Question

I have been using python for a while now and Im happy using it in most forms but I am wondering which form is more pythonic. Is it right to emulate objects and types or is it better to subclass or inherit from these types. I can see advantages for both and also the disadvantages. Whats the correct method to be doing this?

Subclassing method

class UniqueDict(dict):
    def __init__(self, *args, **kwargs):
        dict.__init__(self, *args, **kwargs)

    def __setitem__(self, key, value):
        if key not in self:
            dict.__setitem__(self, key, value)
        else:
            raise KeyError("Key already exists")

Emulating method

class UniqueDict(object):
    def __init__(self, *args, **kwargs):
        self.di = dict(*args, **kwargs)

    def __setitem__(self, key, value):
        if key not in self.di:
            self.di[key] = value
        else:
            raise KeyError("Key already exists")

Answer 1

Key question you have to ask yourself here is:

“How should my class change if the ‘parent’ class changes?”

Imagine new methods are added to dict which you don’t override in your UniqueDict. If you want to express that UniqueDict is simply a small derivation in behaviour from dict‘s behaviour, then you’d go with inheritance since you will get changes to the base class automatically. If you want to express that UniqueDict kinda looks like a dict but actually isn’t, you should go with the ’emulation’ mode.