Boolean tests [was Re: Attack a sacred Python Cow]

[Carl Banks]

On Jul 29, 7:30 pm, Steven D'Aprano <st... at REMOVE-THIS-
cybersource.com.au> wrote:
> On Tue, 29 Jul 2008 10:30:43 -0700, Carl Banks wrote:
> > On Jul 29, 5:15 am, Heiko Wundram <modeln... at modelnine.org> wrote:
> >> I can't dig up a simple example from code I wrote quickly, but because
> >> of the fact that explicit comparisons always hamper polymorphism
>
> > I'm not going to take your word for it.  Do you have code that
> > demonstrates how "if x" improves polymorphism relative to simple
> > explicit tests?
>
> On the rapidly decreasing chance that you're not trolling (looking more
> and more unlikely every time you post):
>
> # The recommended way:
> if x:
>     do_something
>
> # Carl's so-called "simple explicit tests" applied to polymorphic code:

No, the following isn't my way.


> try:
>     # could be a sequence or mapping?
>     # WARNING: must do this test *before* the number test, otherwise
>     # "if [] != 0" will return True, leading to the wrong branch being
>     # taken.
>     if len(x) != 0:
>         do_something
> except AttributeError:
>     # not a sequence or mapping, maybe it's a number of some sort
>     try:
>         int(x)
>     except TypeError:
>         # not convertable to numbers
>         # FIXME: not really sure what to do here for arbitrary types
>         # so fall back on converting to a boolean, and hope that works
>         if bool(x):
>             do_something
>     else:
>         if x != 0:
>               do_something

I say that you will never, ever have to do this because there isn't a
do_something that's actually useful for all these types.

Ok, tell me, oh indignant one, what is do_something?  What could
possibly be the contents of do_something such that it actually does
something useful?  If you can tell me what do_something is you will
have answered my question.


Carl Banks