[Python-ideas] Programming recommendations (PEP 8) and boolean values

[Guido van Rossum]

I'd be strongly against changing that rule. If you don't want other types
than bool, use an isinstance check. Code using "is True" is most likely a
bug. (It's different for None, since that has no second value that is
assumed.)

--Guido

On Wednesday, August 8, 2012, Michael Foord wrote:

> Hey all,
>
> True and False are singletons, so if you want to check precisely for True
> and False then an identity check seems appropriate.
>
> However the "Programming Recommendations" section of PEP 8 has this to say
> on the topic:
>
> Don't compare boolean values to True or False using ==.
>
> Yes:   if greeting:
> No:    if greeting == True:
> Worse: if greeting is True:
>
> http://www.python.org/dev/peps/pep-0008/#programming-recommendations
>
> It seems to me that there is an important distinction between testing that
> an object is either the boolean True or False and merely checking the
> "truthiness" of an object. Many a bug has been caused by an empty container
> object (or some other falsey object) falling into an "if not value" clause
> that was actually meant to check for the presence of False or None.
>
> Why does PEP 8 recommend not testing "boolean values" (which to me implies
> "values you expect to be a bool") using equality or identity, and more
> specifically why does it say that using an identity check is worse than an
> equality check?
>
> As this is Python-ideas and not python-list, my specific suggestion is to
> modify the wording of that section - or just removing it altogether as I
> don't think it can be adequately clarified without using lots of words.
>
> All the best,
>
> Michael Foord
>
> --
>
> http://www.voidspace.org.uk/
>
> May you do good and not evil
>
> May you find forgiveness for yourself and forgive others
> May you share freely, never taking more than you give.
> -- the sqlite blessing http://www.sqlite.org/different.html
>
>
>

-- 
--Guido van Rossum (python.org/~guido)
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