Hi Steve, On 04/07/2016 05:09 PM, Stephen Kelly wrote: > Am I right to think that using 'is' with numbers is expected to work like > this in the DTL? :
The situation with integers is roughly the same as with strings, except
that I believe CPython interns _all_ small integers, not just those that
are literals in the code. This happens to make DTL appear a bit more
"consistent" with CPython for integers, but the correct advice is still
the same: don't ever use `is` with integers, whether in Python code or DTL.
The `is` operator tells you whether two variables refer to the same
object. This is sometimes a meaningful question to ask for instances of
classes with mutable state, or for singletons like None, True, and
False, but it's not a useful or meaningful question to ask about
immutable objects like strings and integers, because there is no
perceptible difference (other than from `is`, `id`, and perhaps memory
usage) resulting from whether the implementation happened to choose to
reuse the same object or not.
> In [120]: c["a"] = 1
>
> In [121]: t = e.from_string("{% if 1 is 1 %}yes{% else %}no{% endif %}")
>
> In [122]: print t.render(c)
> yes
>
> In [123]: t = e.from_string("{% if 1 is a %}yes{% else %}no{% endif %}")
>
> In [124]: print t.render(c)
> yes
>
> In [125]: c["a"] = 1.0
>
> In [126]: t = e.from_string("{% if 1 is a %}yes{% else %}no{% endif %}")
>
> In [127]: print t.render(c)
> no
>
> In [128]: t = e.from_string("{% if 1 is 1.0 %}yes{% else %}no{% endif
> %}")
>
> In [129]: print t.render(c)
> no
>
> In [130]: t = e.from_string("{% if 1.0 is 1.0 %}yes{% else %}no{% endif
> %}")
>
> In [131]: print t.render(c)
> no
>
> Would it be useful to add those as unit tests and document what can be
> expected of 'if ... is'?
No to tests, because we would be adding tests for undefined and
unreliable behavior.
It might be worth adding a short documentation note. We largely want to
avoid documenting Python's behavior in the Django docs, but a short note
in the template `is` docs reminding people not to ever use `is` with
strings or numbers might be worthwhile.
Carl
--
You received this message because you are subscribed to the Google Groups
"Django developers (Contributions to Django itself)" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to django-developers+unsubscr...@googlegroups.com.
To post to this group, send email to django-developers@googlegroups.com.
Visit this group at https://groups.google.com/group/django-developers.
To view this discussion on the web visit
https://groups.google.com/d/msgid/django-developers/5706EAB6.6070207%40oddbird.net.
For more options, visit https://groups.google.com/d/optout.
signature.asc
Description: OpenPGP digital signature
