On Tuesday, 28 February 2017 13:39:21 UTC, Luke Plant wrote:
>
>
>
> On 28/02/17 15:24, Marten Kenbeek wrote:
>
> What about adding a filter |defined that returns True if a variable is
> defined, False otherwise? It may not solve any problems when it's left
> implicit, but at least it allows users the explicitly define the behaviour
> for non-existing template variables, and it makes the intention of the
> template code crystal clear. And of course it's fully backwards compatible.
> The drawback is perhaps the resulting verbosity of if statements:
>
> {% if user.role|defined and roles.staff|defined and user.role ==
> roles.staff %}
> [sensitive information here]
> {% endif %}
>
>
>
> The only way to do it would be to hard code this filter into the template
> engine, because otherwise the 'defined' filter runs too late to know
> whether the variable is defined. This is really hacky and magic.
>
Is that true? In my patch, the undefined variable gets expanded into an
UndefinedVariable sentinel object, which can be detected by the defined
modifier, and can compare == to None (or not, if you wish). Other than the
result of the 'is' operator, I think you can make this fully backward
compatible if you choose. Whether or not this is desirable is another
question, of course.
Tim
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