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

[Ram Rachum]

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.