Python add a variable to a class

Question

I am trying to understand python classes:

I have the following case:

class parent:
    def __init__(self):
        self.a = 1
        self.b = 2
    def printFoo(self):
        print(self.a)
        
        
class child(parent):
    def createC(self, val): # Create a variable inside the derived class?
        self.c = val
    
    def printFoo(self): # overloaded function
        print(self.c)
        
a = parent()
b = child()

b.createC(3)
b.printFoo()
# Edited:
# I can even create variables as: child.d = 10

I would like to define a variable c and store it in the child class. Is it suitable to do this? Should I define an __init__ method to create the variable?

Best regards

score 1 · Answer 1 · answered Aug 12 '22 at 09:52

1

Yes, you can/should definitely have __init__ in the derived classes. You just need to initialize the base class via super method. Here is the example according to your case.

class parent:
    def __init__(self):
        self.a = 1
        self.b = 2
    def printFoo(self):
        print(self.a)
        
        
class child(parent):
    def __init__(self):
        super().__init__() # initialize base class, pass any parameters to base here if it requires any
        self.c = someval
    # def createC(self, val): dont need these
    #    self.c = val
    
    def printFoo(self): # overloaded function
        print(self.c)

If your base class requires some parameters, you can do it like this

class parent:
    def __init__(self, a, b):
        self.a = a
        self.b = b
    def printFoo(self):
        print(self.a)
        
        
class child(parent):
    def __init__(self, a, b, someval):
        super().__init__(a, b) # initialize base class, pass any parameters to base here if it requires any
        self.c = someval
    #def createC(self, val): dont need these
    #    self.c = val
    
    def printFoo(self): # overloaded function
        print(self.c)

answered Aug 12 '22 at 09:52

Ahmad Anis

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Hi thanks, I just noticed that I can even add variables from outside the class. something like `child.c = 30`. – Aug 12 '22 at 09:55
Funnily I thought you had to call `super().__init__()` to initialise the base class, but I just tried the example code above and `self.a` and `self.b` were initialised correctly, in python 3.8.8. – John M. Aug 12 '22 at 09:55
@JohnM. try my 2nd example without `super().__init__()` and you'll know. – Ahmad Anis Aug 12 '22 at 10:02
@ThomasJerry yes you can do it, but you should not do it like this as it is not a good practice. – Ahmad Anis Aug 12 '22 at 10:03
It's clear the second example needs initialisation, but if the base constructor has no input parameters it seems to work OK. I think there is a strong best practice argument. – John M. Aug 12 '22 at 10:04

score 0 · Answer 2 · answered Aug 12 '22 at 10:01

Do you want your variable to change ? You can simply put c inside your new child class, such as a static class variable (have a look here maybe).

class child(parent):
    c = 3

You can access it doing :

child.c

If you want to pass by the __init__() function and define the new variable when you define the class, use the super() method as @Ahmad Anis was suggested.

Python add a variable to a class

2 Answers2