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

[Donald Stufft]

On Mar 5, 2014, at 5:04 PM, M.-A. Lemburg <mal at egenix.com> wrote:

> I don't know what all the fuzz is about. There are lots of types
> in Python that evaluate to False in certain corner cases, e.g.
> "", (), [], {}, 0, 0.0, 0j, None, etc.
> 
> datetime.time(0,0,0) is just another one :-)
> 
> BTW: If you want to test for None, well, then do test for None and
> not for some property of None:
> 
> if x is None:
>    print ("x is None, which implies it's False")
>    print ("Oh, and do note that the inverse is not necessarily true :-)")
> 
> In logic:
> 
> A -> B does not imply B -> A; it only implies (not B) -> (not A).
> 
> in Python again:
> 
> if bool(x) is not False:
>     print ("x is not None")
> 
> -- 
> Marc-Andre Lemburg
> eGenix.com
> 
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Most of those are pretty easy to reason about being “False”, they all have some
concept of empty, nothing, etc. Midnight however is not any more or less a time
than one minute past midnight. Even worse, if you consider not explicitly
checking for None a bug in that application code, debugging that code is made
much more difficult because of the fact that for some reason "midnight" is
considered False.

That is the real problem here, sure people should be using ``is None`` checks
but until people are perfect and never write buggy code, we have to contend
with the fact that sometimes people will not write ``is None``. Making that
particular error harder to debug is doing nothing but wasting their time and
having Python be punitive for what in many cases may be a mistake.

-----------------
Donald Stufft
PGP: 0x6E3CBCE93372DCFA // 7C6B 7C5D 5E2B 6356 A926 F04F 6E3C BCE9 3372 DCFA

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