[Python-ideas] Start argument for itertools.accumulate() [Was: Proposal: A Reduce-Map Comprehension and a "last" builtin]

[Tim Peters]

Just a nit here:

[Tim]
>> ...
>> Arguing that it "has to do" something exactly the way `sum()` happens
>> to be implemented just doesn't follow - not even if they happen to
>> give the same name to an optional argument.  If the function were
>> named `accumulate_sum()`, and restricted to numeric types, maybe - but
>> it's not.

[Nick]
> When first added in 3.2, it did have that restriction, and the default
> behaviour without a function argument still parallels repeated use of
> the sum() builtin.

They're not quite the same today.  For example, if you have an object
`x` of a custom numeric class,

sum([x])

invokes x.__radd__ once (it really does do `x.__radd__(0)`), but

list(itertools.accumulate([x]))

just returns [x] without invoking x.__add__ or x.__radd__ at all.


> Extending the parallel to *also* include a parameter called "start"
> would create the expectation for me that that parameter would adjust
> the partial result calculations, without adding an extra result.
>
> Other parameter names (like the "first_result" I suggested in my reply
> to Guido) wouldn't have the same implications for me, so this
> objection is specific to the use of "start" as the parameter name, not
> to the entire idea.

Yup!  That's fine by me too - and preferable even ;-)