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

Thu Mar 6 10:34:32 CET 2014

On 6 March 2014 09:32, M.-A. Lemburg <mal at egenix.com> wrote:

> On 06.03.2014 09:46, Donald Stufft wrote:
> >
> > On Mar 6, 2014, at 3:35 AM, M.-A. Lemburg <mal at egenix.com> wrote:
> >
> >> On 06.03.2014 03:23, Donald Stufft wrote:
> >>>
> >>> On Mar 5, 2014, at 9:15 PM, Tim Peters <tim.peters at gmail.com> wrote:
> >>>
> >>>> [Donald Stufft]
> >>>>> When the documented behavior is both nonsensical and the cause of
> hard to debug bugs that
> >>>>> is a pretty compelling use case to me, unless you actively enjoy
> being user hostile.
> >>>>
> >>>> Breaking code that currently works is as hostile as hostile gets.
> Where is the evidence that
> >>>> no code relies on the current behavior? For that matter, where is the
> evidence that the
> >>>> current behavior is a significant cause of bugs?  I know I've used
> "if some_time_object:",
> >>>> but it did exactly what I expected it to do.
> >>>
> >>> Forgive me if I'm wrong, but aren't you the author of the date time
> module? If that's the case
> >>> what you expect it to do isn't particularly relevant as you're
> intimately aware of it's
> >>> implementation.
> >>
> >> Just in case you're looking for a relevant use case:
> >>
> >> Having bool(time(0,0,0)) allows you to quickly check whether you are
> >> dealing with a datetime value with (non-trivial) time part or not.
> >
> > I don't see how midnight is any more or less trivial than 12:01.
> >
> >>
> >> You run into situations where you have to test for this in
> >> date/time conversions, arithmetics, etc. e.g. if you need to convert
> >> a datetime value to a date and want to prevent truncation of
> >> information, or if you want to format a datetime value in a user
> >> friendly way by omitting the zero time part, or if you're doing
> >> datetime arithmetic that has to follow special requirements w/r to
> >> days (think interest or insurance math) and you want to
> >> quickly check whether you can use the fast path simple
> >> calculation, or you need to do the full blown complicated part.
> >
> > I think these would be better served by actually checking for what you
> mean.
> > For the record I also think that checking for a date time to not be None
> > is also better served by actually checking for what you mean. The
> > difference is in intent and likelihood of confusion.
>
> FWIW, I expect bool(time(0,0,0)) == False, just like I expect
> bool(0.0) == False. I would find it confusing to have
> bool(zero_element) == True.
>
> > For all of those you're going to be explicitly testing and working with
> > midnight objects, so you're going to test them, you're going to exercise
> > that code. In the ``is None`` case, Midnight is just another value so
> that
> > bug will lay dormant and undetected until someone just happens to
> > hit an unlucky time. And once you get that it's likely to be difficult to
> > reproduce.
>
> datetime values with zero time are *very* common in practice
> (you run into them whenever you convert a date value into a datetime
> value), so the "is None" case is easy to reproduce and will hit
> you more often than you like.
>
> BTW: Not using "is None" will bite you in many other ways as
> well. None it typically used as value for "not initialized" or
> "no value provided". This meaning is completely different from
> "empty selection" or "empty string", so in order to address all
> those cases as well, you'd have to change:
>
> bool(()) == True
> bool('') == True
> etc.
>
> The "if x is None:" test is also faster than "if x:", so actually
> win something by coding correctly ;-)
>
> Please provide a stronger argument for needing to change
> bool(time(0,0,0)) than one which is built on buggy code :-)
> (I'm running out of smileys...)
>
>

I'm very surprised that "midnight evaluates to False is insane" is not in
and of itself a strong enough argument to change it. :-/

Michael

> --
> Marc-Andre Lemburg
> eGenix.com
>
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