Deprecate and remove `OBSERVE_REQUEST_CALLBACK`

[tim-schilling]

With the addition of UPDATE_ON_FETCH from #1843, we no longer need to keep OBSERVE_REQUEST_CALLBACK around. The functionality that can be implemented with that can be redone in SHOW_TOOLBAR_CALLBACK as indicated by @living180 here

[kevalrajpalknight]

@tim-schilling this issue could be marked as closed.

[dennisvang]

(debug_toolbar.W008) The deprecated OBSERVE_REQUEST_CALLBACK setting is present in DEBUG_TOOLBAR_CONFIG.
HINT: Use the UPDATE_ON_FETCH and/or SHOW_TOOLBAR_CALLBACK settings instead.

This deprecation warning appears to suggest that we can remove OBSERVE_REQUEST_CALLBACK from the DEBUG_TOOLBAR_CONFIG dict.

However, that's not the case, because it would break the following:

https://github.com/jazzband/django-debug-toolbar/blob/9e30a06e418ecdd4eeb837530b86be40bb1e3d2d/debug_toolbar/toolbar.py#L182

Moreover, it is not clear to me from the message/docs/PR whether the default values for UPDATE_ON_FETCH and/or SHOW_TOOLBAR_CALLBACK are equivalent to the default value of OBSERVE_REQUEST_CALLBACK.

[tim-schilling]

@dennisvang OBSERVE_REQUEST_CALLBACK defaults to returning [a reference to the function] toolbar.observe_request which returns True. That's why it's safe to remove in a future update of the toolbar.

If you're attempting to have the toolbar reflect the latest request being made, including the AJAX requests while you're on a page, set UPDATE_ON_FETCH to True. If that's not helpful, please provide more information on what you're confused about and what you're trying to do.

[dennisvang]

@tim-schilling Thanks for the explanation.

My point is that we now get a warning if OBSERVE_REQUEST_CALLBACK is in the config dict, but removing it from the config dict causes an error, so it seems like there is no way to act on this warning yet.

[tim-schilling]

@dennisvang It should only log a deprecation warning if you include it in your project's settings' DEBUG_TOOLBAR_CONFIG dict (see https://github.com/jazzband/django-debug-toolbar/blob/9e30a06e418ecdd4eeb837530b86be40bb1e3d2d/debug_toolbar/apps.py#L265). Not including it in that dictionary should be fine (no errors and no warnings).

If you're getting a KeyError, I suspect you're manipulating the toolbar's actual default dict, which you probably shouldn't do unless you have good reason to.

[dennisvang]

@tim-schilling Thanks, you're right:

It turns out we had DEBUG_TOOLBAR_CONFIG = debug_toolbar.settings.CONFIG_DEFAULTS, without making a copy. So, removing the key from there would indeed manipulate the default dict.

Sorry for that.