En Mon, 05 May 2008 11:08:02 -0300, Szabolcs <[EMAIL PROTECTED]> escribió:
On May 5, 12:24 pm, Duncan Booth <[EMAIL PROTECTED]> wrote:
Szabolcs <[EMAIL PROTECTED]> wrote:
> On May 5, 9:37 am, Szabolcs Horvát <[EMAIL PROTECTED]> wrote:
>> Gabriel Genellina wrote:
>> > Python doesn't require __add__ to be associative, so this should
>> > not be
> used as a general sum replacement.
>> It does not _require_ this, but using an __add__ that is not
>> commutative and associative, or has side effects, would qualify as a
>> serious misuse, anyway.
> Sorry, I meant associative only, not commutative.
Unfortunately an __add__ which is not associative is actually perfectly
reasonable.
Well, 'reasonable' is a subjective notion in this case :-) I have
never seen a non-associative use of the plus sign in maths, so I would
tend to take associativity for granted when seeing a + b + c in code,
therefore I would avoid such uses. (Multiplication signs are a
different story.) Of course others may feel differently about this,
and technically there's nothing in the way of using a non-associative
__add__! :)
You're approaching Python from a mathematical point of view, I presume?
Python has a perfectly defined meaning for a + b + c: simply, expressions
are evaluated from left to right
<http://docs.python.org/ref/evalorder.html>.
a+b+c is the same as (a+b)+c and *not* the same thing as a+(b+c).
Parenthesis aren't required in the first case, not because addition is
associative, but because evaluation order is clearly defined.
__add__, __mul__ and all other special methods can be used (and in fact,
*are* used) for many kind of objects, not just numbers; the Tree example
by Duncan Booth is a perfectly valid application. (Of course this usage
can be abused, as any other feature, but it's not so common)
--
Gabriel Genellina
--
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