Names changed to protect the guilty

Aahz aahz at pythoncraft.com
Mon Oct 9 22:53:48 CEST 2006


In article <slrneik8ap.55m.apardon at rcpc42.vub.ac.be>,
Antoon Pardon  <apardon at forel.vub.ac.be> wrote:
>On 2006-10-08, Aahz <aahz at pythoncraft.com> wrote:
>> In article <877izb1vwe.fsf at pobox.com>, John J. Lee <jjl at pobox.com> wrote:
>>>aahz at pythoncraft.com (Aahz) writes:
>>>>
>>>> The following line of lightly munged code was found in a publicly
>>>> available Python library...
>>>> 
>>>>     if schema.elements.has_key(key) is False:
>>>> 
>>>> Sorry, just had to vent.
>>>
>>>I think I was reading the same code recently (epydoc?) and was also
>>>momentarily horrified ;-) until I realized that it was quite
>>>deliberately using three-valued logic (True, False, None) for some
>>>presumably-sensible reason.  Since None is false, they had to use
>>>"is".  So, given the need for three-valued logic, it's not as silly as
>>>it looks.
>>
>> My take is that even in that case, one should use "is" only with None.
>> There is too much ground for bugs with True/False, particularly if you
>> either intend to work across version *or* you might possibly accept a
>> user's object (because *they* might be working across versions and
>> therefore returning 1/0 instead of True/False).  I think it's safest to
>> simply ban this idiom.  No exceptions, never.
>
>The problem is there is also ground for bugs if you don't use "blah is
>True". If some application naturally seems to ask for a variable that
>can be valued False, True or a positive integer then things like "if
>var" or "if not var" may very well be a bug too.

Anyone designing an app like that in Python deserves to lose.  It's just
another way of shooting yourself in the foot.
-- 
Aahz (aahz at pythoncraft.com)           <*>         http://www.pythoncraft.com/

"If you don't know what your program is supposed to do, you'd better not
start writing it."  --Dijkstra



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