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()

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.

Conclusion