Pre-PEP: Dictionary accumulator methods - typing & initialising

Matteo Dell'Amico della at
Sun Mar 20 16:11:55 CET 2005

Kay Schluehr wrote:

> Why do You set
> d.defaultValue(0)
> d.defaultValue(function=list)
> but not
> d.defaultValue(0)
> d.defaultValue([])
> ?

I think that's because you have to instantiate a different object for 
each different key. Otherwise, you would instantiate just one list as a 
default value for *all* default values. In other words, given:

class DefDict(dict):
     def __init__(self, default):
         self.default = default
     def __getitem__(self, item):
             return dict.__getitem__(self, item)
         except KeyError:
             return self.default

you'll get

In [12]: d = DefDict([])

In [13]: d[42].extend(['foo'])

In [14]: d.default
Out[14]: ['foo']

In [15]: d[10].extend(['bar'])

In [16]: d.default
Out[16]: ['foo', 'bar']

In [17]: d[10]
Out[17]: ['foo', 'bar']

In [18]: d[10] is d.default
Out[18]: True

and this isn't what you really wanted.

By the way, to really work, I think that Duncan's proposal should create 
new objects when you try to access them, and to me it seems a bit 
counterintuitive. Nevertheless, I'm +0 on it.

> And why not dict(type=int), dict(type=list) instead where default
> values are instantiated during object creation? A consistent pythonic
> handling of all types should be envisioned not some ad hoc solutions
> that go deprecated two Python releases later.

I don't really understand you. What should 'type' return? A callable 
that returns a new default value? That's exactly what Duncan proposed 
with the "function" keyword argument.


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