[Python-Dev] dict.addlist()

Raymond Hettinger python at rcn.com
Tue Jan 20 13:32:33 EST 2004

> > bookindex = dict()
> (What's wrong with {}?)

Nothing at all.  {} is shorter, faster, and everyone understands it.

When teasing out ideas at the interpreter prompt, I tend to use
dict() because I find it easier to edit the line and add some
initial values using dict(one=1, two=2, s='abc').

> works for me.  (I think setdefault() was already a minor mistake -- by
> the time I've realized it applies I have already written working code
> without it.  And when using setdefault() I always worry about the
> waste of the new empty list passed in each time that's ignored most
> times.)
> If you really want library support for this idiom ...

Not really.  It was more of a time machine question -- if we had
setdefault() to do over again, what would be done differently:

* Keep setdefault().
* Drop it and make do with get(), try/except, or if k in d.
* Martin's idea for dicts to have an optional factory function for
* Have a specialized method that just supports dicts of lists.

After all the discussions, the best solution to the defaulting problem
appears to be some variant of Martin's general purpose approach:

d = {}
for k, v in myitems:
	d[k].append(v)		# dict of lists

d = {}
for v in mydata:
	d[f(v)].add(v)		# partition into equivalence classes

d = {}
for v in mydata:
	d[k] += 1			# bag

Raymond Hettinger

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