What's wrong with this code?
class MyList(list): def __init__(self, li): self = li When I create an instance of MyList with, for example, MyList([1, 2, 3]), and then I print this instance, all I get is an empty list []. If MyDict is subclassing list, isn't MyDict a list itself?
NB: both in Python 2.x and 3.x.
You need to call the list initializer:
class MyList(list): def __init__(self, li): super(MyList, self).__init__(li) Assigning to self in the function just replaces the local variable with the list, not assign anything to the instance:
>>> class MyList(list): ... def __init__(self, li): ... super(MyList, self).__init__(li) ... >>> ml = MyList([1, 2, 3]) >>> ml [1, 2, 3] >>> len(ml) 3 >>> type(ml) <class '__main__.MyList'> 
list.__init__(self) also work when inheriting from list?
list might not be the next class in the MRO in such cases.I figured it out on my own: self is an instance of a subclass of list, so it can't be casted to list still being a MyList object.
self is a reference to an instance of MyList. Python doesn't have casting. See Martijn Pieter's answer.