panels(templates): avoid evaluating LazyObject

[nijel]

Description

LazyObject is typically used for something expensive to evaluate, so avoid evaluating it just for showing it in the debug toolbar.

Fixes # (issue)

Checklist:

• I have added the relevant tests for this change.
• I have added an item to the Pending section of docs/changes.rst.

I will look at writing tests once I get an approval, that this is a viable approach.

[matthiask]

Hey,

Thanks for your contribution!

I have questions:

• SQL queries should be impossible anyway; which other expensive context variables could be wrapped by LazyObject in your experience? I myself am using lazy() for all sorts of things, but mostly to wrap objects which generate SQL queries or translations, so until now evaluating such an object once too much didn't bother me.
• I had to read the code and I think that we still show the object if it has been evaluated already, is that correct?

[nijel]

For me, the reason was an expensive SQL query. I turned something possibly expensive (and rarely used) to a LazyObject (in WeblateOrg/weblate@d49c46e) and the query was still executed when running with Django debug toolbar. That took me some time to figure out that it's not my broken code, but the Django debug toolbar evaluates all template variables to display them.

SimpleLazyObject has __repr__ that shows the value if it has already been evaluated (see https://github.com/django/django/blob/8af3ae4ad9ca475f2428fac950de8df56b575e6a/django/utils/functional.py#L406-L413), that's why I used repr(). This should have been mentioned in the comment, I'll include that.

PS: lazy() seems to do a different thing than LazyObject, so this change covers LazyObject only.

[matthiask]

Thanks for the explanation! Makes sense.

I will look at writing tests once I get an approval, that this is a viable approach.

I approve :)

[matthiask]

Looks good! Two minor comments.

[matthiask]

Thanks!