Request and Response Objects

Note

This chapter is adapted from a portion of the WebOb documentation, originally written by Ian Bicking.

Pyramid uses the WebOb package as a basis for its request and response object implementations. The request object that is passed to a Pyramid view is an instance of the pyramid.request.Request class, which is a subclass of webob.request.Request. The response returned from a Pyramid view renderer is an instance of the pyramid.response.Response class, which is a subclass of the webob.response.Response class. Users can also return an instance of pyramid.response.Response directly from a view as necessary.

WebOb is a project separate from Pyramid with a separate set of authors and a fully separate set of documentation. Pyramid adds some functionality to the standard WebOb request, which is documented in the pyramid.request API documentation.

WebOb provides objects for HTTP requests and responses. Specifically it does this by wrapping the WSGI request environment and response status, header list, and app_iter (body) values.

WebOb request and response objects provide many conveniences for parsing WSGI requests and forming WSGI responses. WebOb is a nice way to represent "raw" WSGI requests and responses. However, we won't cover that use case in this document, as users of Pyramid don't typically need to use the WSGI-related features of WebOb directly. The reference documentation shows many examples of creating requests and using response objects in this manner, however.

Request

The request object is a wrapper around the WSGI environ dictionary. This dictionary contains keys for each header, keys that describe the request (including the path and query string), a file-like object for the request body, and a variety of custom keys. You can always access the environ with req.environ.

Some of the most important and interesting attributes of a request object are below.

req.method

The request method, e.g., GET, POST

req.GET

A multidict with all the variables in the query string.

req.POST

A multidict with all the variables in the request body. This only has variables if the request was a POST and it is a form submission.

req.params

A multidict with a combination of everything in req.GET and req.POST.

req.body

The contents of the body of the request. This contains the entire request body as a string. This is useful when the request is a POST that is not a form submission, or a request like a PUT. You can also get req.body_file for a file-like object.

req.json_body

The JSON-decoded contents of the body of the request. See Dealing with a JSON-Encoded Request Body.

req.cookies

A simple dictionary of all the cookies.

req.headers

A dictionary of all the headers. This dictionary is case-insensitive.

req.urlvars and req.urlargs

req.urlvars are the keyword parameters associated with the request URL. req.urlargs are the positional parameters. These are set by products like Routes and Selector.

Also for standard HTTP request headers, there are usually attributes such as req.accept_language, req.content_length, and req.user_agent. These properties expose the parsed form of each header, for whatever parsing makes sense. For instance, req.if_modified_since returns a datetime object (or None if the header is was not provided).

Note

Full API documentation for the Pyramid request object is available in pyramid.request.

Special Attributes Added to the Request by Pyramid

In addition to the standard WebOb attributes, Pyramid adds special attributes to every request: context, registry, root, subpath, traversed, view_name, virtual_root, virtual_root_path, session, matchdict, and matched_route. These attributes are documented further within the pyramid.request.Request API documentation.

URLs

In addition to these attributes, there are several ways to get the URL of the request and its parts. We'll show various values for an example URL http://localhost/app/blog?id=10, where the application is mounted at http://localhost/app.

req.url

The full request URL with query string, e.g., http://localhost/app/blog?id=10

req.host

The host information in the URL, e.g., localhost

req.host_url

The URL with the host, e.g., http://localhost

req.application_url

The URL of the application (just the SCRIPT_NAME portion of the path, not PATH_INFO), e.g., http://localhost/app

req.path_url

The URL of the application including the PATH_INFO, e.g., http://localhost/app/blog

req.path

The URL including PATH_INFO without the host or scheme, e.g., /app/blog

req.path_qs

The URL including PATH_INFO and the query string, e.g, /app/blog?id=10

req.query_string

The query string in the URL, e.g., id=10

req.relative_url(url, to_application=False)

Gives a URL relative to the current URL. If to_application is True, then resolves it relative to req.application_url.

