1. =============

  2. Writing views

  3. =============

  4. 

  5. A view function, or *view* for short, is a Python function that takes a

  6. web request and returns a web response. This response can be the HTML contents

  7. of a web page, or a redirect, or a 404 error, or an XML document, or an image .

  8. . . or anything, really. The view itself contains whatever arbitrary logic is

  9. necessary to return that response. This code can live anywhere you want, as long

  10. as it's on your Python path. There's no other requirement--no "magic," so to

  11. speak. For the sake of putting the code *somewhere*, the convention is to

  12. put views in a file called ``views.py``, placed in your project or

  13. application directory.

  14. 

  15. A simple view

  16. =============

  17. 

  18. Here's a view that returns the current date and time, as an HTML document::

  19. 

  20.     from django.http import HttpResponse

  21.     import datetime

  22. 

  23.     def current_datetime(request):

  24.         now = datetime.datetime.now()

  25.         html = "<html><body>It is now %s.</body></html>" % now

  26.         return HttpResponse(html)

  27. 

  28. Let's step through this code one line at a time:

  29. 

  30. * First, we import the class :class:`~django.http.HttpResponse` from the

  31.   :mod:`django.http` module, along with Python's ``datetime`` library.

  32. 

  33. * Next, we define a function called ``current_datetime``. This is the view

  34.   function. Each view function takes an :class:`~django.http.HttpRequest`

  35.   object as its first parameter, which is typically named ``request``.

  36. 

  37.   Note that the name of the view function doesn't matter; it doesn't have to

  38.   be named in a certain way in order for Django to recognize it. We're

  39.   calling it ``current_datetime`` here, because that name clearly indicates

  40.   what it does.

  41. 

  42. * The view returns an :class:`~django.http.HttpResponse` object that

  43.   contains the generated response. Each view function is responsible for

  44.   returning an :class:`~django.http.HttpResponse` object. (There are

  45.   exceptions, but we'll get to those later.)

  46. 

  47. .. admonition:: Django's Time Zone

  48. 

  49.     Django includes a :setting:`TIME_ZONE` setting that defaults to

  50.     ``America/Chicago``. This probably isn't where you live, so you might want

  51.     to change it in your settings file.

  52. 

  53. Mapping URLs to views

  54. =====================

  55. 

  56. So, to recap, this view function returns an HTML page that includes the current

  57. date and time. To display this view at a particular URL, you'll need to create a

  58. *URLconf*; see :doc:`/topics/http/urls` for instructions.

  59. 

  60. Returning errors

  61. ================

  62. 

  63. Django provides help for returning HTTP error codes. There are subclasses of

  64. :class:`~django.http.HttpResponse` for a number of common HTTP status codes

  65. other than 200 (which means *"OK"*). You can find the full list of available

  66. subclasses in the :ref:`request/response <ref-httpresponse-subclasses>`

  67. documentation. Return an instance of one of those subclasses instead of a

  68. normal :class:`~django.http.HttpResponse` in order to signify an error. For

  69. example::

  70. 

  71.     from django.http import HttpResponse, HttpResponseNotFound

  72. 

  73.     def my_view(request):

  74.         # ...

  75.         if foo:

  76.             return HttpResponseNotFound('<h1>Page not found</h1>')

  77.         else:

  78.             return HttpResponse('<h1>Page was found</h1>')

  79. 

  80. There isn't a specialized subclass for every possible HTTP response code,

  81. since many of them aren't going to be that common. However, as documented in

  82. the :class:`~django.http.HttpResponse` documentation, you can also pass the

  83. HTTP status code into the constructor for :class:`~django.http.HttpResponse`

  84. to create a return class for any status code you like. For example::

  85. 

  86.     from django.http import HttpResponse

  87. 

  88.     def my_view(request):

  89.         # ...

  90. 

  91.         # Return a "created" (201) response code.

  92.         return HttpResponse(status=201)

  93. 

  94. Because 404 errors are by far the most common HTTP error, there's an easier way

  95. to handle those errors.

  96. 

  97. The ``Http404`` exception

  98. -------------------------

  99. 

  100. .. class:: django.http.Http404()

  101. 

  102. When you return an error such as :class:`~django.http.HttpResponseNotFound`,

  103. you're responsible for defining the HTML of the resulting error page::

  104. 

  105.     return HttpResponseNotFound('<h1>Page not found</h1>')

  106. 

  107. For convenience, and because it's a good idea to have a consistent 404 error page

  108. across your site, Django provides an ``Http404`` exception. If you raise

  109. ``Http404`` at any point in a view function, Django will catch it and return the

  110. standard error page for your application, along with an HTTP error code 404.

