trap setting attribute when the attribute is dict
yosuke at ccwf.cc.utexas.edu
yosuke at ccwf.cc.utexas.edu
Tue Sep 4 04:31:56 CEST 2007
Arnaud Delobelle <arnodel at googlemail.com> wrote:
> On Sep 3, 8:47 pm, <yos... at ccwf.cc.utexas.edu> wrote:
> [...]
>> My intention was to have a propery 'sum' in my object, and which has sum
>> of all the values() of the dict (i have code to make sure that the value
>> of dict are all numeric). I could just the propery being calculated
>> everytime the property got __getattr__. But I thought that it was
>> inefficient to calculate the same number over and over when the value is
>> already known. So I was thiking of calculating the number only when the
>> dict got modified.
>>
>> I also experimented with the idea of subclassing dict itself to
>> calculate the sum. Is this what would be better solution?
> Why not simply have a method to update the dictionary that also keeps
> the sum up to date? Something like that:
>>>> class MyObj(object):
> ... def __init__(self):
> ... self._d = {}
> ... self._sum = 0
> ... def set_key(self, key, val):
> ... self._sum += val - self._d.get(key, 0)
> ... self._d[key] = val
> ... def get_key(self, key):
> ... return self._d[key]
> ... def sum(self):
> ... return self._sum
> ...
>>>> a = MyObj()
>>>> a.set_key(1, 2)
>>>> a.sum()
> 2
>>>> a.set_key('a', 10)
>>>> a.set_key(1, 5)
>>>> a.sum()
> 15
>>>>
> Of course this is only worth it if you need to use the sum often
> enough.
> If you update the dictionary a lot but only need the sum from time to
> time, then it might not be worth it at all.
> Of course you could subclass dict:
> class MyDict(dict):
> def __init__(self, *args, **kwargs):
> self._sum = sum(self.itervalues())
> def __setitem__(self, key, val):
> self._sum += val - self.get(key, 0)
> dict.__setitem__(self, key, val)
> def sum(self):
> return self._sum
> # overload all other methods that mutate the dict
> # to keep _sum up to date
>>>> d = MyDict()
>>>> d.sum()
> 0
>>>> d['a']=5
>>>> d.sum()
> 5
>>>> d['b']=10
>>>> d['a']=8
>>>> d.sum()
> 18
>>>>
Thank you. I will go with subclassing method.
--
yosuke kimura
Center for Energy and Environmental Resources
The Univ. of Texas at Austin, USA
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