = Main()
main.c.setting()
main.run()

answered Oct 12, 2017 at 13:24

Mateusz

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Comments

Adam Barnes · Accepted Answer · 2017-10-10 13:51:49Z

2

You're not inheriting from anything here in Main, you're adding member variables. You're expressing a "has a" relationship, not an "is a".

To inherit, define Main as such:

class Main(Child):

    def run(self):
        self.setting()
        self.getting()

With your current code, you could inspect main_instance.p.parameter after calling main_instance.run() and see that [1, 2, 3] is returned.

answered Oct 10, 2017 at 13:51

Adam Barnes

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