appendixG.rst

Appendix G: Request and Response Objects

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.

We've used these objects often throughout the book; this appendix explains the complete APIs for HttpRequest and HttpResponse objects.

HttpRequest

HttpRequest represents a single HTTP request from some user-agent.

Much of the important information about the request is available as attributes on the HttpRequest instance (see Table G-1). All attributes except session should be considered read-only.

Table G-1. Attributes of HttpRequest Objects

Attribute	Description
path	A string representing the full path to the requested page, not including the domain -- for example, "/music/bands/the_beatles/".
method	A string representing the HTTP method used in the request. This is guaranteed to be uppercase. For example: if request.method == 'GET': do_something() elif request.method == 'POST': do_something_else()
encoding	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 upcoming QueryDict documentation.
POST	A dictionary-like object containing all given HTTP POST parameters. See the upcoming QueryDict documentation. 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 the method entry in this table). Note: POST does not include file-upload information. See FILES.
REQUEST	For convenience, a dictionary-like object that searches POST first, and 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. See Chapter 14 for more on using cookies.
FILES	A dictionary-like object that maps filenames to UploadedFile objects. See the Django documentation for more.
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 QUERY_STRING: The raw unparsed query string REMOTE_ADDR: The IP address of the client REMOTE_HOST: The hostname of the client SERVER_NAME: The hostname of the server. SERVER_PORT: The port of the server Any HTTP headers are available in META as keys prefixed with HTTP_, converted to uppercase and substituting underscores for hyphens. For example: 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 HTTP_X_BENDER: The value of the X-Bender header, if set
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 available only if your Django installation has the AuthenticationMiddleware activated. For the complete details of authentication and users, see Chapter 14.
session	A readable and writable, dictionary-like object that represents the current session. This is available only if your Django installation has session support activated. See Chapter 14.
raw_post_data	The raw HTTP POST data. This is useful for advanced processing.

Request objects also have a few useful methods, as shown in Table G-2.

Table G-2. HttpRequest Methods

Method	Description
__getitem__(key)	Returns the GET/POST value for the given key, checking POST first, and then GET. Raises KeyError if the key doesn't exist. This lets you use dictionary-accessing syntax on an HttpRequest instance. For example, request["foo"] is the same as checking request.POST["foo"] and then request.GET["foo"].
has_key()	Returns True or False, designating whether request.GET or request.POST has the given key.
get_host()	Returns the originating host of the request using information from the HTTP_X_FORWARDED_HOST and HTTP_HOST headers (in that order). If they don't provide a value, the method uses a combination of SERVER_NAME and SERVER_PORT.
get_full_path()	Returns the path, plus an appended query string, if applicable. For 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 in Table G-3.

Table G-3. How QueryDicts Differ from Standard Dictionaries.

Method	Differences from Standard dict Implementation
__getitem__	Works just like a dictionary. However, if the key has more than one value, __getitem__() returns the last value.
__setitem__	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 be called only on a mutable QueryDict (one that was created via copy()).
get()	If the key has more than one value, get() returns the last value just like __getitem__.
update()	Takes either a QueryDict or standard dictionary. Unlike the standard dictionary's update method, this method appends to the current dictionary items rather than replacing them: >>> 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()__: >>> 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()__.

In addition, QueryDict has the methods shown in Table G-4.

G-4. Extra (Nondictionary) QueryDict Methods

Method	Description
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, a)	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 (e.g., "a=2&b=3&b=5").

A Complete Example

For example, given this HTML form:

<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 Note:

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

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 at django.http.HttpResponse.

Construction HttpResponses

Typically, you'll construct an HttpResponse 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 filelike 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 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 filelike object. Doing so will raise Exception.

Finally, note that HttpResponse implements a write() method, which makes is suitable for use anywhere that Python expects a filelike object. See Chapter 8 for some examples of using this technique.

Setting Headers

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."

You can also use has_header(header) to check for the existence of a header.

Avoid setting Cookie headers by hand; instead, see Chapter 14 for instructions on how cookies work in Django.

HttpResponse Subclasses

Django includes a number of HttpResponse subclasses that handle different types of HTTP responses (see Table G-5). Like HttpResponse, these subclasses live in django.http.

Table G-5. HttpResponse Subclasses

Class	Description
HttpResponseRedirect	The constructor takes a single argument: the path to redirect to. This can be a fully qualified URL (e.g., 'http://search.yahoo.com/') 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	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. It 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.

You can, of course, define your own HttpResponse subclass to support different types of responses not supported out of the box.

Returning Errors

Returning HTTP error codes in Django is easy. We've already mentioned the HttpResponseNotFound, HttpResponseForbidden, HttpResponseServerError, and other 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 a 404 error is by far the most common HTTP error, there's an easier way to handle it.

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.

Here's an example:

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(request, '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 it should be located in the top level of your template tree.

Customizing the 404 (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:

from django.conf.urls.defaults import *

urlpatterns = patterns('',
    ...
)

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.

There are 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.

Customizing 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:

from django.conf.urls.defaults import *

urlpatterns = patterns('',
    ...
)

handler500 = 'mysite.views.my_custom_error_view'