[Baypiggies] overriding boolean and,or operators?

[Jeremy Fishman]

Wow, thanks.  It's neat to see the difference between

>>> dis.dis(compile('x if y else z', 'eval', 'eval'))
  1           0 LOAD_NAME                0 (y)
              3 JUMP_IF_FALSE            5 (to 11)
              6 POP_TOP
              7 LOAD_NAME                1 (x)
             10 RETURN_VALUE
        >>   11 POP_TOP
             12 LOAD_NAME                2 (z)
             15 RETURN_VALUE
>>> dis.dis(compile('y and x or z', 'eval', 'eval'))
  1           0 LOAD_NAME                0 (y)
              3 JUMP_IF_FALSE            7 (to 13)
              6 POP_TOP
              7 LOAD_NAME                1 (x)
             10 JUMP_IF_TRUE             4 (to 17)
        >>   13 POP_TOP
             14 LOAD_NAME                2 (z)
        >>   17 RETURN_VALUE

in byte code.  Now if only I weren't stuck in 2.4.3 at work...

  - Jeremy


On Mon, Jul 12, 2010 at 4:48 PM, Aahz <aahz at pythoncraft.com> wrote:

> On Mon, Jul 12, 2010, Alex Martelli wrote:
> >
> > `and` and `or` cannot be overridden; they always perform their
> > short-circuit evaluation functionality, no matter what the types on
> > either side.  I understand your surprise, because you think of them as
> > operator, but they're like other keywords such as `lambda`, `if`,
> > `else`... they **control what gets executed, when, and whether the
> > execution happens at all in a certain circumstance**.
>
> Just to emphasize Alex's point, this is a necessary part of the boolean
> short-circuit capability that allows stuff like this:
>
>    if a is not None and a.valid():
>
> (assume that a contains a method called "valid" if it's not None -- but
> there's no getattr() for "valid" unless a is not None)
> --
> Aahz (aahz at pythoncraft.com)           <*>
> http://www.pythoncraft.com/
>
> "....Normal is what cuts off your sixth finger and your tail..."  --Siobhan
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