Python class inheritance: AttributeError: '[SubClass]' object has no attribute 'xxx'

Question

I have the following base class and subclass:

class Event(object):
    def __init__(self, sr1=None, foobar=None):
        self.sr1 = sr1
        self.foobar = foobar
        self.state = STATE_NON_EVENT

# Event class wrappers to provide syntatic sugar
class TypeTwoEvent(Event):
    def __init__(self, level=None):
        self.sr1 = level
        self.state = STATE_EVENT_TWO

Further on in my code, I am inspecting an instance of a TypeTwoEvent class, checking for a field I know exists in the base class - I expected it to be defaulted to value None. However, my code raises the following exception:

AttributeError: 'TypeTwoEvent' object has no attribute 'foobar'

I was under the impression that the base class fields would be inherited by the subclass and that creating an instance of a subclass will instantiate the base class (and thus invoke its constructor) ...

What am I missing here? Why does TypeTwoEvent not have a foobar attribute - when the base class from which it is derived has a foobar attribute?

Answer 1

Your subclass should be:

class TypeTwoEvent(Event):

    def __init__(self, level=None, *args, **kwargs):
        super(TypeTwoEvent, self).__init__(*args, **kwargs)
        self.sr1 = level
        self.state = STATE_EVENT_TWO

Because you override the __init__ method, so you need to call the parent method if you want the parent behavior to happen.

Remember, __init__ is not a special method dispite its strange name. It's just the method automatically called after the object is created. Otherwise it's an ordinary method, and ordinary inheritance rules apply.

super(ClassName, self).__init__(arguments, that, goes, to, parents)

is the syntax to call the parent version of the method.

For *args and **kwargs, it just ensures we catch all additional arguments passed to __init__ and pass it to the parent method, as you child method signature didn't do it and the parent need these arguments to work.

Answer 2

You're overriding the constructor (__init__) of the parent class. To extend it, you need to explicitly call the constructor of the parent with a super() call.

class TypeTwoEvent(Event):
    def __init__(self, level=None, **kwargs):
        # the super call to set the attributes in the parent class
        super(TypeTwoEvent, self).__init__(**kwargs)
        # now, extend other attributes
        self.sr1 = level
        self.state = STATE_EVENT_TWO

Note that the super call is not always at the top of the __init__ method in your sub-class. Its location depends on your situation and logic.

Answer 3

When the instance is created, its __init__ method is called. In this case, that is TypeTwoEvent.__init__. Superclass methods will not be called automatically because that would be immensely confusing.

You should call Event.__init__(self, ...) from TypeTwoEvent.__init__ (or use super, but if you're not familiar with it, read up on it first so you know what you're doing).

Answer 4

You need to call the __init__ method of the base class from the __init__ method of the inherited class.

See here for how to do this.

Answer 5

I have had problem with this also, but i've putted super().__init__() on the bottom of my derived class and that's why it doesn't work. Because i try to use attributes that is not initialized.