On Sun, 15 Jul 2012 10:19:16 -0600, Ian Kelly wrote:
> On Sun, Jul 15, 2012 at 4:56 AM, Steven D'Aprano
> <steve+comp.lang.pyt...@pearwood.info> wrote:
>> (For the record, I can only think of one trap for the unwary: time
>> objects are false at *exactly* midnight.)
>
> Ugh, that's irritating. I can't think of any scenario where I would
> ever want the semantics "if timeval (is not midnight):".
Yes, it is a genuine gotcha. Time values are numbers, and zero is falsey,
so midnight is falsey even though it shouldn't be.
There's no good solution here, since we have a conflict between treating
time values as time values ("midnight is nothing special") and as numbers
("midnight == 0 which is falsey"). The only "solution" is to throw out
duck-typing of boolean values, which is tossing the baby out with the
bathwater -- bool duck-typing works fine for everything else.
> This
> reinforces the point that if you only want to test whether you have
> None, you should use "is not None" rather than relying on __bool__.
Often you should, but I didn't mention anything about testing for None.
Testing objects in a boolean context is more than just testing for None.
I have just written a bunch of code with about two dozen examples similar
to this:
for item in (seq or []):
do_something_with(item)
iterates over seq if it is non-empty, or the empty list. Writing it like
this would be more painful, more complex, less readable and less
idiomatic:
if seq is not None:
for item in seq:
do_something_with(item)
not to mention completely unnecessary if you have already checked that
seq is either None or a sequence, and not some other arbitrary value.
(If seq really could be any type at all, then an explicit identity test
against None is required.)
One of my favourites:
value = (dict or {}).get('key')
instead of:
value = None if dict is None else dict.get('key')
--
Steven
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