[Python-ideas] Implement __add__ for set and frozenset

Brandon Mintern bmintern at gmail.com
Tue Jun 3 04:28:41 CEST 2008

On Mon, Jun 2, 2008 at 10:06 PM, George Sakkis <george.sakkis at gmail.com> wrote:
> On Mon, Jun 2, 2008 at 9:28 PM, Brandon Mintern <bmintern at gmail.com> wrote:
>> I thought max was implemented using += (i.e. it usually starts at 0
>> and uses += on each item in the iterable). If so, implementing **
>> _iadd_ ** would result in exactly the code you posted. I realize that
>> I said __add__ in the first place, but __iadd__ is really what I
>> meant.
> No, it uses __add__:
> $ python -c "
> class Set(set): __iadd__=set.__ior__
> sum([Set([1]), Set([2])], Set())
> "
> Traceback (most recent call last):
>   File "<string>", line 3, in <module>
> TypeError: unsupported operand type(s) for +: 'Set' and 'Set'
> You can easily see the quadratic behavior of __add__:


Ouch. Never mind my idea then. I do find that rather strange, though.
It seems kind of strange to be able define an initial value but not
use it as an accumulator. This means that sum on list, mutable user
number classes, etc. is bound to be less efficient than it could be.
Perhaps a better proposal would be "change max to use __iadd__ if
available, falling back to __add__ if not", and then maybe we can
revisit this idea at that time. Honestly, what's wrong with sum being
defined as:

def sum (iterable, start=0):
   acc = start
   for i in iteratble:
      acc += i
   return acc

Even though I'm making a lot of bad proposals, I sure am learning a lot.


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