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?
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.