course detail test: NoReverseMatch

    class CourseViewsTests(TestCase):
        def setUp(self):
            self.course = Course.objects.create(
                title="Python Testing",
                description="Learn to write tests in Python"
            )
            self.course2 = Course.objects.create(
                title="New Course",
                description="A new course"
            )
            self.step = Step.objects.create(
                title="Introduction to Doctests",
                description="Learn to write tests in your docstrings.",
                course = self.course
            )

        def test_course_list_view(self):
            resp = self.client.get(reverse('courses:list'))
            self.assertEqual(resp.status_code, 200)
            self.assertIn(self.course, resp.context['courses'])
            self.assertIn(self.course2, resp.context['courses'])

        def test_course_detail_view(self):
            resp = self.client.get(reverse('courses:detail'))
            self.assertEqual(resp.status_code, 200)
            self.assertIn(self.course, resp.context['course'])
            self.assertIn(self.course2, resp.context['course'])

from django.shortcuts import get_object_or_404, render

from .models import Course, Step


def course_list(request):
    courses = Course.objects.all()
    return render(request, 'courses/course_list.html', {'courses': courses})


def course_detail(request, pk):
    course = get_object_or_404(Course, pk=pk)
    return render(request, 'courses/course_detail.html', {'course': course})

def step_detail(request, course_pk, step_pk):
    step = get_object_or_404(Step, course_id=course_pk, pk=step_pk)
    return render(request, 'courses/step_detail.html', {'step': step})

from django.conf.urls import url

from . import views

urlpatterns = [
    url(r'^$', views.course_list, name='list'),
    url(r'(?P<course_pk>\d+)/(?P<step_pk>\d+)/$', views.step_detail, name='step'),
    url(r'(?P<pk>\d+)/$', views.course_detail, name='detail'),
]