[Python-Dev] dict.addlist() (original) (raw)

Fred L. Drake, Jr. fdrake at acm.org
Tue Jan 20 10:11:07 EST 2004

Regarding d.setdefault(k, factory=list).append(v):

Moore, Paul writes:

How is it more expressive than d.setdefault(k, []).append(v)? As far as I can see, it's longer and contains an extra obscure(ish) term (factory).

The advantage I see is that it delays creation of the object being used as the default value; no list is created unless needed, whereas the setdefault() invocation shown always creates a new list, even if it's going to be thrown away. In many situations, if you're really using a list, this doesn't matter, but in other cases, especially if you're using a more complex object (or one with a more expensive constructor), using the factory makes a lot of sense.

The idea that "factory" is an obscure term for Python programmers is scary.

And I agree with the other posters that other defaults are often useful, and further complicating the dictionary interface isn't particularly helpful.

The basic setdefault() certainly remains useful on its own; I don't think anyone suggested otherwise.

My objection is the complication of the dictionary interface; it has a lot of methods now, and avoiding new names with highly specialized meanings is good.

-1 for addlist(k, v) -0 for setdefault(k, factory=...)

If conciseness is important,

def addlist(d, k, v):
    d.setdefault(k, []).append(v)

Yep. Or whatever variation makes sense in context.

-Fred

-- Fred L. Drake, Jr. PythonLabs at Zope Corporation

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