[Python-ideas] Why does `sum` use a default for the `start` parameter?
Ram Rachum
cool-rr at cool-rr.com
Sat Dec 5 19:42:31 CET 2009
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])
2. A smaller problem: The user has to actually create that zero object now, and
for some objects the definition might be lengthy, adding needless complexity to
the code.
Also, using the `start` has some overhead, for creating the zero object and
calling __add__.
Ram.
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