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suppressing import errors

David Riley fraveydank at gmail.com
Tue Nov 15 16:20:33 EST 2011 
On Nov 15, 2011, at 3:01 PM, Chris Angelico wrote:

> On Wed, Nov 16, 2011 at 6:39 AM, David Riley <fraveydank at gmail.com> wrote:
>> True, and that does avoid polluting namespace.  However, you shouldn't be testing for None as a bool; you should instead do an "if <module> is None:" (or, of course, "is not None").
> 
> Why not? Is there some other way for the module object to evaluate as false?

Well, probably not.  It was my understanding that "None" evaluating to a Boolean false was not necessarily guaranteed; I've even gotten some warnings from Python to that effect, though I can't recall the context in which that happened.  In any case, PEP 8 states:

      Comparisons to singletons like None should always be done with
      'is' or 'is not', never the equality operators.

      Also, beware of writing "if x" when you really mean "if x is not None"
      -- e.g. when testing whether a variable or argument that defaults to
      None was set to some other value.  The other value might have a type
      (such as a container) that could be false in a boolean context!

Obviously, that last bit doesn't apply to modules; they're not going to evaluate as False in general.  I just bristle when I see people writing "if x" when they really mean "if x is not None", perhaps because it's not The Right Way(tm)?  It mostly comes down to aesthetics, I guess.  Write what you really mean.

- Dave
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