[Solved]-AttributeError: 'str' object has no attribute 'regex' django 1.9

Also make sure to remove the beginning empty url pattern–can be overlooked when migrating your urls.

urlpatterns = ['', # <== this blank element ('') produces the error.
    ...
]

tl;dr

For the curious, I found this out by adding a warning to the check_pattern_startswith_slash method in the django.core.checks.urls module:

def check_pattern_startswith_slash(pattern):
    """
    Check that the pattern does not begin with a forward slash.
    """
    if not hasattr(pattern, 'regex'):
        warning = Warning(
            "Invalid pattern '%s'" % pattern,
            id="urls.W002",
        )
        return [warning]

And, sure enough, I got a bunch of warnings like this:

?: (urls.W002) Invalid pattern ''

Firstly, remove the django.conf.urls.handler400 from the middle of the urlpatterns. It doesn’t belong there, and is the cause of the error.

Once the error has been fixed, you can make a couple of changes to update your code for Django 1.8+

1. Change urlpatterns to a list, instead of using patterns()

2. Import the views (or view modules), instead of using strings in your urls()

3. You are using the same regex for the start and login views. This means you won’t be able to reach the login views. One fix would be to change the regex for the login view to something like ^login/$

Putting that together, you get something like:

from firstsite.module.views import start
from exam import views as exam_views
from django.contrib.auth import views as auth_views

urlpatterns = [
  url(r'^$', start, name="home"),
  url(r'^admin/', include(admin.site.urls)),
  url(r'^login/$', auth_views.login, {'template_name': 'login.html'}, name='login'),
  url(r'^signup/$', exam_views.signup, name='signup'),
  url(r'^signup/submit/$', exam_views.signup_submit, name='signup_submit'),
]

Remove the beginning empty Url patterns and
also remove

django.conf.urls.handler400,    

from your urls.py this will solve your problem.

For Django 2

from django.urls.resolvers import get_resolver, URLPattern, URLResolver

urls = get_resolver()

def if_none(value):
    if value:
        return value
    return ''

def print_urls(urls, parent_pattern=None):
    for url in urls.url_patterns:
        if isinstance(url, URLResolver):
            print_urls(url, if_none(parent_pattern) + if_none(str(url.pattern)))
        elif isinstance(url, URLPattern):
            print(f"{url} ({url.lookup_str})")
            print('----')
print_urls(urls)