Pre-PEP: Dictionary accumulator methods

Alexander Schmolck a.schmolck at gmx.net
Sun Mar 20 02:26:41 CET 2005


"Raymond Hettinger" <vze4rx4y at verizon.net> writes:

> The rationale is to replace the awkward and slow existing idioms for dictionary
> based accumulation:
>
>     d[key] = d.get(key, 0) + qty
>     d.setdefault(key, []).extend(values)
>
> In simplest form, those two statements would now be coded more readably as:
>
>    d.count(key)
>    d.appendlist(key, value)

Yuck.

The relatively recent "improvement" of the dict constructor signature
(``dict(foo=bar,...)``) obviously makes it impossible to just extend the
constructor to ``dict(default=...)`` (or anything else for that matter) which
would seem much less ad hoc. But why not use a classmethod (e.g.
``d=dict.withdefault(0)``) then?

Or, for the first and most common case, just a bag type?


'as



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