`math.sumprod` should support a `start` parameter like `sum` and `math.prod`

[cake-monotone]

Feature or enhancement

Proposal:

math.sumprod should support a start parameter.

# old
def sumprod(p, q, /)

# proposal
def sumprod(p, q, /, *, start=0)

Reason:

from __future__ import annotations

from typing import Any
from math import sumprod

class Foo:
    def __add__(self, other: Any) -> Foo:
        return Foo()

    def __mul__(self, other: Any) -> Foo:
        return Foo()

sumprod([Foo()], [Foo()])
# TypeError: unsupported operand type(s) for +: 'int' and 'Foo'

For now, math.sumprod starts its calculation from 0 by default, and there seems to be no way to customize this value.

I understand that alternatives like itertools.starmap or defining def __radd__(self, o) exist. But I believe sumprod should support a start parameter for consistency with sum and math.prod.

[cake-monotone]

Ahh, I made a mistake when creating this issue. It's not a bug—it should be labeled as type-feature

[rhettinger]

FWIW, it was an intentional decision to not have a start argument. The use cases are somewhat different than for sum or prod. See numpy's dot() function for comparison. Even in their highly numeric world, a start argument was not needed. Also, see the sumproduct() function in MS Excel which also does not have a start argument. Also, this function has already had a long life both as an itertools recipe and as production code in the more-itertools module. In neither case was a start argument warranted.

Another reason to exclude the start was the it would further complicate an already messy set of code paths that special case different input datatypes. If someone does want to use an exotic "numeric" object that won't add on to zero, it is really easy to just spell this out: sum([x*y for x, y in zip(p, q)], start=IdentityElement).

[encukou]

Thanks for this suggestion! But, for next time:

Ahh, I made a mistake when creating this issue. It's not a bug—it should be labeled as type-feature

You've also missed the issue template, which asks to discuss non-trivial feature requests on the forum first. GitHub issues aren't the right place for the initial discussion.