[Python-Dev] Rationale for sum()'s design?

[Nick Coghlan]

Guido van Rossum wrote:
> I think the conclusion should be that sum() is sufficiently
> constrained by backwards compatibility to make "fixing" it impossible
> before 3.0. But in 3.0 I'd like to fix it so that the 2nd argument is
> only used for empty lists.

Two questions about this:

1. When omitting the second argument, would supplying an empty list return 0, 
None or raise an exception?

The last seems most reasonable, as otherwise the function is guessing about what 
the programmer wants.

2. How would the initial value that forms the basis of summation be built for 
non-empty sequences?

The first element can't be used directly, as that would mutate the first element 
for a sequence of lists or other mutable objects.

Using the default constructor for the type of the first element in the iterable 
has its own problem. With the second argument being ignored for non-empty 
iterables, it makes it impossible to use sum() with classes that do have an 
additive identity, but it isn't created by the default constructor. At the 
moment, such classes can be used by supplying the additive identity as the 
second argument to sum().

Both of the above cases can be supported by an approach that says:

a. If a second argument is not supplied, and the iterable is empty, raise an 
exception.

b. If a second argument is not supplied, and the iterable is not empty, use the 
default constructor of the type of the first argument as the initial value

c. If a second argument is supplied, and the iterable is empty, return the 
second argument.

d. If a second argument is supplied, and the iterable is not empty, use the 
second argument as the initial value.

This scheme can be made backwards compatible with the current sum() by switching 
point a. from 'raise an exception' to 'return zero'.

With the above compatibility change, getting all the way back to the existing 
sum() behaviour only requires changing point b. to say "use zero as the initial 
value"

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at email.com   |   Brisbane, Australia
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