[Python-ideas] Please reconsider the Boolean evaluation of midnight

[Stephen J. Turnbull]

Ethan Furman writes:
 > On 03/05/2014 07:42 AM, Paul Moore wrote:
 > > On 5 March 2014 15:08, Yann Kaiser <kaiser.yann at gmail.com> wrote:
 > >>   Let me show you an actual zero:
 > >>
 > >>      if event.num_attendants:
 > >>          prepare_cake()
 > >>      else:
 > >>          cancel_event()

 > > I'm sorry, are you really trying to say that the above code is better than
 > >
 > >      if event.num_attendants != 0:
 > >          prepare_cake()
 > >      else:
 > >          cancel_event()
 > >
 > > ?

 > Yes.  This is Python.  blah blah blah != 0 is unnecessary boiler-plate.

I wouldn't call that *better*, just a different style.  I agree I
prefer "if blah.count:" to "if blah.count != 0:", but I might very
well write "if blah.count > 0:".  I don't see anything "wrong" with
using an explicit comparison, but I don't have a problem reading the
abbreviated style in this situation.[1]

Personally I would *always* use the "implied boolean" style for
emptiness tests (I'm an old Lisper), but I might very well use
explicit comparisons against zero.  For example, I would be likely to
write the party example with a reversed test (to keep 'if' and 'else'
close together):

    if event.num_attendants == 0:    # nobody's coming
        cancel_event()
    else:
        # several-pages-long suite of preparations
        prepare_cake()

and I think that looks better than

    if not event.num_attendants:     # it's a count, not a boolean
    # etc

But actually I'd probably design the event class differently, and
write it as:

    if not event.list_of_attendants: # attendance list is empty
    # etc

or perhaps

    if event.list_of_attendants:     # attendance list is non-empty
    # reverse the suites

(depending on the phase of the moon and whether the trout are running
in Esopus Creek to choose between the last two :-).


Footnotes: 
[1]  I do have a real issue with the "if x:" to test for a sentinel
meaning "uninitialized" that happens to evaluate to false.  There's
objectively a probability that it would pass an error that would be
caught if the "is None" form were used, but that probability is
admittedly low.  Tastes may differ regarding "importance" here.