lib/django-1.3/docs/topics/http/views.txt - external/googleappengine/python - Git at Google

 =============
 Writing views
 =============

 A view function, or *view* for short, is simply a Python function that takes a
 Web request and returns a Web response. This response can be the HTML contents
 of a Web page, or a redirect, or a 404 error, or an XML document, or an image .
 . . or anything, really. The view itself contains whatever arbitrary logic is
 necessary to return that response. This code can live anywhere you want, as long
 as it's on your Python path. There's no other requirement--no "magic," so to
 speak. For the sake of putting the code *somewhere*, the convention is to
 put views in a file called ``views.py``, placed in your project or
 application directory.

 A simple view
 =============

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

 .. code-block:: python

     from django.http import HttpResponse
     import datetime

     def current_datetime(request):
         now = datetime.datetime.now()
         html = "<html><body>It is now %s.</body></html>" % now
         return HttpResponse(html)

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

     * First, we import the class :class:`~django.http.HttpResponse` from the
       :mod:`django.http` module, along with Python's ``datetime`` library.

     * Next, we define a function called ``current_datetime``. This is the view
       function. Each view function takes an :class:`~django.http.HttpRequest`
       object as its first parameter, which is typically named ``request``.

       Note that the name of the view function doesn't matter; it doesn't have to
       be named in a certain way in order for Django to recognize it. We're
       calling it ``current_datetime`` here, because that name clearly indicates
       what it does.

     * The view returns an :class:`~django.http.HttpResponse` object that
       contains the generated response. Each view function is responsible for
       returning an :class:`~django.http.HttpResponse` object. (There are
       exceptions, but we'll get to those later.)

 .. admonition:: Django's Time Zone

     Django includes a :setting:`TIME_ZONE` setting that defaults to
     ``America/Chicago``. This probably isn't where you live, so you might want
     to change it in your settings file.

 Mapping URLs to views
 =====================

 So, to recap, this view function returns an HTML page that includes the current
 date and time. To display this view at a particular URL, you'll need to create a
 *URLconf*; see :doc:`/topics/http/urls` for instructions.

 Returning errors
 ================

 Returning HTTP error codes in Django is easy. There are subclasses of
 :class:`~django.http.HttpResponse` for a number of common HTTP status codes
 other than 200 (which means *"OK"*). You can find the full list of available
 subclasses in the :ref:`request/response <ref-httpresponse-subclasses>`
 documentation.  Just return an instance of one of those subclasses instead of
 a normal :class:`~django.http.HttpResponse` in order to signify an error. For
 example::

     def my_view(request):
         # ...
         if foo:
             return HttpResponseNotFound('<h1>Page not found</h1>')
         else:
             return HttpResponse('<h1>Page was found</h1>')

 There isn't a specialized subclass for every possible HTTP response code,
 since many of them aren't going to be that common. However, as documented in
 the :class:`~django.http.HttpResponse` documentation, you can also pass the
 HTTP status code into the constructor for :class:`~django.http.HttpResponse`
 to create a return class for any status code you like. For example::

     def my_view(request):
         # ...

         # Return a "created" (201) response code.
         return HttpResponse(status=201)

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

 The Http404 exception
 ---------------------

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

 When you return an error such as :class:`~django.http.HttpResponseNotFound`,
 you're responsible for defining the HTML of the resulting error page::

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

 For convenience, and because it's a good idea to have a consistent 404 error page
 across your site, Django provides an ``Http404`` exception. If you raise
 ``Http404`` at any point in a view function, Django will catch it and return the
 standard error page for your application, along with an HTTP error code 404.

 Example usage::

     from django.http import Http404

     def detail(request, poll_id):
         try:
             p = Poll.objects.get(pk=poll_id)
         except Poll.DoesNotExist:
             raise Http404
         return render_to_response('polls/detail.html', {'poll': p})

 In order to use the ``Http404`` exception to its fullest, you should create a
 template that is displayed when a 404 error is raised. This template should be
 called ``404.html`` and located in the top level of your template tree.

 Customizing error views
 =======================

 The 404 (page not found) view
 -----------------------------

 When you raise an ``Http404`` exception, Django loads a special view devoted
 to handling 404 errors. By default, it's the view
 ``django.views.defaults.page_not_found``, which loads and renders the template
 ``404.html``.

 This means you need to define a ``404.html`` template in your root template
 directory. This template will be used for all 404 errors.

