When DEBUG is True, raising Http404 in a path converter's to_python method does not result in a technical response

[Antoine Humeau]

This is the response I get (plain text):

A server error occurred.  Please contact the administrator.

I understand a ValueError should be raised which tells the URL resolver "this path does not match, try next one" but Http404 is what came to my mind intuitively and the error message was not very helpful.

One could also make a point that raising a Http404 should be valid way to tell the resolver "this is indeed the right path but the current parameter value does not match anything so stop what you are doing and let the handler return the 404 page (including a helpful error message when DEBUG is True instead of the default 'Django tried these URL patterns')".

This would prove useful for example to implement a path converter that uses get_object_or_404.

[Tim Graham]

It seems that other exceptions correctly result in a technical 500 response.

[Antoine Humeau]

The technical_404_response view performs a new URL resolving (cf ​https://github.com/django/django/blob/a8e492bc81fca829f5d270e2d57703c02e58701e/django/views/debug.py#L482) which will obviously raise a new Http404 which won't be caught as only Resolver404 is checked. That means the WSGI handler fails and the WSGI server returns the previously described default error message (indeed the error message is the default one from wsgiref.handlers.BaseHandler ​https://docs.python.org/3.6/library/wsgiref.html#wsgiref.handlers.BaseHandler.error_body).

The solution seems to be to catch Http404 instead of Resolver404 in technical_404_response. This will result in a technical 404 page with the Http404's displayed and will match the behaviour when DEBUG is False.

[Ngalim Siregar]

Created ​PR , but I am not sure how to write the tests.

I've looking about the response before and after catch Http404 instead of Resolver404, and there is no difference. Should I also change the technical_404.html for response?

[Ngalim Siregar]

I've added test to the patch, but not sure if it is correct.

[Ngalim Siregar]

I have made the requested changes; please review again

[Mariusz Felisiak <felisiak.mariusz@…>]

In 503f60ff:

Fixed #29008 -- Fixed crash of 404 debug page when URL path converter raises Http404.