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
<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
<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.
I meant above that in the {{ }} context 'default_if_none' doesn't return
the default value, but just returns the empty string input value.
I also had to read the docs to understand the behaviour - it is
mentioned in the 3rd paragraph down in the docs linked above, "How
invalid variables are handled" - the `if`, `for` and `regroup` tags are
specifically mentioned. I think the docs could be clearly about how
things work with variables passed to template tags etc. "In general" as
you say is rather vague. Definitely room for improvement here!
Luke
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