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# Using the Twisted Web Client

## Overview

This document describes how to use the HTTP client included in Twisted Web. After reading it, you should be able to make HTTP and HTTPS requests using Twisted Web. You will be able to specify the request method, headers, and body and you will be able to retrieve the response code, headers, and body.

A number of higher-level features are also explained, including proxying, automatic content encoding negotiation, and cookie handling.

### 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:

`request.py`

    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:

`stringprod.py`

    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:

`sendbody.py`

    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.

`filesendbody.py`

    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.

If the connection or the request take too much time, you can cancel the `Deferred` returned by the `Agent.request` method. This will abort the connection, and the `Deferred` will errback with CancelledError .

### Receiving Responses

So far, the examples have demonstrated how to issue a request. However, they have ignored the response, except for showing that it is a `Deferred` which seems to fire when the response has been received. Next we’ll cover what that response is and how to interpret it.

`Agent.request` , as with most `Deferred` -returning APIs, can return a `Deferred` which fires with a `Failure` . If the request fails somehow, this will be reflected with a failure. This may be due to a problem looking up the host IP address, or it may be because the HTTP server is not accepting connections, or it may be because of a problem parsing the response, or any other problem which arises which prevents the response from being received. It does not include responses with an error status.

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. In addition to these, the `Response` also contains a reference to the request that it is a response to; one particularly useful attribute on the request is absoluteURI : The absolute URI to which the request was made. The `Response` object 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:

`response.py`

    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.

If the body of the response isn’t going to be consumed incrementally, then readBody can be used to get the body as a byte-string. This function returns a `Deferred` that fires with the body after the request has been completed.

`responseBody.py`

    from sys import argv
    from pprint import pformat

    from twisted.internet.task import react
    from twisted.web.client import Agent, readBody
    from twisted.web.http_headers import Headers

    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()))
        d = readBody(response)
        d.addCallback(cbBody)
        return d

    def cbBody(body):
        print 'Response body:'
        print body

    def main(reactor, url=b"http://example.com/"):
        agent = Agent(reactor)
        d = agent.request(
            'GET', url,
            Headers({'User-Agent': ['Twisted Web Client Example']}),
            None)
        d.addCallback(cbRequest)
        return d

    react(main, argv[1:])

### HTTP over SSL

Everything you’ve read so far applies whether the scheme of the request URI is HTTP or HTTPS . However, to control the SSL negotiation performed when an HTTPS URI is requested, there’s one extra object to pay attention to: the SSL context factory.

`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.

    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.

### HTTP Persistent Connection

HTTP persistent connections use the same TCP connection to send and receive multiple HTTP requests/responses. This reduces latency and TCP connection establishment overhead.

The constructor of twisted.web.client.Agent takes an optional parameter pool, which should be an instance of HTTPConnectionPool , which will be used to manage the connections. If the pool is created with the parameter `persistent` set to `True` (the default), it will not close connections when the request is done, and instead hold them in its cache to be re-used.

Here’s an example which sends requests over a persistent connection:

    from twisted.internet import reactor
    from twisted.internet.defer import Deferred, DeferredList
    from twisted.internet.protocol import Protocol
    from twisted.web.client import Agent, HTTPConnectionPool

    class IgnoreBody(Protocol):
        def __init__(self, deferred):
            self.deferred = deferred

        def dataReceived(self, bytes):
            pass

        def connectionLost(self, reason):
            self.deferred.callback(None)

    def cbRequest(response):
        print 'Response code:', response.code
        finished = Deferred()
        response.deliverBody(IgnoreBody(finished))
        return finished

    pool = HTTPConnectionPool(reactor)
    agent = Agent(reactor, pool=pool)

    def requestGet(url):
        d = agent.request('GET', url)
        d.addCallback(cbRequest)
        return d

    # Two requests to the same host:
    d = requestGet('http://localhost:8080/foo').addCallback(
        lambda ign: requestGet("http://localhost:8080/bar"))
    def cbShutdown(ignored):
        reactor.stop()
    d.addCallback(cbShutdown)

    reactor.run()

Here, the two requests are to the same host, one after the each other. In most cases, the same connection will be used for the second request, instead of two different connections when using a non-persistent pool.

### Multiple Connections to the Same Server

twisted.web.client.HTTPConnectionPool instances have an attribute called `maxPersistentPerHost` which limits the number of cached persistent connections to the same server. The default value is 2. This is effective only when the persistent option is True. You can change the value like bellow:

    from twisted.web.client import HTTPConnectionPool

    pool = HTTPConnectionPool(reactor, persistent=True)
    pool.maxPersistentPerHost = 1

With the default value of 2, the pool keeps around two connections to the same host at most. Eventually the cached persistent connections will be closed, by default after 240 seconds; you can change this timeout value with the `cachedConnectionTimeout` attribute of the pool. To force all connections to close use the closeCachedConnections method.

#### Automatic Retries

If a request fails without getting a response, and the request is something that hopefully can be retried without having any side-effects (e.g. a request with method GET), it will be retried automatically when sending a request over a previously-cached persistent connection. You can disable this behavior by setting retryAutomatically to `False` . Note that each request will only be retried once.

### 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.

    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()

In contrast, twisted.web.client.BrowserLikeRedirectAgent implements more lenient behaviour that closely emulates what web browsers do; in other words 301 and 302 `POST` redirects are treated like 303, meaning the method is changed to `GET` before making the redirect request.

As mentioned previously, Response contains a reference to both the request that it is a response to, and the previously received response , accessible by previousResponse . In most cases there will not be a previous response, but in the case of `RedirectAgent` the response history can be obtained by following the previous responses from response to response.

### 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.

    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:

`cookies.py`

    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.

`gzipdecoder.py`

    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

You should now understand the basics of the Twisted Web HTTP client. In particular, you should understand:

- How to issue requests with arbitrary methods, headers, and bodies.
- How to access the response version, code, phrase, headers, and body.
- How to store, send, and track cookies.
- How to control the streaming of the response body.
- How to enable the HTTP persistent connection, and control the number of connections.