CODEWITHSUNDEEP
pythoninheritanceclass-attributes
Homunculus Reticulli
Homunculus Reticulli

Reputation: 68476

Why don't my subclass instances contain the attributes from the base class (causing an AttributeError when I try to use them)?

I have this base class and subclass:

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


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

Later on, when I try to check the foobar attribute of a TypeTwoEvent instance, I get an exception. For example, testing this at the REPL:

>>> event = TypeTwoEvent()
>>> event.foobar
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'TypeTwoEvent' object has no attribute 'foobar'

I thought that the base class attributes would be inherited by the subclass and that creating an instance of a subclass would instantiate the base class (and thus invoke its constructor). Therefore, I expected the foobar attribute value to be defaulted to None.

Why do TypeTwoEvent instances not have a foobar attribute, even though Event instances do?

Upvotes: 48

Views: 61004

Answers (6)

Daniel Donnelly
Daniel Donnelly

Reputation: 610

None of the above fixed my problem. Apparently sometimes Python doesn't like the subclass to inherit @properties. See this screenshot:

enter image description here

parent_graph used to be an @property and the same analogous error you see displayed in the picture occured. Now I made it into a regular method. So now it gets past that error and moves onto the next uninherited @property which is ambient_space. So now you can imagine what I have to do: take away @property's in practically my whole project.

Arrow is using single inheritance from base class Base which in turn singly inherits from QGraphicsObject. I am definitely calling the super().__init__() appropriately.

Upvotes: 0

Bite code
Bite code

Reputation: 597291

The subclass should be:

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

Because __init__ is overridden, the base class' __init__ code will only run if it is explicitly requested.

Despite its strange name, __init__ is not specially treated. It gets called automatically after the object is created; but otherwise it's an ordinary method, and ordinary inheritance rules apply.

super().__init__(arguments, that, go, to, parents)

is the syntax to call the parent version of the method. Using *args and **kwargs allows us to catch additional arguments passed to __init__ and pass them to the parent method; this way, when a TypeTwoEvent is created, a value can be specified for the foobar, along with anything else specific to the base class.

Upvotes: 48

М.Б.
М.Б.

Reputation: 1420

I've had the same problem, but in my case I put super().__init__() on the bottom of my derived class and that's why it doesn't work. Because I tried to use attributes that are not initialized.

Upvotes: 0

Praveen Gollakota
Praveen Gollakota

Reputation: 38990

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().__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.

Upvotes: 4

Chris Morgan
Chris Morgan

Reputation: 90852

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).

Upvotes: 2

alan
alan

Reputation: 4852

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.

Upvotes: 1

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