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============================
Request and response objects
============================
Quick overview
==============
Django uses request and response objects to pass state through the system.
When a page is requested, Django creates an ``HttpRequest`` object that
contains metadata about the request. Then Django loads the appropriate view,
passing the ``HttpRequest`` as the first argument to the view function. Each
view is responsible for returning an ``HttpResponse`` object.
This document explains the APIs for ``HttpRequest`` and ``HttpResponse``
objects.
HttpRequest objects
===================
Attributes
----------
All attributes except ``session`` should be considered read-only.
``path``
A string representing the full path to the requested page, not including
the domain.
Example: ``"/music/bands/the_beatles/"``
``method``
A string representing the HTTP method used in the request. This is
guaranteed to be uppercase. Example::
if request.method == 'GET':
do_something()
elif request.method == 'POST':
do_something_else()
``encoding``
**New in Django development version**
A string representing the current encoding used to decode form submission
data (or ``None``, which means the ``DEFAULT_CHARSET`` setting is used).
You can write to this attribute to change the encoding used when accessing
the form data. Any subsequent attribute accesses (such as reading from
``GET`` or ``POST``) will use the new ``encoding`` value. Useful if you
know the form data is not in the ``DEFAULT_CHARSET`` encoding.
``GET``
A dictionary-like object containing all given HTTP GET parameters. See the
``QueryDict`` documentation below.
``POST``
A dictionary-like object containing all given HTTP POST parameters. See the
``QueryDict`` documentation below.
It's possible that a request can come in via POST with an empty ``POST``
dictionary -- if, say, a form is requested via the POST HTTP method but
does not include form data. Therefore, you shouldn't use ``if request.POST``
to check for use of the POST method; instead, use ``if request.method ==
"POST"`` (see above).
Note: ``POST`` does *not* include file-upload information. See ``FILES``.
``REQUEST``
For convenience, a dictionary-like object that searches ``POST`` first,
then ``GET``. Inspired by PHP's ``$_REQUEST``.
For example, if ``GET = {"name": "john"}`` and ``POST = {"age": '34'}``,
``REQUEST["name"]`` would be ``"john"``, and ``REQUEST["age"]`` would be
``"34"``.
It's strongly suggested that you use ``GET`` and ``POST`` instead of
``REQUEST``, because the former are more explicit.
``COOKIES``
A standard Python dictionary containing all cookies. Keys and values are
strings.
``FILES``
A dictionary-like object containing all uploaded files. Each key in
``FILES`` is the ``name`` from the ``<input type="file" name="" />``. Each
value in ``FILES`` is a standard Python dictionary with the following three
keys:
* ``filename`` -- The name of the uploaded file, as a Python string.
* ``content-type`` -- The content type of the uploaded file.
* ``content`` -- The raw content of the uploaded file.
Note that ``FILES`` will only contain data if the request method was POST
and the ``<form>`` that posted to the request had
``enctype="multipart/form-data"``. Otherwise, ``FILES`` will be a blank
dictionary-like object.
``META``
A standard Python dictionary containing all available HTTP headers.
Available headers depend on the client and server, but here are some
examples:
* ``CONTENT_LENGTH``
* ``CONTENT_TYPE``
* ``HTTP_ACCEPT_ENCODING``
* ``HTTP_ACCEPT_LANGUAGE``
* ``HTTP_HOST`` -- The HTTP Host header sent by the client.
* ``HTTP_REFERER`` -- The referring page, if any.
* ``HTTP_USER_AGENT`` -- The client's user-agent string.
* ``QUERY_STRING`` -- The query string, as a single (unparsed) string.
* ``REMOTE_ADDR`` -- The IP address of the client.
* ``REMOTE_HOST`` -- The hostname of the client.
* ``REQUEST_METHOD`` -- A string such as ``"GET"`` or ``"POST"``.
* ``SERVER_NAME`` -- The hostname of the server.
* ``SERVER_PORT`` -- The port of the server.