 This ``page_not_found`` view should suffice for 99% of Web applications, but if
 you want to override the 404 view, you can specify ``handler404`` in your
 URLconf, like so::

     handler404 = 'mysite.views.my_custom_404_view'

 Behind the scenes, Django determines the 404 view by looking for ``handler404``.
 By default, URLconfs contain the following line::

     from django.conf.urls.defaults import *

 That takes care of setting ``handler404`` in the current module. As you can see
 in ``django/conf/urls/defaults.py``, ``handler404`` is set to
 ``'django.views.defaults.page_not_found'`` by default.

 Three things to note about 404 views:

     * The 404 view is also called if Django doesn't find a match after checking
       every regular expression in the URLconf.

     * If you don't define your own 404 view -- and simply use the
       default, which is recommended -- you still have one obligation:
       you must create a ``404.html`` template in the root of your
       template directory. The default 404 view will use that template
       for all 404 errors. The default 404 view will pass one variable
       to the template: ``request_path``, which is the URL that resulted
       in the 404.

     * The 404 view is passed a :class:`~django.template.RequestContext` and
       will have access to variables supplied by your
       :setting:`TEMPLATE_CONTEXT_PROCESSORS` setting (e.g.,
       :setting:`MEDIA_URL`).

     * If :setting:`DEBUG` is set to ``True`` (in your settings module), then
       your 404 view will never be used, and the traceback will be displayed
       instead.

 The 500 (server error) view
 ----------------------------

 Similarly, Django executes special-case behavior in the case of runtime errors
 in view code. If a view results in an exception, Django will, by default, call
 the view ``django.views.defaults.server_error``, which loads and renders the
 template ``500.html``.

 This means you need to define a ``500.html`` template in your root template
 directory. This template will be used for all server errors. The default 500
 view passes no variables to this template and is rendered with an empty
 ``Context`` to lessen the chance of additional errors.

 This ``server_error`` view should suffice for 99% of Web applications, but if
 you want to override the view, you can specify ``handler500`` in your
 URLconf, like so::

     handler500 = 'mysite.views.my_custom_error_view'

 Behind the scenes, Django determines the error view by looking for ``handler500``.
 By default, URLconfs contain the following line::

     from django.conf.urls.defaults import *

 That takes care of setting ``handler500`` in the current module. As you can see
 in ``django/conf/urls/defaults.py``, ``handler500`` is set to
 ``'django.views.defaults.server_error'`` by default.
	=============
	Writing views
	=============

	A view function, or view for short, is simply a Python function that takes a
	Web request and returns a Web response. This response can be the HTML contents
	of a Web page, or a redirect, or a 404 error, or an XML document, or an image .
	. . or anything, really. The view itself contains whatever arbitrary logic is
	necessary to return that response. This code can live anywhere you want, as long
	as it's on your Python path. There's no other requirement--no "magic," so to
	speak. For the sake of putting the code somewhere, the convention is to
	put views in a file called ``views.py``, placed in your project or
	application directory.

	A simple view
	=============

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

	.. code-block:: python

	from django.http import HttpResponse
	import datetime

	def current_datetime(request):
	now = datetime.datetime.now()
	html = "<html><body>It is now %s.</body></html>" % now
	return HttpResponse(html)

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

	* First, we import the class :class:`~django.http.HttpResponse` from the
	:mod:`django.http` module, along with Python's ``datetime`` library.

	* Next, we define a function called ``current_datetime``. This is the view
	function. Each view function takes an :class:`~django.http.HttpRequest`
	object as its first parameter, which is typically named ``request``.

	Note that the name of the view function doesn't matter; it doesn't have to
	be named in a certain way in order for Django to recognize it. We're
	calling it ``current_datetime`` here, because that name clearly indicates
	what it does.

	* The view returns an :class:`~django.http.HttpResponse` object that
	contains the generated response. Each view function is responsible for
	returning an :class:`~django.http.HttpResponse` object. (There are
	exceptions, but we'll get to those later.)

	.. admonition:: Django's Time Zone

	Django includes a :setting:`TIME_ZONE` setting that defaults to
	``America/Chicago``. This probably isn't where you live, so you might want
	to change it in your settings file.

	Mapping URLs to views
	=====================

	So, to recap, this view function returns an HTML page that includes the current
	date and time. To display this view at a particular URL, you'll need to create a
	URLconf; see :doc:`/topics/http/urls` for instructions.