Methods

There are methods of request objects documented in pyramid.request.Request but you'll find that you won't use very many of them. Here are a couple that might be useful:

Request.blank(base_url)

Creates a new request with blank information, based at the given URL. This can be useful for subrequests and artificial requests. You can also use req.copy() to copy an existing request, or for subrequests req.copy_get() which copies the request but always turns it into a GET (which is safer to share for subrequests).

req.get_response(wsgi_application)

This method calls the given WSGI application with this request, and returns a pyramid.response.Response object. You can also use this for subrequests or testing.

Text (Unicode)

Most of the properties of the request object will be text values. The values in req.POST, req.GET, req.params, and req.cookies will contain text and are generated assuming a UTF-8 charset. The client can indicate the charset with something like Content-Type: application/x-www-form-urlencoded; charset=utf8, but browsers seldom set this. You can reset the charset of an existing request with newreq = req.decode('utf-8'), or during instantiation with Request(environ, charset='utf8').

Multidict

Several attributes of a WebOb request are multidict structures (such as request.GET, request.POST, and request.params). A multidict is a dictionary where a key can have multiple values. The quintessential example is a query string like ?pref=red&pref=blue; the pref variable has two values: red and blue.

In a multidict, when you do request.GET['pref'], you'll get back only "blue" (the last value of pref). This returned result might not be expected—sometimes returning a string, and sometimes returning a list—and may be cause of frequent exceptions. If you want all the values back, use request.GET.getall('pref'). If you want to be sure there is one and only one value, use request.GET.getone('pref'), which will raise an exception if there is zero or more than one value for pref.

When you use operations like request.GET.items(), you'll get back something like [('pref', 'red'), ('pref', 'blue')]. All the key/value pairs will show up. Similarly request.GET.keys() returns ['pref', 'pref']. Multidict is a view on a list of tuples; all the keys are ordered, and all the values are ordered.

API documentation for a multidict exists as pyramid.interfaces.IMultiDict.

Dealing with a JSON-Encoded Request Body

New in version 1.1.

pyramid.request.Request.json_body is a property that returns a JSON-decoded representation of the request body. If the request does not have a body, or the body is not a properly JSON-encoded value, an exception will be raised when this attribute is accessed.

This attribute is useful when you invoke a Pyramid view callable via, for example, jQuery's $.ajax function, which has the potential to send a request with a JSON-encoded body.

Using request.json_body is equivalent to:

from json import loads
loads(request.body, encoding=request.charset)

Here's how to construct an AJAX request in JavaScript using jQuery that allows you to use the request.json_body attribute when the request is sent to a Pyramid application:

jQuery.ajax({type:'POST',
             url: 'http://localhost:6543/', // the pyramid server
             data: JSON.stringify({'a':1}),
             contentType: 'application/json'});

When such a request reaches a view in your application, the request.json_body attribute will be available in the view callable body.

@view_config(renderer='string')
def aview(request):
    print(request.json_body)
    return 'OK'

For the above view, printed to the console will be:

{'a': 1}

For bonus points, here's a bit of client-side code that will produce a request that has a body suitable for reading via request.json_body using Python's urllib2 instead of a JavaScript AJAX request:

import urllib2
import json

json_payload = json.dumps({'a':1})
headers = {'Content-Type':'application/json'}
req = urllib2.Request('http://localhost:6543/', json_payload, headers)
resp = urllib2.urlopen(req)

If you are doing Cross-origin resource sharing (CORS), then the standard requires the browser to do a pre-flight HTTP OPTIONS request. The easiest way to handle this is to add an extra view_config for the same route, with request_method set to OPTIONS, and set the desired response header before returning. You can find examples of response headers Access control CORS, Preflighted requests.

Cleaning up after a Request

Sometimes it's required to perform some cleanup at the end of a request when a database connection is involved.

For example, let's say you have a mypackage Pyramid application package that uses SQLAlchemy, and you'd like the current SQLAlchemy database session to be removed after each request. Put the following in the mypackage.__init__ module:

from mypackage.models import DBSession

from pyramid.events import subscriber
from pyramid.events import NewRequest

def cleanup_callback(request):
    DBSession.remove()

@subscriber(NewRequest)
def add_cleanup_callback(event):
    event.request.add_finished_callback(cleanup_callback)

Registering the cleanup_callback finished callback at the start of a request (by causing the add_cleanup_callback to receive a pyramid.events.NewRequest event at the start of each request) will cause the DBSession to be removed whenever request processing has ended. Note that in the example above, for the pyramid.events.subscriber decorator to work, the pyramid.config.Configurator.scan() method must be called against your mypackage package during application initialization.

