On 01/03/17 01:53, Tim Martin wrote:
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 |definedthat 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.
I was talking about with the current behaviour of template engine i.e.
without changing how undefined variables are handled. For this to work
as you proposed, you would have to be passing the UndefinedVariable
object into these filters. Having checked out your branch, I can see
that is what you are doing.
This is a really big backwards incompatibility, as far as I can see. It
means that any filter may now get passed `UndefinedVariable` instead of
being passed `None` or string_if_invalid. So for example, the following
template behaves oppositely with your patch, if variable 'missing' is
undefined:
{% if missing|yesno == "maybe" %}true{% else %}false{% endif %}
There are likely many other filters that will behave differently e.g.:
{{ missing|add:"x" }}
Without the patch, if 'missing' is undefined this returns "x", but with
the patch it returns "". It's not just builtin filters I'm thinking
about, it's everyone else's that would also have to be made aware of
this new false-y value that it will start receiving. This is a really
big backwards incompatible change, I don't see how we could justify
this. If we were starting from scratch it would be another matter.
Luke
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