Autoreloader hides exceptions from urlconf module

[Austin Morton]

Initialize an empty project

$ uv init . && uv add django && uv run django-admin startproject repro .

Initialized project `django-repro` at `/Users/amorton/src/django-repro`
Using CPython 3.14.0
Creating virtual environment at: .venv
Resolved 5 packages in 132ms
Installed 3 packages in 97ms
 + asgiref==3.11.1
 + django==6.0.2
 + sqlparse==0.5.5

Edit urls.py to the following:

from django.urls import include, path, register_converter


class MyConverter:
    # forgot to specify regex
    # regex = "[^/]+"

    def to_python(self, value):
        return value

    def to_url(self, value):
        return value


register_converter(MyConverter, "mine")

urlpatterns = [
    path("<mine:foo>", include([])),
]

Run the development server:

$ uv run manage.py runserver
Watching for file changes with StatReloader
Performing system checks...

Exception in thread django-main-thread:
Traceback (most recent call last):
   ...
  File "/Users/amorton/src/django-repro/repro/urls.py", line 15, in <module>
    register_converter(MyConverter, "mine")
    ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
  File "/Users/amorton/src/django-repro/.venv/lib/python3.14/site-packages/django/urls/converters.py", line 57, in register_converter
    raise ValueError(f"Converter {type_name!r} is already registered.")
ValueError: Converter 'mine' is already registered.

What's going on here is that the original exception is being swallowed in django.utils.autoreload.BaseReloader.run.
The exception gets thrown after the call to register_converter succeeds, but before the URLResolver.urlconf_module cached property returns.
The urlconf module is executed a second time later, and the original exception is never shown to the user.

Ultimately this was a user-error on my part, but was very difficult to debug.

[Jacob Walls]

Thanks for the report. We could at least include the original error in the cause without altering the behavior.

Something like this looks promising, would you like to polish and submit a PR?

django/utils/autoreload.py

@@ -33 +33 @@
 # file paths to allow watching them in the future.
 _error_files = []
 _exception = None
+# A nice comment.
+_url_module_exception = None
 
 try:
     import termios
@@ -62 +64 @@
         global _exception
         try:
             fn(*args, **kwargs)
-        except Exception:
+        except Exception as e:
             _exception = sys.exc_info()
 
             et, ev, tb = _exception
@@ -75 +77 @@
 
             if filename not in _error_files:
                 _error_files.append(filename)
+            if _url_module_exception is not None:
+                raise e from _url_module_exception
 
-            raise
+            raise e
 
     return wrapper
 
@@ -339 +343 @@
             return False
 
     def run(self, django_main_thread):
+        global _url_module_exception
         logger.debug("Waiting for apps ready_event.")
         self.wait_for_apps_ready(apps, django_main_thread)
         from django.urls import get_resolver
@@ -347 +352 @@
         # reloader starts by accessing the urlconf_module property.
         try:
             get_resolver().urlconf_module
-        except Exception:
+        except Exception as e:
             # Loading the urlconf can result in errors during development.
-            # If this occurs then swallow the error and continue.
-            pass
+            # If this occurs then store the error and continue.
+            _url_module_exception = e
         logger.debug("Apps ready_event triggered. Sending autoreload_started signal.")
         autoreload_started.send(sender=self)
         self.run_loop()

[Varun Kasyap Pentamaraju]

​https://github.com/django/django/pull/20821

[Varun Kasyap Pentamaraju]

​https://github.com/django/django/pull/20877

[Jacob Walls <jacobtylerwalls@…>]

In 3483bfc:

Fixed #36943 -- Preserved any exception from URLconf module in autoreloader.

Co-authored-by: Jacob Walls <jacobtylerwalls@…>