  111. 

  112. Example usage::

  113. 

  114.     from django.http import Http404

  115.     from django.shortcuts import render

  116.     from polls.models import Poll

  117. 

  118.     def detail(request, poll_id):

  119.         try:

  120.             p = Poll.objects.get(pk=poll_id)

  121.         except Poll.DoesNotExist:

  122.             raise Http404("Poll does not exist")

  123.         return render(request, 'polls/detail.html', {'poll': p})

  124. 

  125. In order to show customized HTML when Django returns a 404, you can create an

  126. HTML template named ``404.html`` and place it in the top level of your

  127. template tree. This template will then be served when :setting:`DEBUG` is set

  128. to ``False``.

  129. 

  130. When :setting:`DEBUG` is ``True``, you can provide a message to ``Http404`` and

  131. it will appear in the standard 404 debug template. Use these messages for

  132. debugging purposes; they generally aren't suitable for use in a production 404

  133. template.

  134. 

  135. .. _customizing-error-views:

  136. 

  137. Customizing error views

  138. =======================

  139. 

  140. The default error views in Django should suffice for most web applications,

  141. but can easily be overridden if you need any custom behavior. Specify the

  142. handlers as seen below in your URLconf (setting them anywhere else will have no

  143. effect).

  144. 

  145. The :func:`~django.views.defaults.page_not_found` view is overridden by

  146. :data:`~django.conf.urls.handler404`::

  147. 

  148.     handler404 = 'mysite.views.my_custom_page_not_found_view'

  149. 

  150. The :func:`~django.views.defaults.server_error` view is overridden by

  151. :data:`~django.conf.urls.handler500`::

  152. 

  153.     handler500 = 'mysite.views.my_custom_error_view'

  154. 

  155. The :func:`~django.views.defaults.permission_denied` view is overridden by

  156. :data:`~django.conf.urls.handler403`::

  157. 

  158.     handler403 = 'mysite.views.my_custom_permission_denied_view'

  159. 

  160. The :func:`~django.views.defaults.bad_request` view is overridden by

  161. :data:`~django.conf.urls.handler400`::

  162. 

  163.     handler400 = 'mysite.views.my_custom_bad_request_view'

  164. 

  165. .. seealso::

  166. 

  167.     Use the :setting:`CSRF_FAILURE_VIEW` setting to override the CSRF error

  168.     view.

  169. 

  170. Testing custom error views

  171. --------------------------

  172. 

  173. To test the response of a custom error handler, raise the appropriate exception

  174. in a test view. For example::

  175. 

  176.     from django.core.exceptions import PermissionDenied

  177.     from django.http import HttpResponse

  178.     from django.test import SimpleTestCase, override_settings

  179.     from django.urls import path

  180. 

  181. 

  182.     def response_error_handler(request, exception=None):

  183.         return HttpResponse('Error handler content', status=403)

  184. 

  185. 

  186.     def permission_denied_view(request):

  187.         raise PermissionDenied

  188. 

  189. 

  190.     urlpatterns = [

  191.         path('403/', permission_denied_view),

  192.     ]

  193. 

  194.     handler403 = response_error_handler

  195. 

  196. 

  197.     # ROOT_URLCONF must specify the module that contains handler403 = ...

  198.     @override_settings(ROOT_URLCONF=__name__)

  199.     class CustomErrorHandlerTests(SimpleTestCase):

  200. 

  201.         def test_handler_renders_template_response(self):

  202.             response = self.client.get('/403/')

  203.             # Make assertions on the response here. For example:

  204.             self.assertContains(response, 'Error handler content', status_code=403)

  205. 

  206. .. _async-views:

  207. 

  208. Async views

  209. ===========

  210. 

  211. As well as being synchronous functions, views can also be asynchronous

  212. ("async") functions, normally defined using Python's ``async def`` syntax.

  213. Django will automatically detect these and run them in an async context.

  214. However, you will need to use an async server based on ASGI to get their

  215. performance benefits.

  216. 

  217. Here's an example of an async view::

  218. 

  219.     import datetime

  220.     from django.http import HttpResponse

  221. 

  222.     async def current_datetime(request):

  223.         now = datetime.datetime.now()

  224.         html = '<html><body>It is now %s.</body></html>' % now

  225.         return HttpResponse(html)

  226. 

  227. You can read more about Django's async support, and how to best use async

  228. views, in :doc:`/topics/async`.
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