On Friday, 6 January 2017 10:15:22 UTC, Alasdair Nicol wrote:
>
> Hi,
>
> On Thursday, 5 January 2017 17:51:39 UTC, Tim Martin wrote:
>>
>>
>>
>> On Thursday, 5 January 2017 04:15:31 UTC, Carl Meyer wrote:
>>>
>>> Hi Tim,
>>>
>>> On 01/04/2017 03:39 PM, Tim Martin wrote:
>>>
>>> > 1) There are test cases where we have templates that should treat "x
>>> > is y" as True where x and y are both undefined.
>>>
>>> Really? Is this an accidental side effect of some historical change, or
>>> really the intent of the test? That behavior seems buggy to me; more
>>> likely to be a bug-magnet than useful.
>>>
>>
>> There are a couple of test cases in test_if.py:
>> test_if_is_both_variables_missing and
>> test_if_is_not_both_variables_missing. These are verifying this specific
>> case, so they haven't been introduced by accident. I haven't got as far as
>> figuring out whether it was a fully thought through decision or whether
>> someone just created these cases for completeness.
>>
>
> I added the tests in ticket 26479 (see comment 5 onwards) [1]. I believe I
> added them for consistency with == and != operators, which have similar
> tests [2], [3] (trickier to spot because they are numbered). I apologise if
> adding the tests has made it harder to improve the behaviour of the tag.
>
Thanks for the background info Alasdair, that saved me lots of time hunting
around.
I agree you were right to add the tests, which were useful since without
them I wouldn't have realised the potential behaviour change I was
introducing. As I see it, these tests don't in themselves form a reason not
to change the behaviour (since AIUI you originally added them for
completeness, rather than because this specific behaviour was desired).
Does anyone think changing the behaviour of {% if a is None %} is the wrong
thing to do? I realise there can potentially be template code out there
relying on this, but after a quick scout through the documentation I can't
see anywhere that the behaviour on undefined variables is specified
officially.
For what it's worth, the same issue doesn't come up with ==, because
although the existing template behaviour has the same pattern, it's
possible to override == and != so that the Undefined object gives the same
behaviour as None did before. It's only for 'is' that this can't be
achieved. I don't know how for to go with this: preserving the existing ==
behaviour but changing it for 'is' leaves the two operations superficially
inconsistent (though there's nothing fundamentally wrong with two things
that satisfy equality but not 'is'). I can see a couple of alternatives (in
all cases all variables are undefined):
* "x is y" is false, but "x == y" is true and "x != y" is false (minimal
difference from the current behaviour)
* "x is y" and "x == y" are both false, "x != y" is true (probably the most
mutually consistent?)
* "x is y", "x == y" and "x != y" are all false (the SQL NULL alternative -
I'm not sure if I like this, but it has the merit that wrongly skipping
sections of a template is usually less bad than wrongly including parts of
a template)
Tim
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