[Python-Dev] Proposal: defaultdict

skip at pobox.com skip at pobox.com
Fri Feb 17 16:05:27 CET 2006

    Guido> Over lunch with Alex Martelli, he proposed that a subclass of
    Guido> dict with this behavior (but implemented in C) would be a good
    Guido> addition to the language.

Instead, why not define setdefault() the way it should have been done in the
first place?  When you create a dict it has the current behavior.  If you
then call its setdefault() method that becomes the default value for missing

    d = {'a': 1}'
    d['b']              # raises KeyError
    d.get('c')          # evaluates to None
    d['b']              # evaluates to 42
    d.get('c')          # evaluates to 42

For symmetry, setdefault() should probably be undoable: deldefault(),
removedefault(), nodefault(), default_free(), whatever.

The only question in my mind is whether or not getting a non-existent value
under the influence of a given default value should stick that value in the
dictionary or not.

down-with-more-builtins-ly, y'rs,


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