Why does this code pass a parameter to a constructor when there is no __init__ method?

Question

So, I am reading some code online and I came across the following class definition and I'm a little confused;

class MyClass(OrderedDict):
    def __hash__(self):
        return hash(tuple(self.iteritems()))

Elsewhere in the code there is the following line;

MyClass(my_OD)

Where my_OD is an ordered dictionary. My question is, how can you pass an argument to this class when there is no __init__ method? Where is this variable being assigned within the class? I'm coming from Java and I'm fairly certain that in Java you cannot pass an argument to a class without a constructor so this behavior is foreign to me.

Answer 1

The class MyClass inherits from OrderedDict:

class MyClass(OrderedDict):

Since MyClass doesn't have an __init__ method specified, it calls the init method of the OrderedDict class. So the my_OD argument to the constructor gets passed on to the OrderedDict. Btw, __init__ is not technically the constructor.

The purpose of this MyClass is to be an OrderedDict which computes the hash of its instances in a different way than OrderedDict does. Specifically, OrderedDict doesn't have a __hash__, that's defined on dicts and in that case, the hash is defined as None - so dicts are un-hashable. MyClass changes that adds a way to get the hash, while the rest of the functionality is the same OrderedDicts and dicts.