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?
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.