Unexpected output in class inheritance code-CodePudding

I tried this inheritance code and got an unexpected output. Kindly guide me where is my fault.

Child.py

from ParentClass import Parent


class ChildImp(Parent):
    var2 = 200

    def __init__(self):
        Parent.__init__(self, 8, 3)


    def getData(self):
        self.var1   self.Sub()   self.Add()


obj = ChildImp()
print(obj.getData())

ParentClass.py

class Parent:
    var1 = 100

    def __init__(self, a, b):
        self.firstNUm = a
        self.secondNUm = b


    def Add(self):
        return self.firstNUm   self.secondNUm

    def Sub(self):
       return self.firstNUm - self.secondNUm


obj1 = Parent(4, 6)
print(obj1.Add())
obj2 = Parent(9, 2)
print(obj2.Sub())

output:

10
7
None

Process finished with exit code 0

Where does this 10 and 7 come from? Why there is a None in the output?

CodePudding user response：

The output is from each of the following calls

10     # print(obj1.Add())
7      # print(obj2.Sub())
None   # print(obj.getData())

note that Add and Sub will return the computed value, btu getData has no return so will implicitly return None hence the last output.

Note that the reason those Add and Sub lines are executing is because of the import statement. If you only want those to run when that script is directly invoked you would modify that block to

if __name__ == '__main__':
    obj1 = Parent(4, 6)
    print(obj1.Add())
    obj2 = Parent(9, 2)
    print(obj2.Sub())

CodePudding user response：

Output is from the 3 print statements.

print(obj1.Add())    # prints 10
print(obj2.Sub())    # prints 7
print(obj.getData()) # prints None

Need a return on ChildImp getData() function otherwise returns None.

from ParentClass import Parent

class ChildImp(Parent):
    var2 = 200

    def __init__(self):
        Parent.__init__(self, 8, 3)

    def getData(self):
        return self.var1   self.Sub()   self.Add() # <== added return

obj = ChildImp()
print(obj.getData())

Also good practice to add if name == __main__ check in the parent class so only the main in the class called is executed.

# ParentClass.py
class Parent:
    ...

if __name__ == '__main__':

    obj1 = Parent(4, 6)
    print(obj1.Add())
    obj2 = Parent(9, 2)
    print(obj2.Sub())

CodePudding user response：

10 and 7 are the return values of the Add and Sub methods

The None is there because getData does not return anything

For all three values there is a print call.