On Monday, 27 February 2017 10:43:02 UTC, Luke Plant wrote:
>
>
>
> On 05/01/17 02:39, Tim Martin wrote:
>
> 2) There appears to be an inconsistency in the default_if_none
> modifier. If I have a template with
>
> x|default_if_none:y = {{x|default_if_none:y}}
> {% if x|default_if_none:y %}
> x|default_if_none:y is apparently True
> {% endif %}
>
> with y defined to True and x undefined, then it produces:
>
> x|default_if_none:y =
> x|default_if_none:y is apparently True
>
> IOW it appears that the default_if_none doesn't cause y's value to
> be used in the case where the variable is being printed, even
> though it causes the expression to have y's value when evaluated in
> an if. I think this is something to do with the fact that the two
> ways of handling failures (ignore_failures and string_if_invalid)
> do different things.
>
> I don't see a way to make backward compatible code here.
>
> I think this is just a bug, does anyone think otherwise?
>
>
> This seems to be working exactly as it should.
>
> In the context of template variable interpolation i.e. `{{ }}` syntax, it
> is defined that `x` in this case will get converted to the empty string (or
> your 'string_if_invalid' setting) -
> https://docs.djangoproject.com/en/dev/ref/templates/api/#how-invalid-variables-are-handled
>
> - unlike the case for `if` tags.
>
> When this value (the empty string) is passed to `default_if_none`, the
> value is not changed, since it is not None. See
> https://docs.djangoproject.com/en/1.10/ref/templates/builtins/#default-if-none
>
The missing piece of information for me (and even reading the docs it's far
from obvious) is that undefined variables mean two different things in the
different contexts.
The value *is* changed in the second case, because it *is* None. Undefined
values are '' in {{ }} context but are None in {% %} context. Maybe that's
obvious to you, but it's not to me. In fact, after a few minutes searching
I still can't find where this is mentioned in the documentation. Only the
vague statement that invalid variables are "generally" replaced with
string_if_invalid.
Tim
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