``user``
A ``django.contrib.auth.models.User`` object representing the currently
logged-in user. If the user isn't currently logged in, ``user`` will be set
to an instance of ``django.contrib.auth.models.AnonymousUser``. You
can tell them apart with ``is_authenticated()``, like so::
if request.user.is_authenticated():
# Do something for logged-in users.
else:
# Do something for anonymous users.
``user`` is only available if your Django installation has the
``AuthenticationMiddleware`` activated. For more, see
`Authentication in Web requests`_.
.. _Authentication in Web requests: ../authentication/#authentication-in-web-requests
``session``
A readable-and-writable, dictionary-like object that represents the current
session. This is only available if your Django installation has session
support activated. See the `session documentation`_ for full details.
.. _`session documentation`: ../sessions/
``raw_post_data``
The raw HTTP POST data. This is only useful for advanced processing. Use
``POST`` instead.
Methods
-------
``__getitem__(key)``
Returns the GET/POST value for the given key, checking POST first, then
GET. Raises ``KeyError`` if the key doesn't exist.
This lets you use dictionary-accessing syntax on an ``HttpRequest``
instance. Example: ``request["foo"]`` would return ``True`` if either
``request.POST`` or ``request.GET`` had a ``"foo"`` key.
``has_key()``
Returns ``True`` or ``False``, designating whether ``request.GET`` or
``request.POST`` has the given key.
``get_full_path()``
Returns the ``path``, plus an appended query string, if applicable.
Example: ``"/music/bands/the_beatles/?print=true"``
``is_secure()``
Returns ``True`` if the request is secure; that is, if it was made with
HTTPS.
QueryDict objects
-----------------
In an ``HttpRequest`` object, the ``GET`` and ``POST`` attributes are instances
of ``django.http.QueryDict``. ``QueryDict`` is a dictionary-like
class customized to deal with multiple values for the same key. This is
necessary because some HTML form elements, notably
``<select multiple="multiple">``, pass multiple values for the same key.
``QueryDict`` instances are immutable, unless you create a ``copy()`` of them.
That means you can't change attributes of ``request.POST`` and ``request.GET``
directly.
``QueryDict`` implements the all standard dictionary methods, because it's a
subclass of dictionary. Exceptions are outlined here:
* ``__getitem__(key)`` -- Returns the value for the given key. If the key
has more than one value, ``__getitem__()`` returns the last value.
* ``__setitem__(key, value)`` -- Sets the given key to ``[value]``
(a Python list whose single element is ``value``). Note that this, as
other dictionary functions that have side effects, can only be called on
a mutable ``QueryDict`` (one that was created via ``copy()``).
* ``__contains__(key)`` -- Returns ``True`` if the given key is set. This
lets you do, e.g., ``if "foo" in request.GET``.
* ``get(key, default)`` -- Uses the same logic as ``__getitem__()`` above,
with a hook for returning a default value if the key doesn't exist.
* ``has_key(key)``
* ``setdefault(key, default)`` -- Just like the standard dictionary
``setdefault()`` method, except it uses ``__setitem__`` internally.
* ``update(other_dict)`` -- Takes either a ``QueryDict`` or standard
dictionary. Just like the standard dictionary ``update()`` method, except
it *appends* to the current dictionary items rather than replacing them.
For example::
>>> q = QueryDict('a=1')
>>> q = q.copy() # to make it mutable
>>> q.update({'a': '2'})
>>> q.getlist('a')
['1', '2']
>>> q['a'] # returns the last
['2']
* ``items()`` -- Just like the standard dictionary ``items()`` method,
except this uses the same last-value logic as ``__getitem()__``. For
example::
>>> q = QueryDict('a=1&a=2&a=3')
>>> q.items()
[('a', '3')]
* ``values()`` -- Just like the standard dictionary ``values()`` method,
except this uses the same last-value logic as ``__getitem()__``. For
example::
>>> q = QueryDict('a=1&a=2&a=3')
>>> q.values()
['3']
In addition, ``QueryDict`` has the following methods:
* ``copy()`` -- Returns a copy of the object, using ``copy.deepcopy()``
from the Python standard library. The copy will be mutable -- that is,
you can change its values.
* ``getlist(key)`` -- Returns the data with the requested key, as a Python
list. Returns an empty list if the key doesn't exist. It's guaranteed to
return a list of some sort.
