A Bug By Any Other Name ...

rwwh rob.hooft at gmail.com
Thu Jul 16 11:25:27 CEST 2009


On Jul 7, 2:00 pm, Steven D'Aprano <st... at REMOVE-THIS-
cybersource.com.au> wrote:
> On Mon, 06 Jul 2009 22:18:20 -0700, Chris Rebert wrote:
> >> Not so rare. Decimal uses unary plus. Don't assume +x is a no-op.
> [...]
> > Well, yes, but when would you apply it twice in a row?
>
> My point was that unary + isn't a no-op, and therefore neither is ++. For
> Decimal, I can't think why you'd want to apply ++x, but for other
> objects, who knows?
>
> Here's a toy example:
>
> >>> class Spam(str):
>
> ...     def __pos__(self):
> ...         return self.__class__("spam " + self)
> ...>>> s = Spam("")
> >>> ++++s
>
> 'spam spam spam spam '

Here's another toy example:

class Toy(int):
    def __init__(self, value):
        self._incrd = False
        int.__init__(self, value)

    def incrHalf(self):
        self._incrd = True

    def __pos__(self):
        if self._incrd:
            return self.__class__(self+1)
        else:
            p = self.__class__(self)
            p.incrHalf()
            return p

    def __add__(self, other):
        return self.__class__(int(self)+other)

nows122[126]~% python -i toy.py
>>> i=Toy(5)
>>> +i
5
>>> ++i
6
>>> +++i
6
>>> +i++i
10
>>> +(+i++i)
10
>>> (++i)++i
11



More information about the Python-list mailing list