Python inheritance not working as I expected

Question

I have a class -

class Start(object):
 def __init__(self):
 self.flag = False
 self.my_list = []
 def set_up(self):
 self.flag = True
 def end_set_up(self):
 self.my_list = [i*2 for i in self.my_list]

And another class which inherits from this class -

class Process(Start):
 def __init__(self):
 super(Process, self).__init__()
 def check_flag(self):
 if self.flag:
 self.my_list = range(1, 10)

And in the third class, I want to do some operations on my_list

class Operation(Start):
 def __init__(self):
 super(Operation, self).__init__() 
 def perform_op(self):
 self.my_list = [i*2 for i in self.my_list]

Now these classes are used in a code snippet as -

start_ob = Start()
start_ob.set_up()
process_ob = Process()
process_ob.check_flag()
op_ob = Operation()
op_ob.perform_op()

My understanding of classes is not that strong. What I thought of achieving with this was -

• Set up class Start()
• Inherit flag from class Start() into Process() which should be True now since I called set_up() function here start_ob.set_up()
• Set my_list in base class to be [1,2....9]
• Inherit Start() into Operation() and modify list [1,2....9] that I created in the object Process()

But things are not moving according to my understanding. my_list is empty as set_up is False for classes Process and Operation. How do I change my code to make it work according to what my understanding is?

Edit- In the base class, there are two methods, one has to run when the object is initialised, right at the beginning. It will set a flag to True. After which another method in the same base class needs to run according to that flag

• juanpa.arrivillaga: All your objects are independent with their own instance variables. It seems like you are expecting to some form of prototypal inheritance (a la javascript) but class-based inheritance is different.
• Sushant: Thing is in the base class, there are two methods, one has to run when the object is initialised, right at the beginning. It will set a flag to True. After which another method in the same base class needs to run according to that flag
• juanpa.arrivillaga: Yes, but you have created three independent objects.
• Sushant: @juanpa.arrivillaga I think I understand what you are saying. So how do I go about solving this?
• TomasCarvalho: If you want a flag set to True when you create an object, why don't you set self.flag = True?

Answer 1

What you’re doing here:

start_ob = Start()
start_ob.set_up()
process_ob = Process()
process_ob.check_flag()
op_ob = Operation()
op_ob.perform_op()

... is creating three entirely separate objects. Each one has its own my_list. Just like you can have three different int objects and they’re all separate values, you can have three different Start objects and they’re all separate values.

What you probably wanted is:

ob = Operation()
ob.set_up()
ob.check_flag()
ob.perform_op()

Now you have a single object, which is an Operation, and therefore a Process, and therefore a Start, so you can call methods from any of those three types and they will affect your object’s value. And now you’re using inheritance.

• juanpa.arrivillaga: Note, Operation isn't inheriting from Process, they are sister classes. It won't have check_flag
• Sushant: Hi, thanks for the quick answer. Can you see whether my edited question affects your answer?
• Sushant: But we can't call check_flag from Operation class as check_flag is a function of Process not Operation?