sum and strings

[Paddy]

I was browsing the Voidspace blog item on "Flattening Lists", and
followed up on the use of sum to do the flattening.
A solution was:

>>> nestedList = [[1, 2], [3, 4], [5, 6]]
>>> sum(nestedList,[])
[1, 2, 3, 4, 5, 6]

I would not have thought of using sum in this way. When I did help(sum)
the docstring was very number-centric: It went further, and precluded
its use on strings:

>>> help(sum)
Help on built-in function sum in module __builtin__:

sum(...)
    sum(sequence, start=0) -> value

    Returns the sum of a sequence of numbers (NOT strings) plus the
value
    of parameter 'start'.  When the sequence is empty, returns start.

The string preclusion would not help with duck-typing (in general), so
I decided to consult the ref doc on sum:

sum( sequence[, start])

Sums start and the items of a sequence, from left to right, and returns
the total. start defaults to 0. The sequence's items are normally
numbers, and are not allowed to be strings. The fast, correct way to
concatenate sequence of strings is by calling ''.join(sequence). Note
that sum(range(n), m) is equivalent to reduce(operator.add, range(n),
m) New in version 2.3.


The above was a lot better description of sum for me, and with an
inquisitive mind, I like to think that I might have come up with using
sum to flatten nestedList :-)
But there was still that warning about using strings.

I therefore  tried sum versus their reduce "equivalent" for strings:

>>> import operator
>>> reduce(operator.add, "ABCD".split(), '')
'ABCD'
>>> sum("ABCD".split(), '')
Traceback (most recent call last):
  File "<interactive input>", line 1, in ?
TypeError: sum() can't sum strings [use ''.join(seq) instead]
>>>

Well, after all the above, there is a question:

  Why not make sum work for strings too?

It would remove what seems like an arbitrary restriction and aid
duck-typing. If the answer is that the sum optimisations don't work for
the string datatype, then wouldn't it be better to put a trap in the
sum code diverting strings to the reduce equivalent?

Just a thought,

- Paddy.