[Python-ideas] Why does `sum` use a default for the `start` parameter?

[Georg Brandl]

Ram Rachum schrieb:
>> Sometimes you might find that the list you're summing is empty. Because
>> 'sum' is most often used with numbers, the default sum of a list is 0.
>> If you want to sum a list of non-numbers, provide a suitable start
>> value. For example, to sum a list of lists a suitable start value is []:
>> 
>> >>> sum([[0, 1], [2, 3]], [])
>> [0, 1, 2, 3]
>> 
>> I agree that it would be nice if the start value could just be omitted,
>> but then what should 'sum' return if the list is empty?
> 
> 
> I see the problem. I think a good solution would be to tell the user, "If you
> want `sum` to be able to handle a non-empty list, you must supply `start`."
> Users that want to add up a (possibly empty) sequence of numbers will have to 
> specify `start`.
> 
> If start is supplied, it will work like it does now. If start isn't supplied, it 
> will add up all the elements without adding any `start` to them.
> 
> What do you think?

There is a choice between these two variants:

a) require start for non-numerical sequences
b) require start for possibly empty sequences

I don't have a preference for either, so for compatibility's sake I would
vote to keep the current one, which is a).  It also stands to reason that case b)

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