[Django] #34427: Improve error message for bad context processors

[Django]

#34427: Improve error message for bad context processors
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Reporter: Adam Johnson | Owner: nobody
Type: New feature | Status: new
Component: Template system | Version: dev
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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If you write a template context processor but return the wrong type, for
example `None` by missing the return statement:

{{{
def data(request):
data = {"something": True}
}}}

Then currently the error message is very mysterious:

{{{
Internal Server Error: /
Traceback (most recent call last):
...
File "/..../site-packages/django/template/backends/django.py", line 61,
in render
return self.template.render(context)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/..../site-packages/django/template/base.py", line 173, in render
with context.bind_template(self):
File "/.../lib/python3.11/contextlib.py", line 137, in __enter__
return next(self.gen)
^^^^^^^^^^^^^^
File "/..../site-packages/django/template/context.py", line 254, in
bind_template
updates.update(processor(self.request))
TypeError: 'NoneType' object is not iterable
}}}

If a view returns the wrong type, Django raises a nice error message:

{{{
The view example.views.index didn't return an HttpResponse object. It
returned None instead.
}}}

I suggest we do the same for context processors. If we use try/except
around the `updates.update()` line, it will not slow down normal
processing any noticeable amount, thanks to Python 3.11's “zero-cost”
exception handling: https://docs.python.org/3.11/whatsnew/3.11.html#misc

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Ticket URL: <https://code.djangoproject.com/ticket/34427>
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