[Python-Dev] Re: PEP 285: Adding a bool type (original) (raw)

M.-A. Lemburg mal@lemburg.com
Tue, 02 Apr 2002 20:49:41 +0200

I haven't followed this thread for quite some time, but since the PEP still seems alive, let me add some experience I've had with using the already existing singletons (Py_True and Py_False) in Python for recognizing truth values.

Py_True and Py_False can be exposed in Python via

True = (1==1) False = (1!=1)

and most comparison operations and quite a few other APIs returning truth values make use of these singletons.

I thought it would be a good idea to use those two singletons for protocols which use booleans such as XML-RPC. My experiences with this approach are, well, not so good :-/

The reason is that True and False are really integers and not of a special new type. Now they are singletons, which is good, since they represent the only states a boolean can have and in many cases work reasonably well as boolean representative, but not always (e.g. arithmetic operations such as True - True == False). Also, when trying to recognize the two singletons you have to use the "is" comparison -- "==" will fail to differentiate between 1 and True...

def isboolean(x): return x in (True, False)

...doesn't work...

def isboolean(x): return (x is True) or (x is False)

..does.

As a conclusion, I think it would be better to make bool() a new type which does not inherit from integers, but which does know how deal with other types which are commonly used together with booleans such as integers. However, the type should implement boolean algebra and not try to mimic integer arithemtic, i.e. True - True raises an exception.

Py_True and Py_False should then be made singletons of this new type (possibly after a transition phase which replaces the singletons with a version that doesn't raise exceptions but instead issues warnings).

This may sound painful at first, but in the long run, I believe, it'll result in the same benefits as other painful changes have or will (e.g. the change from integer division to floating point division).

-- Marc-Andre Lemburg CEO eGenix.com Software GmbH

Company & Consulting: http://www.egenix.com/ Python Software: http://www.egenix.com/files/python/