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 | [EMAIL PROTECTED] | Brisbane, Australia
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http://boredomandlaziness.skystorm.net
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