* ``setlist(key, list_)`` -- Sets the given key to ``list_`` (unlike
``__setitem__()``).
* ``appendlist(key, item)`` -- Appends an item to the internal list
associated with key.
* ``setlistdefault(key, default_list)`` -- Just like ``setdefault``, except
it takes a list of values instead of a single value.
* ``lists()`` -- Like ``items()``, except it includes all values, as a list,
for each member of the dictionary. For example::
>>> q = QueryDict('a=1&a=2&a=3')
>>> q.lists()
[('a', ['1', '2', '3'])]
* ``urlencode()`` -- Returns a string of the data in query-string format.
Example: ``"a=2&b=3&b=5"``.
Examples
--------
Here's an example HTML form and how Django would treat the input::
<form action="/foo/bar/" method="post">
<input type="text" name="your_name" />
<select multiple="multiple" name="bands">
<option value="beatles">The Beatles</option>
<option value="who">The Who</option>
<option value="zombies">The Zombies</option>
</select>
<input type="submit" />
</form>
If the user enters ``"John Smith"`` in the ``your_name`` field and selects both
"The Beatles" and "The Zombies" in the multiple select box, here's what
Django's request object would have::
>>> request.GET
{}
>>> request.POST
{'your_name': ['John Smith'], 'bands': ['beatles', 'zombies']}
>>> request.POST['your_name']
'John Smith'
>>> request.POST['bands']
'zombies'
>>> request.POST.getlist('bands')
['beatles', 'zombies']
>>> request.POST.get('your_name', 'Adrian')
'John Smith'
>>> request.POST.get('nonexistent_field', 'Nowhere Man')
'Nowhere Man'
Implementation notes
--------------------
The ``GET``, ``POST``, ``COOKIES``, ``FILES``, ``META``, ``REQUEST``,
``raw_post_data`` and ``user`` attributes are all lazily loaded. That means
Django doesn't spend resources calculating the values of those attributes until
your code requests them.
HttpResponse objects
====================
In contrast to ``HttpRequest`` objects, which are created automatically by
Django, ``HttpResponse`` objects are your responsibility. Each view you write
is responsible for instantiating, populating and returning an ``HttpResponse``.
The ``HttpResponse`` class lives in the ``django.http`` module.
Usage
-----
Passing strings
~~~~~~~~~~~~~~~
Typical usage is to pass the contents of the page, as a string, to the
``HttpResponse`` constructor::
>>> response = HttpResponse("Here's the text of the Web page.")
>>> response = HttpResponse("Text only, please.", mimetype="text/plain")
But if you want to add content incrementally, you can use ``response`` as a
file-like object::
>>> response = HttpResponse()
>>> response.write("<p>Here's the text of the Web page.</p>")
>>> response.write("<p>Here's another paragraph.</p>")
You can add and delete headers using dictionary syntax::
>>> response = HttpResponse()
>>> response['X-DJANGO'] = "It's the best."
>>> del response['X-PHP']
>>> response['X-DJANGO']
"It's the best."
Note that ``del`` doesn't raise ``KeyError`` if the header doesn't exist.
Passing iterators
~~~~~~~~~~~~~~~~~
Finally, you can pass ``HttpResponse`` an iterator rather than passing it
hard-coded strings. If you use this technique, follow these guidelines:
* The iterator should return strings.
* If an ``HttpResponse`` has been initialized with an iterator as its
content, you can't use the ``HttpResponse`` instance as a file-like
object. Doing so will raise ``Exception``.
Methods
-------
``__init__(content='', mimetype=None, status=200, content_type=DEFAULT_CONTENT_TYPE)``
Instantiates an ``HttpResponse`` object with the given page content (a
string) and MIME type. The ``DEFAULT_CONTENT_TYPE`` is ``'text/html'``.
``content`` can be an iterator or a string. If it's an iterator, it should
return strings, and those strings will be joined together to form the
content of the response.
``status`` is the `HTTP Status code`_ for the response.
