[Python-Dev] Proposal: defaultdict (original) (raw)

Jack Diederich jack at performancedrivers.com
Fri Feb 17 17:19:32 CET 2006

On Thu, Feb 16, 2006 at 01:11:49PM -0800, Guido van Rossum wrote: [snip]

Google has an internal data type called a DefaultDict which gets passed a default value upon construction. Its getitem method, instead of raising KeyError, inserts a shallow copy (!) of the given default value into the dict when the value is not found. So the above code, after

d = DefaultDict([]) can be written as simply d[key].append(value) Note that of all the possible semantics for getitem that could have produced similar results (e.g. not inserting the default in the underlying dict, or not copying the default value), the chosen semantics are the only ones that makes this example work.

Having getitem insert the returned default value allows it to work with a larger variety of classes. My own ForgivingDict does not do this and works fine for ints and lists but not much else.

fd = ForgivingDict(list) fd[key] += [val] # extends the list and does a setitem

The += operator isn't useful for dicts.

How can you make a defaultdict with a defaultdict as the default? My head asploded when I tried it with the constructor arg. It does seem possible with the 'd.default = func' syntax

empty defaultdict constructor

d = defaultdict() d.default = d tree = defaultdict() tree.default = d.copy

-jackdied

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