[Python-Dev] defaultdict proposal round three

Brett Cannon brett at python.org
Tue Feb 21 00:04:57 CET 2006

On 2/20/06, Steven Bethard <steven.bethard at gmail.com> wrote:
> I wrote:
> >    # I want to do ``dd[item] += 1``
> Guido van Rossum wrote:
> > You don't need a new feature for that use case; d[k] = d.get(k, 0) + 1
> > is perfectly fine there and hard to improve upon.
> Alex Martelli wrote:
> > I see d[k]+=1 as a substantial improvement -- conceptually more
> > direct, "I've now seen one more k than I had seen before".
> Guido van Rossum wrote:
> > Yes, I now agree. This means that I'm withdrawing proposal A (new
> > method) and championing only B (a subclass that implements
> > __getitem__() calling on_missing() and on_missing() defined in that
> > subclass as before, calling default_factory unless it's None).
> Probably already obvious from my previous post, but FWIW, +1.
> Two unaddressed issues:
> * What module should hold the type?  I hope the collections module
> isn't too controversial.
> * Should default_factory be an argument to the constructor?  The three
> answers I see:
>   - "No."  I'm not a big fan of this answer.  Since the whole point of
> creating a defaultdict type is to provide a default, requiring two
> statements (the constructor call and the default_factory assignment)
> to initialize such a dictionary seems a little inconvenient.
>   - "Yes and it should be followed by all the normal dict constructor
> arguments."  This is okay, but a few errors, like
> ``defaultdict({1:2})`` will pass silently (until you try to use the
> dict, of course).
>   - "Yes and it should be the only constructor argument."  This is my
> favorite mainly because I think it's simple, and I couldn't think of
> good examples where I really wanted to do ``defaultdict(list,
> some_dict_or_iterable)`` or ``defaultdict(list,
> **some_keyword_args)``.  It's also forward compatible if we need to
> add some of the dict constructor args in later.

While #3 is my preferred solution as well, it does pose a Liskov
violation if this is a direct dict subclass instead of storing a dict
internally (can't remember the name of the design pattern that does
this).  But I think it is good to have the constructor be different
since it does also help drive home the point that this is not a
standard dict.


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