子类是否需要初始化一个空的超类？

Question

This just came into my mind.

class Parent:
    pass

class Child(Parent):
    def __init__(self):
        # is this necessary?
        super().__init__()

When a class inherits an empty class, do the subclass need to initialize it and why?

Answer 1

This is just fine:

class Parent:
    # the __init__ is inherited from parent
    pass

class Child(Parent):
    # the __init__ is inherited from parent
    pass

This is also fine:

class Parent:
    # the __init__ is inherited from parent
    pass

class Child(Parent):
    def __init__(self):
        # __init__ is called on parent
        super().__init__()

This may seem ok, and will usually work fine, but not always:

class Parent:
    # the __init__ is inherited from parent
    pass

class Child(Parent):
    def __init__(self):
        # this does not call parent's __init__, 
        pass

Here is one example where it goes wrong:

class Parent2:
    def __init__(self):
        super().__init__()
        print('Parent2 initialized')


class Child2(Child, Parent2):
    pass

# you'd expect this to call Parent2.__init__, but it won't:
Child2()

This is because the MRO of Child2 is: Child2 -> Child -> Parent -> Parent2 -> object.

Child2.__init__ is inherited from Child and that one does not call Parent2.__init__ , because of the missing call to super().__init__ .

Answer 2

No it isn't necessary. It is necessary when you want the parent's logic to run as well.

class Parent:
    def __init__(self):
        self.some_field = 'value'    

class Child(Parent):
    def __init__(self):
        self.other_field = 'other_value'
        super().__init__()
child = Child()
child.some_field # 'value'

Answer 3

There is no requirement in the language that subclass need to call __init__ of superclass. Despite this, it is almost always needed because superclass initializes some base attributes and the subclass expects them to be initialized. So, if the superclass __init__ is empty, you don't need to call it, otherwise you need to.