Override 'and' and 'or'

[Kay Schluehr]

On Oct 7, 4:48 pm, Dekker <m.aschwan... at gmail.com> wrote:
> On 7 Okt., 16:19, Steven D'Aprano <st... at REMOVE-THIS-
>
> cybersource.com.au> wrote:
> > On Sun, 07 Oct 2007 13:52:15 +0000, Dekker wrote:
> > > Is it possible to override 'and' and/or 'or'?
>
> > Not without hacking the Python source code, in which case what you've got
> > is no longer Python.
>
> > Why do you want to do so?
>
> > --
> > Steven.
>
> Well I think it is not possible what I wanted to achieve. By
> overriding the "and" and "or" keyword I wanted to return a new object:
>
> SqlValueInt(4) and SqlValueInt(5) --> SqlOpAnd(SqlValueInt(4),
> SqlValueInt(5))
>
> This is only possible for: +, -, /, *, >, >=, ...
>
> Well... I have to live with the (binary) __and__, __or__ option and
> the user has to write:
>
> SqlValueInt(4) & SqlValueInt(5) --> SqlOpAnd(SqlValueInt(4),
> SqlValueInt(5))
>
> Thanks for your input, but __nonzero__ is not of any help in this
> case... I want to abuse the "magic" functions for some transformations
> and not some evaluation.
>
> Marco

You can see what "and" and "or" are actually doing:

import dis
dis.dis(lambda: x or y and z)

  1           0 LOAD_GLOBAL              0 (x)
              3 JUMP_IF_TRUE            11 (to 17)
              6 POP_TOP
              7 LOAD_GLOBAL              1 (y)
             10 JUMP_IF_FALSE            4 (to 17)
             13 POP_TOP
             14 LOAD_GLOBAL              2 (z)
        >>   17 RETURN_VALUE

Here you can see nicely that they are not implemented as specialized
opcodes but being compiled to jumps. This causes their lazy nature. If
"and" would be implemented as a normal ( eager ) operator a statement
like:

if l and l[0] == 2:
    BLOCK

would raise an IndexError if l is empty.