[Baypiggies] overriding boolean and,or operators?

[Stephen Lacy]

Ah, that Infix example is really cute, but not exactly what I was hoping
for.

I'm actually playing with a bit of trickery -- the __eq__() operator doesn't
need to return a boolean type (like I think __bool__() does). It can return
any object.  You can use this trick to make __eq__() into a factory method
and have it return an object that *represents* the comparison without
actually *doing* the comparison.  This is useful for things like ORM
mappers, where you take the Python expression, traverse the expression tree,
translate it into SQL, and then execute it.  I'm basing my ideas on the
implementation of Column comparators in SQLAlchemy, which you can see some
examples of here:
http://www.sqlalchemy.org/docs/sqlexpression.html#operators  But I'm not
working with SQL, I'm trying to do this for another query language.

I'm having trouble expressing "and" and "or" in a nice Pythonic way, and if
you look at the SQLAlchemy source, they use and_() and or_() methods, which
I'd like to try to avoid. I was hoping for some trick, akin to the Infix
thing you mentioned, but that would be a little more pythonic.  (And, I'm
surprised that you can override pretty much everything except for 'and' and
'or')

Here's a code example I wrote to get the juices flowing:

http://dpaste.com/217476/

Line 60 is where things get interesting, I have to use bitwise "and" instead
of boolean and, because as far as I can tell, there's no override for
boolean and....

Steve

On Mon, Jul 12, 2010 at 2:14 PM, Brent Pedersen <bpederse at gmail.com> wrote:

> On Mon, Jul 12, 2010 at 11:21 AM, Stephen Lacy <slacy at slacy.com> wrote:
> > Hi,
> >
> > According to http://docs.python.org/library/operator.html boolean
> operators
> > ==, <, >, etc. can be overridden via __eq__(), __lt__(), __gt__(), etc.
> >
> > They also mention *bitwise* and/or operators via __and__() and __or__().
> >
> > But, I'd like to override the *boolean* operators 'and' and 'or'.  Is
> this
> > possible?
> >
> > Background:
> >
> > I'm experimenting with a library that takes a python expression like
> this:
> >
> > "(a == b) or (c and d)"
> >
> > And instead of evaluating it via the interpreter, creates (and returns) a
> > representation of said boolean decision tree such that it can be
> evaluated
> > dynamically and/or converted to some other form, like prefix or postfix
> > (this is just an example and for my own exploration and deeper
> understanding
> > of operator overrides).
> >
> > I can get this to work if I use the syntax "(a == b) | (c & d)" but I
> would
> > prefer to use the more pythonic and syntactically correct "and" and "or".
> > Is this possible?
> >
> > Steve
> >
> > _______________________________________________
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> >
>
> not quite sure i follow your use-case, but...
> you can also override __nonzero__ which i think is what gets called
> when you use bool(),
> but you'll probably still need a bit more, you can add some sugar using
> this:
> http://code.activestate.com/recipes/384122-infix-operators/
>
> so you'd have your classes implement __bool_or__ (or whatever you want
> to call it) and then use:
>
> OR = Infix(lambda a, b: a.__bool_or__(b))
>
> MyClass(22) |OR| MyClass(0)
>
> where MyClass implements __bool_or__
>
> -brent
>
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