Django Trailing Slash URL Behavior

This is an interesting issue with Django's URL handling and error pages. Let me explain what's likely happening and suggest some solutions.

Understanding the Behavior

Django's APPEND_SLASH setting is designed to work with the CommonMiddleware to redirect URLs without trailing slashes to URLs with trailing slashes. However, this redirection only happens when:

1. The requested URL doesn't match any pattern in your URLconf
2. The URL with a trailing slash added would match a URL pattern

When a 500 error occurs, it's likely interrupting this normal flow of URL processing.

Possible Solutions

1. Ensure CommonMiddleware is Properly Configured

Make sure CommonMiddleware is in your MIDDLEWARE setting:

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MIDDLEWARE = [
    # other middleware...
    'django.middleware.common.CommonMiddleware',
    # other middleware...
]

2. Explicitly Define URL Patterns Both Ways

You could define your URL patterns to accept both with and without trailing slashes:

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urlpatterns = [
    path('myapp/page/', view_function, name='page_with_slash'),
    path('myapp/page', view_function, name='page_without_slash'),
    # other URL patterns...
]

3. Use a Custom Middleware

Create a custom middleware that handles the trailing slash issue before any errors occur:

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class TrailingSlashMiddleware:
    def __init__(self, get_response):
        self.get_response = get_response

    def __call__(self, request):
        # Check if the URL doesn't end with a slash and isn't a file
        if not request.path.endswith('/') and not '.' in request.path.split('/')[-1]:
            new_url = request.path + '/'
            if request.GET:
                new_url += '?' + request.GET.urlencode()
            return HttpResponseRedirect(new_url)
        
        return self.get_response(request)

Add this middleware before other middleware in your settings.

4. Review Your 500.html Template

Ensure your 500.html template doesn't contain any code that might interfere with URL processing. Keep it simple and static.

5. Check for URL-related Settings

Review other URL-related settings that might be affecting this behavior:

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# Make sure these are set correctly
APPEND_SLASH = True
PREPEND_WWW = False

The fact that removing the 500.html template "fixes" the issue suggests there might be something in your error handling flow that's bypassing the normal URL processing when the custom error page is present.