__init__ of Child Class - Printable Version

__init__ of Child Class - Printable Version

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__init__ of Child Class - zero_fX0 - Mar-22-2023

Hello all,
Base is parent and Child inherited .
Should argument(s) in the __init__of the child class be same as what we have in the __init__ 's parent?
I get an error.

class Base():
    
    def __init__(self,a,b):
        self.a = a
        self.b = b
        print("From Parent Class")

class Child(Base):
    
    def __init__(self):
        super().__init__(self,a,b) # or Base.__init__(self,a,b)
        print("From Child Class")

childobject=Child()

After trying to make an object from Child Class :

Error:NameError                                 Traceback (most recent call last)
<ipython-input-80-3762dc95e937> in <module>
----> 1 childobject=Child()

<ipython-input-79-b57528e5fe56> in __init__(self)
      2 
      3     def __init__(self):
----> 4         super().__init__(self,a,b)
      5         print("From Child Class")

NameError: name 'a' is not defined

RE: __init__ of Child Class - deanhystad - Mar-22-2023

In Base.__init__(), a and b are positional arguments, meaning you are required to pass values for a and b. You try to do that, but you don't have any values to pass. As far as Child.__init__() is concerned, a and b are unassigned local variables. They have no value.

If a and b are important attributes in Base, You should pass values for these to Child.__init__(). That way you can properly initialize Base using the values you pass to Child(a_value, b_value).

class Child(Base):
    def __init__(self, a. b):
        super().__init__(a, b)  # Notice that self should not be included as an arg.
        print("From Child Class")

Another optiion is to pass default values for a and b.

class Child(Base):
    def __init__(self):
        super().__init__(0, 0)  # Whatever values make sense as defaults for a and b
        print("From Child Class")

RE: __init__ of Child Class - zero_fX0 - Mar-22-2023

Thanks for your response.

(Mar-22-2023, 03:01 PM)deanhystad Wrote: In Base.__init__(), a and b are positional arguments, meaning you are required to pass values for a and b. You try to do that, but you don't have any values to pass

You mean I should create an object for Parent firstly which make me able to pass values?But how passing variables through a Parent class can help for second Child Class?

Here:

super().__init__(a, b)  # Notice that self should not be included as an arg.

Why self keyword should not be included there? In many examples I've seen it.

Your explanation is excellent but I m getting confused why still needs to be passed a , b in Child.__init__ ( ) . It is already passed in parent, why we have to concern it for Child Class ? If we want to pass a , b in Constructor of Child Class , why we create a inheritance class from Base class?
To be honest, I still have problem of this part of code:

def __init__(self, a, b):

Thanks in-advance If you provide more details .

RE: __init__ of Child Class - zero_fX0 - Mar-22-2023

If I pass a , b in Child.__init__( ) as you mentioned :

For creating an object from Child class like CHILD_OBJECT=Child() also should pass two variables(a and b) but in action I won't want that.Parent class should concern for those two not Child class .
How should solve this conflict ?

RE: __init__ of Child Class - deanhystad - Mar-22-2023

Quote:Why self keyword should not be included there? In many examples I've seen it.

You have not seen it. Calling an instance method using instance.method(args) gets automatically translated to instance.__class__.method(instance, args). If you call super().__init__(self, a, b) this is automatically translated to Base.__init__(self, self, a, b). What you saw in examples was probably this:

class Child(Base):
    def __init__(self):
        Base.__init__(self, a, b)
        print("From Child Class")

This is not using an instance to call Base.__init__(), so the translation does not occur.

Quote:You mean I should create an object for Parent firstly which make me able to pass values?But how passing variables through a Parent class can help for second Child Class?

No. I am saying that you have to pass values for a and b when you call Base.__init__(). Child.__init__() has to do this:

super().__init__(a, b)

You have to decide where a and b come from. If the values are important, I pass them as arguments to Child.

class Child(Base):
    def __init__(sel, a, b):
        Base.__init__(self, a, b)
        print("From Child Class")

If I don't want to always provide values, I might make a and b named arguments and give them default values. Now I can provide a, b values when creating Child(4, "Goodbye"), or I can use the default values Child().

class Child(Base):
    def __init__(self, a=5, b="Hello"):
        Base.__init__(self, a, b)
        print("From Child Class")

Or you can hide a and b inside Child and use default values when calling Base.__init__().

class Child(Base):
    def __init__(self):
        Base.__init__(self, 5, "Hello")
        print("From Child Class")

To change the value for Base.a or Base.b I would either set the attributes or call instance methods after the Child object is created.