Proto-PEP: Overloadable Boolean Operators

[Michael Hudson]

aleaxit at yahoo.com (Alex Martelli) writes:

> Andrew Durdin <adurdin at gmail.com> wrote:
> 
> > On Tue, 07 Sep 2004 16:37:18 +1200, Greg Ewing
> > <greg at cosc.canterbury.ac.nz> wrote:
> > > Alex Martelli wrote:
> > > > Problem is, __nonzero__ is currently
> > > > typechecked -- it has to return an integer.
> > > 
> > > Yes, that's the problem. I should probably elaborate
> > > on that a bit in the PEP.
> > 
> > That is not the only issue with __nonzero__ versus __not__ -- in some
> > cases (e.g. the symbolic algebra or SQL query constructor) it is
> > useful to determine when an explicit "not" operator has been used.
> 
> Yes, you're right, and my assertion was flawed.  Typechecking is only
> part of the issue.
> 
> 
> > I'm not at a machine with the patch installed on it at the moment, but
> > I just began to whether this patch would have an effect on expressions
> > like (a < b < c) (which are also short-circuiting)...  Come to think
> > of it, how do objects which override __gt__ and the other comparisons
> > (particularly for expression construction) work in that case?
> 
> I believe a < b < c has exactly the semantics of [but with no doube
> evaluation of b...]:
>     (a < b) and (b < c)
> 
> Overriding _lt_ to give a print seems to confirm that:

dis.dis can be used to give the same impression:

>>> def f(a, b, c):
...     return a < b < c
... 
>>> def g(a, b, c):
...     return a < b and b < c
... 
>>> dis.dis(f)
  2           0 LOAD_FAST                0 (a)
              3 LOAD_FAST                1 (b)
              6 DUP_TOP             
              7 ROT_THREE           
              8 COMPARE_OP               0 (<)
             11 JUMP_IF_FALSE           10 (to 24)
             14 POP_TOP             
             15 LOAD_FAST                2 (c)
             18 COMPARE_OP               0 (<)
             21 JUMP_FORWARD             2 (to 26)
        >>   24 ROT_TWO             
             25 POP_TOP             
        >>   26 RETURN_VALUE        
             27 LOAD_CONST               0 (None)
             30 RETURN_VALUE        
>>> dis.dis(g)
  2           0 LOAD_FAST                0 (a)
              3 LOAD_FAST                1 (b)
              6 COMPARE_OP               0 (<)
              9 JUMP_IF_FALSE           10 (to 22)
             12 POP_TOP             
             13 LOAD_FAST                1 (b)
             16 LOAD_FAST                2 (c)
             19 COMPARE_OP               0 (<)
        >>   22 RETURN_VALUE        
             23 LOAD_CONST               0 (None)
             26 RETURN_VALUE        

Cheers,
mwh

-- 
  at any rate, I'm satisfied that not only do they know which end of
  the pointy thing to hold, but where to poke it for maximum effect.
                                  -- Eric The Read, asr, on google.com