[Python-Dev] collections module

Raymond Hettinger python at rcn.com
Sat Jan 10 14:26:17 EST 2004

> I do believe that bag classes
> got into libraries only for academic reasons such as symmetry. In
> particular, they got into Smalltalk because of that reason, and
> everybody copies it because Smalltalk has it. This has to stop :-)

Some much for "everybody else has one" justification ;-)

> Both applications could be implemented if dictionaries had
> the notion of a default value, which would be generated
> by a callable (whether with or without argument is debatable)
> # count things
> d = {}
> d.defaultmaker = lambda k:0
> for t in things:
>    d[t] += 1
> # sort things by category
> d = {}
> d.defaultmaker = lambda k:[]
> for t in things:
>    d[category(t)].append(t)

These both read cleanly.

I really like this idea as a general purpose solution with broad
applicability.  It fully encapsulates the ideas behind bag construction
and building dicts of lists.

Instead of a method, it may be better to use a keyword argument in the

d = dict(default = lambda k:[])
for t in things:

If lambda is made parameterless, it allows type constructors to be used
for the most common cases:

dict(default=list)	# this is clear enough
dict(default=int)       # this may be too cute

Or, lambda could be avoided altogether by cloning a default value

dict(default=[])		# individual default values computed by

> Having a key in the lambda is probably better, as it would
> also allow lazily-filled dictionaries.

Can you give an example?

Raymond Hettinger

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