[Python-Dev] ANN: PEP 335: Overloadable Boolean Operators (original) (raw)

Phillip J. Eby pje at telecommunity.com
Fri Sep 10 20🔞08 CEST 2004

At 12:46 AM 9/9/04 -0700, Michel Pelletier wrote:

> Message: 4 > Date: Fri, 10 Sep 2004 12:58:23 +1200 > From: Greg Ewing <greg at cosc.canterbury.ac.nz> > Subject: [Python-Dev] ANN: PEP 335: Overloadable Boolean Operators > To: python-dev at python.org > Message-ID: > <200409100058.i8A0wNIV002743 at cosc353.cosc.canterbury.ac.nz> > > > Python does not currently provide any 'xxx' special methods > corresponding to the 'and', 'or' and 'not' boolean operators.

I like the PEP with 'and' and 'or', but isn't the 'not' special method essentially the inverse of nonzero?

There isn't such a method currently. Also, note that the expression 'not x' is currently guaranteed to return a boolean value. The purpose of the PEP is to allow 'not x' to potentially return an arbitrary object, as for use in algebraic and query systems that want to use Python code as their syntax. Such systems currently use e.g. '~x' instead of 'not x' because the former allows return of arbitrary objects.

IMO, the algebraic/query use cases would be better served by some sort of "code literal" or "AST literal" syntax, rather than adding more special methods. The reason is that all too often you want to include "normal" Python values in such an expression, but still manipulate them symbolically, or have some other sort of special treatment. A literal syntax for Python expressions is more useful for this, which is why I've moved to using strings and the parser module to accomplish such processing. At that level, boolean operator methods are moot.

(Code literals would be useful primarily in the ability to have them parsed and syntax checked at import time, rather than waiting until runtime. This consideration also applies to PEP 335, but PEP 335 may consume all of its compilation performance gains by losing runtime performance at all boolean operation sites.)

But anyway, I digress. Since PEP 335 doesn't significantly help (IMO) with algebraic and query systems, that leaves the numeric use cases, which I don't have enough experience to comment on.

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