Using the Twisted Web Client

Overview

Prerequisites

This document assumes that you are familiar with Deferreds and Failures, and producers and consumers. It also assumes you are familiar with the basic concepts of HTTP, such as requests and responses, methods, headers, and message bodies. The HTTPS section of this document also assumes you are somewhat familiar with SSL and have read about using SSL in Twisted.

The Agent

Issuing Requests

The twisted.web.client.Agent class is the entry point into the client API. Requests are issued using the request method, which takes as parameters a request method, a request URI, the request headers, and an object which can produce the request body (if there is to be one). The agent is responsible for connection setup. Because of this, it requires a reactor as an argument to its initializer. An example of creating an agent and issuing a request using it might look like this:

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from twisted.internet import reactor
from twisted.web.client import Agent
from twisted.web.http_headers import Headers

agent = Agent(reactor)

d = agent.request(
    'GET',
    'http://example.com/',
    Headers({'User-Agent': ['Twisted Web Client Example']}),
    None)

def cbResponse(ignored):
    print 'Response received'
d.addCallback(cbResponse)

def cbShutdown(ignored):
    reactor.stop()
d.addBoth(cbShutdown)

reactor.run()

As may be obvious, this issues a new GET request for / to the web server on example.com. Agent is responsible for resolving the hostname into an IP address and connecting to it on port 80 (for HTTP URIs), port 443 (for HTTPS URIs), or on the port number specified in the URI itself. It is also responsible for cleaning up the connection afterwards. This code sends a request which includes one custom header, User-Agent. The last argument passed to Agent.request is None, though, so the request has no body.

Sending a request which does include a body requires passing an object providing twisted.web.iweb.IBodyProducer to Agent.request. This interface extends the more general IPushProducer by adding a new length attribute and adding several constraints to the way the producer and consumer interact.

• The length attribute must be a non-negative integer or the constant twisted.web.iweb.UNKNOWN_LENGTH. If the length is known, it will be used to specify the value for the Content-Length header in the request. If the length is unknown the attribute should be set to UNKNOWN_LENGTH. Since more servers support Content-Length, if a length can be provided it should be.
• An additional method is required on IBodyProducer implementations: startProducing. This method is used to associate a consumer with the producer. It should return a Deferred which fires when all data has been produced.
• IBodyProducer implementations should never call the consumer's unregisterProducer method. Instead, when it has produced all of the data it is going to produce, it should only fire the Deferred returned by startProducing.

For additional details about the requirements of IBodyProducer implementations, see the API documentation.

Here's a simple IBodyProducer implementation which writes an in-memory string to the consumer:

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from zope.interface import implements

from twisted.internet.defer import succeed
from twisted.web.iweb import IBodyProducer

class StringProducer(object):
    implements(IBodyProducer)

    def __init__(self, body):
        self.body = body
        self.length = len(body)

    def startProducing(self, consumer):
        consumer.write(self.body)
        return succeed(None)

    def pauseProducing(self):
        pass

    def stopProducing(self):
        pass

This producer can be used to issue a request with a body:

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from twisted.internet import reactor
from twisted.web.client import Agent
from twisted.web.http_headers import Headers

from stringprod import StringProducer

agent = Agent(reactor)
body = StringProducer("hello, world")
d = agent.request(
    'GET',
    'http://example.com/',
    Headers({'User-Agent': ['Twisted Web Client Example'],
             'Content-Type': ['text/x-greeting']}),
    body)

def cbResponse(ignored):
    print 'Response received'
d.addCallback(cbResponse)

def cbShutdown(ignored):
    reactor.stop()
d.addBoth(cbShutdown)

reactor.run()

If you want to upload a file or you just have some data in a string, you don't have to copy StringProducer though. Instead, you can use FileBodyProducer. This IBodyProducer implementation works with any file-like object (so use it with a StringIO if your upload data is already in memory as a string); the idea is the same as StringProducer from the previous example, but with a little extra code to only send data as fast as the server will take it.

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from StringIO import StringIO

from twisted.internet import reactor
from twisted.web.client import Agent
from twisted.web.http_headers import Headers

from twisted.web.client import FileBodyProducer

agent = Agent(reactor)
body = FileBodyProducer(StringIO("hello, world"))
d = agent.request(
    'GET',
    'http://example.com/',
    Headers({'User-Agent': ['Twisted Web Client Example'],
             'Content-Type': ['text/x-greeting']}),
    body)

def cbResponse(ignored):
    print 'Response received'
d.addCallback(cbResponse)

def cbShutdown(ignored):
    reactor.stop()
d.addBoth(cbShutdown)

reactor.run()

FileBodyProducer closes the file when it no longer needs it.

