Using reverse() in django forms

I'm trying to use django's reverse() function in definition of django form for my custom widget, but am getting an error:

ImproperlyConfigured
The included urlconf urls doesn't have any patterns in it

Here is the code:

class WorkForm(forms.Form):
    # ...
    category = forms.ChoiceField(
        required=True,
        label=_('Category'),
        help_text=_('Select most appropriate category for your work.')
    )
    subcategory = forms.ChoiceField(
        widget=DependantChoiceWidget(
            default_value=_('Select category first'),
            data_source_url=reverse('works-json-categories'),
            # data_source_url='', -- it works this way
            depends_on='category_id'
        ),
        required=True,
        label=_('SubCategory'),
        help_text=_('Which subcategory suits your work best.')
    )

I am pretty sure, that my 'works.urls' is configured properly, since all other pages work as expected.

Is there a reason, why I cannot use reverse() in form definition? Does it have something to do with when this code runs? Is there a way to fix this, or the only choice here is to hardcode the URL?

Here is full error dump:

Environment:

Request Method: GET
Request URL: http://127.0.0.1:8000/works/add?category=1&subcategory=1

Django Version: 1.4 pre-alpha
Python Version: 2.7.1
Installed Applications:
['django.contrib.auth',
 'django.contrib.contenttypes',
 'django.contrib.sessions',
 'django.contrib.sites',
 'django.contrib.messages',
 'django.contrib.staticfiles',
 'django.contrib.admin',
 'django.contrib.admindocs',
 'social_auth',
 'sorl.thumbnail',
 'helpers',
 'users',
 'works',
 'debug_toolbar']
Installed Middleware:
('django.middleware.common.CommonMiddleware',
 'django.contrib.sessions.middleware.SessionMiddleware',
 'django.middleware.transaction.TransactionMiddleware',
 'django.middleware.csrf.CsrfViewMiddleware',
 'django.contrib.auth.middleware.AuthenticationMiddleware',
 'django.contrib.messages.middleware.MessageMiddleware',
 'debug_toolbar.middleware.DebugToolbarMiddleware')


Traceback:
File "/usr/local/lib/python2.7/dist-packages/django/core/handlers/base.py" in get_response
  89.                     response = middleware_method(request)
File "/usr/local/lib/python2.7/dist-packages/django/middleware/common.py" in process_request
  67.             if (not _is_valid_path(request.path_info, urlconf) and
File "/usr/local/lib/python2.7/dist-packages/django/middleware/common.py" in _is_valid_path
  164.         urlresolvers.resolve(path, urlconf)
File "/usr/local/lib/python2.7/dist-packages/django/core/urlresolvers.py" in resolve
  416.     return get_resolver(urlconf).resolve(path)
File "/usr/local/lib/python2.7/dist-packages/django/core/urlresolvers.py" in resolve
  298.             for pattern in self.url_patterns:
File "/usr/local/lib/python2.7/dist-packages/django/core/urlresolvers.py" in url_patterns
  328.         patterns = getattr(self.urlconf_module, "urlpatterns", self.urlconf_module)
File "/usr/local/lib/python2.7/dist-packages/django/core/urlresolvers.py" in urlconf_module
  323.             self._urlconf_module = import_module(self.urlconf_name)
File "/usr/local/lib/python2.7/dist-packages/django/utils/importlib.py" in import_module
  35.     __import__(name)
File "/var/www/megenius/trunk/urls.py" in <module>
  27.     url(r'^works/', include('works.urls')),
File "/usr/local/lib/python2.7/dist-packages/django/conf/urls/__init__.py" in include
  24.         urlconf_module = import_module(urlconf_module)
File "/usr/local/lib/python2.7/dist-packages/django/utils/importlib.py" in import_module
  35.     __import__(name)
File "/var/www/megenius/trunk/works/urls.py" in <module>
  2. from works.views import *
File "/var/www/megenius/trunk/works/views.py" in <module>
  9. from works.forms import WorkForm
File "/var/www/megenius/trunk/works/forms.py" in <module>
  10. class WorkForm(forms.Form):
File "/var/www/megenius/trunk/works/forms.py" in WorkForm
  31.             data_source_url=reverse('works-json-categories'),
File "/usr/local/lib/python2.7/dist-packages/django/core/urlresolvers.py" in reverse
  473.                       (prefix, resolver.reverse(view, *args, **kwargs)))
File "/usr/local/lib/python2.7/dist-packages/django/core/urlresolvers.py" in reverse
  360.         possibilities = self.reverse_dict.getlist(lookup_view)
File "/usr/local/lib/python2.7/dist-packages/django/core/urlresolvers.py" in 开发者_运维知识库reverse_dict
  276.             self._populate()
File "/usr/local/lib/python2.7/dist-packages/django/core/urlresolvers.py" in _populate
  242.         for pattern in reversed(self.url_patterns):
File "/usr/local/lib/python2.7/dist-packages/django/core/urlresolvers.py" in url_patterns
  332.             raise ImproperlyConfigured("The included urlconf %s doesn't have any patterns in it" % self.urlconf_name)

Exception Type: ImproperlyConfigured at /works/add
Exception Value: The included urlconf urls doesn't have any patterns in it

---

The problem might be that the form is defined before the urls have been loaded.

Django 1.4 will have a reverse_lazy feature that would solve this problem. You could implement it in your project yourself (see changeset 16121).

Alternatively, you could set the widget in your forms __init__ method instead. Then the reverse call happens when the form is created, after the urls have loaded.

class WorkForm(forms.Form):
    # ...
    subcategory = forms.ChoiceField(
        required=True,
        label=_('SubCategory'),
        help_text=_('Which subcategory suits your work best.')
    )
    def __init__(self, *args, **kwargs):
        super(WorkForm, self).__init__(*args, **kwargs)
        self.fields['subcategory'].widget=DependantChoiceWidget(
            default_value=_('Select category first'),
            data_source_url=reverse('works-json-categories'),
            depends_on='category_id'
        ),