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

Georg Brandl g.brandl at gmx.net
Sat Dec 5 21:59:36 CET 2009

Vitor Bosshard schrieb:
> 2009/12/5 Ram Rachum <cool-rr at cool-rr.com>:
>> MRAB <python at ...> writes:
>>> > I prefer (b). The problem with requiring `start` for sequences of non-
>> numerical
>>> > objects is that you now have to go out and create a "zero object" of the
>> same
>>> > type as your other objects. The object class might not even have a concept
>> of a
>>> > "zero object".
>>> >
>>> If the objects can be summed, shouldn't there also be a zero object?
>>> Does anyone have an example when that's not possible?
>> You're right MRAB, probably almost every object type that has a concept of
>> "addition" will have a concept of a zero element.
>> BUT, that zero object has to be created by the user of `sum`, and that has two
>> problems:
>> 1. The user might not know from beforehand which type of object he's adding.
>> Even within the same type there might be problems. What happens when the user is
>> using `sum` to add a bunch of vectors, and he doesn't know from beforehand what
>> the dimensions of the vectors are? How will he know if his zero element should
>> be Vector([0, 0]) or Vector([0, 0, 0])
> Ugly, but works:
> itr = iter(sequence)
> sum(itr, itr.next())

Or, for sequences:

sum(islice(seq, 1), seq[0])

which clearly communicates the need for a non-empty sequence.


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