Receiving Responses

If the request succeeds, though, the Deferred will fire with a Response. This happens as soon as all the response headers have been received. It happens before any of the response body, if there is one, is processed. The Response object has several attributes giving the response information: its code, version, phrase, and headers, as well as the length of the body to expect. The Response object also has a method which makes the response body available: deliverBody. Using the attributes of the response object and this method, here's an example which displays part of the response to a request:

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from pprint import pformat

from twisted.internet import reactor
from twisted.internet.defer import Deferred
from twisted.internet.protocol import Protocol
from twisted.web.client import Agent
from twisted.web.http_headers import Headers

class BeginningPrinter(Protocol):
    def __init__(self, finished):
        self.finished = finished
        self.remaining = 1024 * 10

    def dataReceived(self, bytes):
        if self.remaining:
            display = bytes[:self.remaining]
            print 'Some data received:'
            print display
            self.remaining -= len(display)

    def connectionLost(self, reason):
        print 'Finished receiving body:', reason.getErrorMessage()
        self.finished.callback(None)

agent = Agent(reactor)
d = agent.request(
    'GET',
    'http://example.com/',
    Headers({'User-Agent': ['Twisted Web Client Example']}),
    None)

def cbRequest(response):
    print 'Response version:', response.version
    print 'Response code:', response.code
    print 'Response phrase:', response.phrase
    print 'Response headers:'
    print pformat(list(response.headers.getAllRawHeaders()))
    finished = Deferred()
    response.deliverBody(BeginningPrinter(finished))
    return finished
d.addCallback(cbRequest)

def cbShutdown(ignored):
    reactor.stop()
d.addBoth(cbShutdown)

reactor.run()

The BeginningPrinter protocol in this example is passed to Response.deliverBody and the response body is then delivered to its dataReceived method as it arrives. When the body has been completely delivered, the protocol's connectionLost method is called. It is important to inspect the Failure passed to connectionLost. If the response body has been completely received, the failure will wrap a twisted.web.client.ResponseDone exception. This indicates that it is known that all data has been received. It is also possible for the failure to wrap a twisted.web.http.PotentialDataLoss exception: this indicates that the server framed the response such that there is no way to know when the entire response body has been received. Only HTTP/1.0 servers should behave this way. Finally, it is possible for the exception to be of another type, indicating guaranteed data loss for some reason (a lost connection, a memory error, etc).

Just as protocols associated with a TCP connection are given a transport, so will be a protocol passed to deliverBody. Since it makes no sense to write more data to the connection at this stage of the request, though, the transport only provides IPushProducer. This allows the protocol to control the flow of the response data: a call to the transport's pauseProducing method will pause delivery; a later call to resumeProducing will resume it. If it is decided that the rest of the response body is not desired, stopProducing can be used to stop delivery permanently; after this, the protocol's connectionLost method will be called.

An important thing to keep in mind is that the body will only be read from the connection after Response.deliverBody is called. This also means that the connection will remain open until this is done (and the body read). So, in general, any response with a body must have that body read using deliverBody. If the application is not interested in the body, it should issue a HEAD request or use a protocol which immediately calls stopProducing on its transport.

HTTP over SSL

Agent's constructor takes an optional second argument, a context factory. This is an object like the context factory described in Using SSL in Twisted but has one small difference. The getContext method of this factory accepts the address from the URL being requested. This allows it to return a context object which verifies that the server's certificate matches the URL being requested.

Here's an example which shows how to use Agent to request an HTTPS URL with no certificate verification.

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from twisted.python.log import err
from twisted.web.client import Agent
from twisted.internet import reactor
from twisted.internet.ssl import ClientContextFactory

class WebClientContextFactory(ClientContextFactory):
    def getContext(self, hostname, port):
        return ClientContextFactory.getContext(self)

def display(response):
    print "Received response"
    print response

def main():
    contextFactory = WebClientContextFactory()
    agent = Agent(reactor, contextFactory)
    d = agent.request("GET", "https://example.com/")
    d.addCallbacks(display, err)
    d.addCallback(lambda ignored: reactor.stop())
    reactor.run()

if __name__ == "__main__":
    main()

The important point to notice here is that getContext now accepts two arguments, a hostname and a port number. These two arguments, a str and an int, give the address to which a connection is being established to request an HTTPS URL. Because an agent might make multiple requests over a single connection, getContext may not be called once for each request. A second or later request for a URL with the same hostname as a previous request may re-use an existing connection, and therefore will re-use the previously returned context object.

To configure SSL options or enable certificate verification or hostname checking, provide a context factory which creates suitably configured context objects.

