Improve error message for incorrect context processors

[Adam Johnson]

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

[David Sanders]

Thanks Adam, good idea 👍

I've taken a stab, left it as a TypeError but just improved the message. I'm not sure about the "It returned <blah> instead." part. With templates the "None" is hardcoded as it's a literal None check. For this I'm not sure what the best way to generalise the return value/type in English is 😁. If you have any ideas I'm open.

And as I write this the ever so quick felixx has already started reviewing though so it may all get resolved by the time you read this 🤣

[GitHub <noreply@…>]

In 5dba5fd:

Fixed #34427 -- Improved error message when context processor does not return a dict.