Note

This is only an example. In particular, it is not necessary to cause DBSession.remove to be called in an application generated from the Pyramid cookiecutter, because these all use the pyramid_tm package. The cleanup done by DBSession.remove is unnecessary when pyramid_tm middleware is configured into the application.

More Details

More detail about the request object API is available as follows.

• pyramid.request.Request API documentation

• WebOb documentation. All methods and attributes of a webob.Request documented within the WebOb documentation will work with request objects created by Pyramid.

Response

The Pyramid response object can be imported as pyramid.response.Response. This class is a subclass of the webob.reponse.Response class. The subclass does not add or change any functionality, so the WebOb Response documentation will be completely relevant for this class as well.

A response object has three fundamental parts:

response.status

The response code plus reason message, like 200 OK. To set the code without a message, use status_int, i.e., response.status_int = 200.

response.headerlist

A list of all the headers, like [('Content-Type', 'text/html')]. There's a case-insensitive multidict in response.headers that also allows you to access these same headers.

response.app_iter

An iterable (such as a list or generator) that will produce the content of the response. This is also accessible as response.body (bytes), response.text (a Unicode string, informed by response.charset), and response.body_file (a file-like object; writing to it appends to app_iter).

Everything else in the object typically derives from this underlying state. Here are some highlights:

response.content_type

The content type not including the charset parameter.

Typical use: response.content_type = 'text/html'.

Default value: response.content_type = 'text/html'.

response.charset

The charset parameter of the content-type, it also informs encoding in response.text. response.content_type_params is a dictionary of all the parameters.

response.set_cookie(name, value, max_age=None, path='/', ...)

Set a cookie. The keyword arguments control the various cookie parameters. The max_age argument is the length for the cookie to live in seconds (you may also use a timedelta object). The Expires key will also be set based on the value of max_age.

response.delete_cookie(name, path='/', domain=None)

Delete a cookie from the client. This sets max_age to 0 and the cookie value to ''.

response.cache_expires(seconds=0)

This makes the response cacheable for the given number of seconds, or if seconds is 0 then the response is uncacheable (this also sets the Expires header).

response(environ, start_response)

The response object is a WSGI application. As an application, it acts according to how you create it. It can do conditional responses if you pass conditional_response=True when instantiating (or set that attribute later). It can also do HEAD and Range requests.

Headers

Like the request, most HTTP response headers are available as properties. These are parsed, so you can do things like response.last_modified = os.path.getmtime(filename).

The details are available in the webob.response API documentation.

Instantiating the Response

Of course most of the time you just want to make a response. Generally any attribute of the response can be passed in as a keyword argument to the class, e.g.:

from pyramid.response import Response
response = Response(body='hello world!', content_type='text/plain')

The status defaults to '200 OK'.

The value of content_type defaults to webob.response.Response.default_content_type, which is text/html. You can subclass pyramid.response.Response and set default_content_type to override this behavior.

Exception Responses

To facilitate error responses like 404 Not Found, the module pyramid.httpexceptions contains classes for each kind of error response. These include boring but appropriate error bodies. The exceptions exposed by this module, when used under Pyramid, should be imported from the pyramid.httpexceptions module. This import location contains subclasses and replacements that mirror those in the webob.exc module.

Each class is named pyramid.httpexceptions.HTTP*, where * is the reason for the error. For instance, pyramid.httpexceptions.HTTPNotFound subclasses pyramid.response.Response, so you can manipulate the instances in the same way. A typical example is:

from pyramid.httpexceptions import HTTPNotFound
from pyramid.httpexceptions import HTTPMovedPermanently

response = HTTPNotFound('There is no such resource')
# or:
response = HTTPMovedPermanently(location=new_url)

More Details

More details about the response object API are available in the pyramid.response documentation. More details about exception responses are in the pyramid.httpexceptions API documentation. The WebOb documentation is also useful.