CODEWITHSUNDEEP
pythonclassinitsuper
zeno
zeno

Reputation: 391

Why allow to return a value from __init__ with super in Python?

From the documentation of __init__

no non-None value may be returned by __init__(); doing so will cause a TypeError to be raised at runtime.

and I found this code from a framework's extendable base class.

It runs without a runtime error.

class Parent():
    def __init__(self, x):
        self.x = x
        return self.x + 10

class Child(Parent):
    def __init__(self, x):
        y = super(Child, self).__init__(x)
        print(y)

c = Child(5)

I searched about it and agreed that could be nice to pass data from super().__init__ to the derived class without having to relay it through an instance.

But, Why python allows the __init__ return value?

Is it a widely known pattern?

Upvotes: 0

Views: 525

Answers (1)

match
match

Reputation: 11070

When a class is instantiated, python looks for a method called __init__ and uses it in setting up the new class. If this method returns anything other than None here, it will raise an error:

class Parent():
    def __init__(self, x):
        self.x = x
        return self.x + 10

Parent(5)
# TypeError: __init__() should return None, not 'int'

However, in your child class you don't let Python use the __init__ method in Parent - instead you override it by creating your own __init__ which doesn't return anything.

You then call the Parent __init__ method (via super) but here you are just calling it as a 'normal' method, rather than using it as part of the class setup, so it won't raise the error.

Upvotes: 1

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