CODEWITHSUNDEEP
pythonclassinheritance
Todd
Todd

Reputation: 101

Attribute inherited from child class prints child object not string from parent

I know many topics around class inheritance in Python have been addressed, but I could not find a thread that addressed this specific issue.

Edit: I'm running Python 3.5.5.

Code:

class Parent():
    def __init__(self, parentParam="parent param"):
        self.parentParam = parentParam

class Child(Parent):
    def __init__(self, childParam = "child param"):
        self.childParam = childParam
        super().__init__(self)

child = Child()
print(child.childParam)
print(child.parentParam)

Output:

child param
<__main__.Child object at 0x0000017CE7C0CAC8>

Why does does child.parentParam return the child object and not the string "parent param"? I feel like it should print out the default string set for the Parent class. This appears to be the same syntax as I have been following in this tutorial.

Thanks, everyone.

Upvotes: 3

Views: 689

Answers (2)

martineau
martineau

Reputation: 123521

Your call to super is wrong. It's not necessary to explicitly pass a self argument when using it to delegate a method call to a superclass (see the typical usage example shown in the documentation).

In Python 3, calls to super() with no arguments are equivalent to super(CurrentClass, self).method(arg)—which was the only way it could be done in Python 2—making it no longer necessary to specify it at all when making calls to superclass methods.

So what's happening, since you passed it in your code, is that it gets interpreted as overriding the default value specified for the parentParam argument.

Here's doing it properly and the result:

class Parent:
    def __init__(self, parentParam="parent param"):
        self.parentParam = parentParam


class Child(Parent):
    def __init__(self, childParam="child param"):
        self.childParam = childParam
        super().__init__()  # NO NEED TO PASS self.


child = Child()
print(child.childParam)   # -> child param
print(child.parentParam)  # -> parent param

Upvotes: 4

Patrick Artner
Patrick Artner

Reputation: 51683

Because you provide the instance of child (aka self) to the super call:

class Child(Parent):
    def __init__(self, childParam = "child param"):
        self.childParam = childParam
        super().__init__(self) # here you override the default by supplying this instance

Use: 

class Child(Parent):
    def __init__(self, childParam = "child param"):
        self.childParam = childParam
        super().__init__()

instead and you get this output instead:

child param
parent param

Upvotes: 5

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