	Returning errors
	================

	Returning HTTP error codes in Django is easy. There are subclasses of
	:class:`~django.http.HttpResponse` for a number of common HTTP status codes
	other than 200 (which means "OK"). You can find the full list of available
	subclasses in the :ref:`request/response <ref-httpresponse-subclasses>`
	documentation. Just return an instance of one of those subclasses instead of
	a normal :class:`~django.http.HttpResponse` in order to signify an error. For
	example::

	def my_view(request):
	# ...
	if foo:
	return HttpResponseNotFound('<h1>Page not found</h1>')
	else:
	return HttpResponse('<h1>Page was found</h1>')

	There isn't a specialized subclass for every possible HTTP response code,
	since many of them aren't going to be that common. However, as documented in
	the :class:`~django.http.HttpResponse` documentation, you can also pass the
	HTTP status code into the constructor for :class:`~django.http.HttpResponse`
	to create a return class for any status code you like. For example::

	def my_view(request):
	# ...

	# Return a "created" (201) response code.
	return HttpResponse(status=201)

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

	The Http404 exception
	---------------------

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

	When you return an error such as :class:`~django.http.HttpResponseNotFound`,
	you're responsible for defining the HTML of the resulting error page::

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

	For convenience, and because it's a good idea to have a consistent 404 error page
	across your site, Django provides an ``Http404`` exception. If you raise
	``Http404`` at any point in a view function, Django will catch it and return the
	standard error page for your application, along with an HTTP error code 404.

	Example usage::

	from django.http import Http404

	def detail(request, poll_id):
	try:
	p = Poll.objects.get(pk=poll_id)
	except Poll.DoesNotExist:
	raise Http404
	return render_to_response('polls/detail.html', {'poll': p})

	In order to use the ``Http404`` exception to its fullest, you should create a
	template that is displayed when a 404 error is raised. This template should be
	called ``404.html`` and located in the top level of your template tree.

	Customizing error views
	=======================

	The 404 (page not found) view
	-----------------------------

	When you raise an ``Http404`` exception, Django loads a special view devoted
	to handling 404 errors. By default, it's the view
	``django.views.defaults.page_not_found``, which loads and renders the template
	``404.html``.

	This means you need to define a ``404.html`` template in your root template
	directory. This template will be used for all 404 errors.

	This ``page_not_found`` view should suffice for 99% of Web applications, but if
	you want to override the 404 view, you can specify ``handler404`` in your
	URLconf, like so::

	handler404 = 'mysite.views.my_custom_404_view'

	Behind the scenes, Django determines the 404 view by looking for ``handler404``.
	By default, URLconfs contain the following line::

	from django.conf.urls.defaults import *

	That takes care of setting ``handler404`` in the current module. As you can see
	in ``django/conf/urls/defaults.py``, ``handler404`` is set to
	``'django.views.defaults.page_not_found'`` by default.

	Three things to note about 404 views:

	* The 404 view is also called if Django doesn't find a match after checking
	every regular expression in the URLconf.

	* If you don't define your own 404 view -- and simply use the
	default, which is recommended -- you still have one obligation:
	you must create a ``404.html`` template in the root of your
	template directory. The default 404 view will use that template
	for all 404 errors. The default 404 view will pass one variable
	to the template: ``request_path``, which is the URL that resulted
	in the 404.

	* The 404 view is passed a :class:`~django.template.RequestContext` and
	will have access to variables supplied by your
	:setting:`TEMPLATE_CONTEXT_PROCESSORS` setting (e.g.,
	:setting:`MEDIA_URL`).

	* If :setting:`DEBUG` is set to ``True`` (in your settings module), then
	your 404 view will never be used, and the traceback will be displayed
	instead.

	The 500 (server error) view
	----------------------------

	Similarly, Django executes special-case behavior in the case of runtime errors
	in view code. If a view results in an exception, Django will, by default, call
	the view ``django.views.defaults.server_error``, which loads and renders the
	template ``500.html``.

	This means you need to define a ``500.html`` template in your root template
	directory. This template will be used for all server errors. The default 500
	view passes no variables to this template and is rendered with an empty
	``Context`` to lessen the chance of additional errors.

	This ``server_error`` view should suffice for 99% of Web applications, but if
	you want to override the view, you can specify ``handler500`` in your
	URLconf, like so::

	handler500 = 'mysite.views.my_custom_error_view'

	Behind the scenes, Django determines the error view by looking for ``handler500``.
	By default, URLconfs contain the following line::

	from django.conf.urls.defaults import *

	That takes care of setting ``handler500`` in the current module. As you can see
	in ``django/conf/urls/defaults.py``, ``handler500`` is set to
	``'django.views.defaults.server_error'`` by default.