**(New in Django development version)** ``content_type`` is an alias for
``mimetype``. Historically, the parameter was only called ``mimetype``,
but since this is actually the value included in the HTTP ``Content-Type``
header, it can also include the character set encoding, which makes it
more than just a MIME type specification. If ``mimetype`` is specifiedi
(not None), that value is used. Otherwise, ``content_type`` is used. If
neither is given, the ``DEFAULT_CONTENT_TYPE`` setting is used.
``__setitem__(header, value)``
Sets the given header name to the given value. Both ``header`` and
``value`` should be strings.
``__delitem__(header)``
Deletes the header with the given name. Fails silently if the header
doesn't exist. Case-sensitive.
``__getitem__(header)``
Returns the value for the given header name. Case-sensitive.
``has_header(header)``
Returns ``True`` or ``False`` based on a case-insensitive check for a
header with the given name.
``set_cookie(key, value='', max_age=None, expires=None, path='/', domain=None, secure=None)``
Sets a cookie. The parameters are the same as in the `cookie Morsel`_
object in the Python standard library.
* ``max_age`` should be a number of seconds, or ``None`` (default) if
the cookie should last only as long as the client's browser session.
* ``expires`` should be a string in the format
``"Wdy, DD-Mon-YY HH:MM:SS GMT"``.
* Use ``domain`` if you want to set a cross-domain cookie. For example,
``domain=".lawrence.com"`` will set a cookie that is readable by
the domains www.lawrence.com, blogs.lawrence.com and
calendars.lawrence.com. Otherwise, a cookie will only be readable by
the domain that set it.
.. _`cookie Morsel`: http://www.python.org/doc/current/lib/morsel-objects.html
``delete_cookie(key, path='/', domain=None)``
Deletes the cookie with the given key. Fails silently if the key doesn't
exist.
Due to the way cookies work, ``path`` and ``domain`` should be the same
values you used in ``set_cookie()`` -- otherwise the cookie may not be deleted.
``content``
Returns the content as a Python string, encoding it from a Unicode object
if necessary. Note this is a property, not a method, so use ``r.content``
instead of ``r.content()``.
``write(content)``, ``flush()`` and ``tell()``
These methods make an ``HttpResponse`` instance a file-like object.
.. _HTTP Status code: http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10
HttpResponse subclasses
-----------------------
Django includes a number of ``HttpResponse`` subclasses that handle different
types of HTTP responses. Like ``HttpResponse``, these subclasses live in
``django.http``.
``HttpResponseRedirect``
The constructor takes a single argument -- the path to redirect to. This
can be a fully qualified URL (e.g. ``'http://www.yahoo.com/search/'``) or an
absolute URL with no domain (e.g. ``'/search/'``). Note that this returns
an HTTP status code 302.
``HttpResponsePermanentRedirect``
Like ``HttpResponseRedirect``, but it returns a permanent redirect (HTTP
status code 301) instead of a "found" redirect (status code 302).
``HttpResponseNotModified``
The constructor doesn't take any arguments. Use this to designate that a
page hasn't been modified since the user's last request.
``HttpResponseBadRequest``
**New in Django development version.**
Acts just like ``HttpResponse`` but uses a 400 status code.
``HttpResponseNotFound``
Acts just like ``HttpResponse`` but uses a 404 status code.
``HttpResponseForbidden``
Acts just like ``HttpResponse`` but uses a 403 status code.
``HttpResponseNotAllowed``
Like ``HttpResponse``, but uses a 405 status code. Takes a single,
required argument: a list of permitted methods (e.g. ``['GET', 'POST']``).
``HttpResponseGone``
Acts just like ``HttpResponse`` but uses a 410 status code.
``HttpResponseServerError``
Acts just like ``HttpResponse`` but uses a 500 status code.
Returning errors
================
Returning HTTP error codes in Django is easy. We've already mentioned the
``HttpResponseNotFound``, ``HttpResponseForbidden``,
``HttpResponseServerError``, etc., subclasses; just return an instance of one
of those subclasses instead of a normal ``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>')
Because 404 errors are by far the most common HTTP error, there's an easier way
to handle those errors.
The Http404 exception
---------------------
When you return an error such as ``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: To create a
``404.html`` template in the root of your template directory. The default
404 view will use that template for all 404 errors.
* If ``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.
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.
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