dictionary of dictionaries

[Marc 'BlackJack' Rintsch]

On Mon, 10 Dec 2007 23:51:00 -0800, kettle wrote:

> On Dec 10, 6:58 pm, Peter Otten <__pete... at web.de> wrote:
>> Well, there's also dict.setdefault()
>>
>> >>> pairs = ["ab", "ab", "ac", "bc"]
>> >>> outer = {}
>> >>> for a, b in pairs:
>>
>> ...     inner = outer.setdefault(a, {})
>> ...     inner[b] = inner.get(b, 0) + 1
>> ...>>> outer
>>
>> {'a': {'c': 1, 'b': 2}, 'b': {'c': 1}}
>>
>> and it's not hard to write your own defaultdict
>>
>> >>> class Dict(dict):
>>
>> ...     def __getitem__(self, key):
>> ...             return self.get(key, 0)
>> ...>>> d = Dict()
>> >>> for c in "abbbcdeafgh": d[c] += 1
>> ...
>> >>> d
>>
>> {'a': 2, 'c': 1, 'b': 3, 'e': 1, 'd': 1, 'g': 1, 'f': 1, 'h': 1}
>>
>> Peter
> 
> One last question.  I've heard the 'Explicit vs. Implicit' argument
> but this seems to boil down to a question of general usage case
> scenarios and what most people 'expect' for default behavior.  The
> above defaultdict implementation defining the __getitem__ method seems
> like it is more generally useful than the real default.  What is the
> reasoning behind NOT using this as the default implementation for a
> dict in python?

How's that more useful in the general case?  Maybe if you come from a
language where some default value pops up if the key is not present you
are used to write code in a way that exploits this fact.  But in the
general case!?  I need `defaultdict` not very often but want to know if a
key is not present in a dictionary.  Because most of the time that's a
special condition or error that has to be handled or signaled up the call
chain.

Ciao,
	Marc 'BlackJack' Rintsch