Following redirects

By itself, Agent doesn't follow HTTP redirects (responses with 301, 302, 303, 307 status codes and a location header field). You need to use the twisted.web.client.RedirectAgent class to do so. It implements a rather strict behavior of the RFC, meaning it will redirect 301 and 302 as 307, only on GET and HEAD requests.

The following example shows how to have a redirect-enabled agent.

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from twisted.python.log import err
from twisted.web.client import Agent, RedirectAgent
from twisted.internet import reactor

def display(response):
    print "Received response"
    print response

def main():
    agent = RedirectAgent(Agent(reactor))
    d = agent.request("GET", "http://example.com/")
    d.addCallbacks(display, err)
    d.addCallback(lambda ignored: reactor.stop())
    reactor.run()

if __name__ == "__main__":
    main()

Using a HTTP proxy

To be able to use HTTP proxies with an agent, you can use the twisted.web.client.ProxyAgent class. It supports the same interface as Agent, but takes the endpoint of the proxy as initializer argument.

Here's an example demonstrating the use of an HTTP proxy running on localhost:8000.

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from twisted.python.log import err
from twisted.web.client import ProxyAgent
from twisted.internet import reactor
from twisted.internet.endpoints import TCP4ClientEndpoint

def display(response):
    print "Received response"
    print response

def main():
    endpoint = TCP4ClientEndpoint(reactor, "localhost", 8000)
    agent = ProxyAgent(endpoint)
    d = agent.request("GET", "https://example.com/")
    d.addCallbacks(display, err)
    d.addCallback(lambda ignored: reactor.stop())
    reactor.run()

if __name__ == "__main__":
    main()

Please refer to the endpoints documentation for more information about how they work and the twisted.internet.endpoints API documentation to learn what other kinds of endpoints exist.

Handling HTTP cookies

An existing agent instance can be wrapped with twisted.web.client.CookieAgent to automatically store, send and track HTTP cookies. A CookieJar instance, from the Python standard library module cookielib, is used to store the cookie information. An example of using CookieAgent to perform a request and display the collected cookies might look like this:

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from cookielib import CookieJar

from twisted.internet import reactor
from twisted.python import log
from twisted.web.client import Agent, CookieAgent

def displayCookies(response, cookieJar):
    print 'Received response'
    print response
    print 'Cookies:', len(cookieJar)
    for cookie in cookieJar:
        print cookie

def main():
    cookieJar = CookieJar()
    agent = CookieAgent(Agent(reactor), cookieJar)

    d = agent.request('GET', 'http://www.google.com/')
    d.addCallback(displayCookies, cookieJar)
    d.addErrback(log.err)
    d.addCallback(lambda ignored: reactor.stop())
    reactor.run()

if __name__ == "__main__":
    main()

Automatic Content Encoding Negotiation

twisted.web.client.ContentDecoderAgent adds support for sending Accept-Encoding request headers and interpreting Content-Encoding response headers. These headers allow the server to encode the response body somehow, typically with some compression scheme to save on transfer costs. ContentDecoderAgent provides this functionality as a wrapper around an existing agent instance. Together with one or more decoder objects (such as twisted.web.client.GzipDecoder), this wrapper automatically negotiates an encoding to use and decodes the response body accordingly. To application code using such an agent, there is no visible difference in the data delivered.

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from twisted.python import log
from twisted.internet import reactor
from twisted.internet.defer import Deferred
from twisted.internet.protocol import Protocol
from twisted.web.client import Agent, ContentDecoderAgent, GzipDecoder

class BeginningPrinter(Protocol):
    def __init__(self, finished):
        self.finished = finished
        self.remaining = 1024 * 10


    def dataReceived(self, bytes):
        if self.remaining:
            display = bytes[:self.remaining]
            print 'Some data received:'
            print display
            self.remaining -= len(display)


    def connectionLost(self, reason):
        print 'Finished receiving body:', reason.type, reason.value
        self.finished.callback(None)



def printBody(response):
    finished = Deferred()
    response.deliverBody(BeginningPrinter(finished))
    return finished


def main():
    agent = ContentDecoderAgent(Agent(reactor), [('gzip', GzipDecoder)])

    d = agent.request('GET', 'http://www.yahoo.com/')
    d.addCallback(printBody)
    d.addErrback(log.err)
    d.addCallback(lambda ignored: reactor.stop())
    reactor.run()

if __name__ == "__main__":
    main()

Implementing support for new content encodings is as simple as writing a new class like GzipDecoder that can decode a response using the new encoding. As there are not many content encodings in widespread use, gzip is the only encoding supported by Twisted itself.

Conclusion