Fixed #32260 handler500 as a Class-based view raises SystemCheckError

[abbasidaniyal]

_check_custom_error_handlers in django/urls/resolver.py now checks if the handler is a function based view or class based view.
For class based view, num_parameters needs to be 2 as class based views take two parameters (self, request).

[charettes]

You should add tests for CBV handlers as well

[charettes]

I think you want to allow an extra parameter when it's a class based view

In other words

# Class based views always take two arguements, (self, request).

Is not not true

Suggested change

	if signature.parameters.get('self'):
	args = [None] * 2 # Class based views always take two arguements, (self, request).
	else:
	args = [None] * num_parameters
	if signature.parameters.get('self'):
	num_parameters += 1
	args = [None] * num_parameters

[abbasidaniyal]

Thank you for your suggestion!
Simply added +1 to the num_parameters will make the value of num_parameters for handler404, handler403 and handler400 as 3 which is not correct.

django/django/urls/resolvers.py

Line 421 in 8d7085e

for status_code, num_parameters in [(400, 2), (403, 2), (404, 2), (500, 1)]:

I can change it as if signature.parameters.get('self') and status_code==500: but that would be the greatest solution in my opinion.

[abbasidaniyal]

You should add tests for CBV handlers as well

Yes I will do that right away!

[adamchainz]

I think this ticket is going in the wrong direction and the check has really exposed a bug in the return value of View.as_view().

View.as_view() returns a function that has signature def view(request, *args, **kwargs): - it does not take a self argument. The self is created per-request inside that view() function. However the functools.update_wrapper() calls inside as_view() incorrectly copy the function signature from the class methods, including their self argument, onto the view() function.

I think we should fix View.as_view() to drop the first positional argument (self) from the copied signature. Then the check will not need changing, and any other tools can see the true function signature.

[abbasidaniyal]

I think this ticket is going in the wrong direction and the check has really exposed a bug in the return value of View.as_view().

View.as_view() returns a function that has signature def view(request, *args, **kwargs): - it does not take a self argument. The self is created per-request inside that view() function. However the functools.update_wrapper() calls inside as_view() incorrectly copy the function signature from the class methods, including their self argument, onto the view() function.

I think we should fix View.as_view() to drop the first positional argument (self) from the copied signature. Then the check will not need changing, and any other tools can see the true function signature.

That does make a lot of sense. Self should not be present on the signature in the first place!
I'll create a separate PR with the fix soon. Will close this as soon as a new one is created!

[felixxm]

@abbasidaniyal Thanks, feel-free to send a new PR.

[abbasidaniyal]

@felixxm Yes. I'm extremely sorry for delaying this. I'll be sending a new PR soon!