Inefficient summing

[Terry Reedy]

Matt Nordhoff wrote:
> Chris Rebert wrote:
>> I personally would probably do:
>>
>> from collections import defaultdict
>>
>> label2sum = defaultdict(lambda: 0)
> 
> FWIW, you can just use:
> 
> label2sum = defaultdict(int)
> 
> You don't need a lambda.

Indeed, in this case, with two known keys, the defaultdict is not needed 
either, since the following should work as well to initialize

label2sum = {'F1':0,'F2':0}

>> for r in rec:
>>     for key, value in r.iteritems():
>>         label2sum[key] += value
>>
>> ratio = label2sum["F1"] / label2sum["F2"]