dict.setdefault() (Patch#101102) (was: Re: [Python-Dev] Re: A small proposed change to dictionaries' "get" method...)

[Greg Ewing]

artcom0!pf@artcom-gmbh.de:
>	dict.setdefault('key', [])
>	dict['key'].append('bar')

I would agree with this more if it said

   dict.setdefault([])
   dict['key'].append('bar')

But I have a problem with all of these proposals: they require
implicitly making a copy of the default value, which violates
the principle that Python never copies anything unless you
tell it to. The default "value" should really be a thunk, not
a value, e.g.

   dict.setdefault(lambda: [])
   dict['key'].append('bar')

or

   dict.get_or_add('key', lambda: []).append('bar')

But I don't really like that, either, because lambdas look
ugly to me, and I don't want to see any more builtin
constructs that more-or-less require their use.

I keep thinking that the solution to this lies somewhere
in the direction of short-circuit evaluation techniques and/or
augmented assignment, but I can't quite see how yet.

Greg Ewing, Computer Science Dept, +--------------------------------------+
University of Canterbury,	   | A citizen of NewZealandCorp, a	  |
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greg@cosc.canterbury.ac.nz	   +--------------------------------------+