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 _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
