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

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 keys.

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

Skip

More information about the Python-Dev mailing list