[Django] #36943: "Converter is already registered" incorrectly raised when registering a malformed converter.

[Django]

#36943: "Converter is already registered" incorrectly raised when registering a
malformed converter.
-------------------------------+---------------------------------------
Reporter: Austin Morton | Type: Bug
Status: new | Component: Core (URLs)
Version: 6.0 | Severity: Normal
Keywords: | Triage Stage: Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------+---------------------------------------
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.
--
Ticket URL: <https://code.djangoproject.com/ticket/36943>
Django <https://code.djangoproject.com/>
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