~certify-web-dev/twisted/certify-trunk

"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml" lang="en"><head><title>Twisted Documentation: The Twisted FAQ</title><link href="../howto/stylesheet.css" type="text/css" rel="stylesheet" /></head><body bgcolor="white"><h1 class="title">The Twisted FAQ</h1><div class="toc"><ol><li><a href="#auto0">General</a></li><ul><li><a href="#auto1">What is Twisted?</a></li><li><a href="#auto2">Why should I use Twisted?</a></li><li><a href="#auto3">I have a problem getting Twisted.</a></li><li><a href="#auto4">Why is Twisted so big?</a></li><li><a href="#auto5">But won't Twisted bloat my program, since it's so big?</a></li></ul><li><a href="#auto6">Stability</a></li><ul><li><a href="#auto7">Does the 1.0 release mean that all of Twisted's APIs are stable?</a></li><li><a href="#auto8">Which parts of Twisted are stable?</a></li></ul><li><a href="#auto9">Installation</a></li><ul><li><a href="#auto10">I run mktap (from site-packages/twisted/scripts/mktap.py)

and nothing happens!</a></li><li><a href="#auto11">Why do the Debian packages for Alphas and Release Candidates have weird versions containing old version numbers?</a></li></ul><li><a href="#auto12">Core Twisted</a></li><ul><li><a href="#auto13">How can I access self.factory from my Protocol's __init__?</a></li><li><a href="#auto14">Where can I find out how to write Twisted servers?</a></li><li><a href="#auto15">When I try to install my reactor, I get errors about a reactor

already being installed. What gives?</a></li><li><a href="#auto16">twistd won't load my .tap file! What's this

Ephemeral nonsense?</a></li><li><a href="#auto17">I get Interrupted system call errors when I use os.popen2. How do I read results from a sub-process in

Twisted?</a></li><li><a href="#auto18">Why don't my spawnProcess programs see my environment variables?</a></li><li><a href="#auto19">My Deferred or DeferredList never fires, so my program just mysteriously

hangs! What's wrong?</a></li><li><a href="#auto20">My exceptions and tracebacks aren't getting printed!</a></li><li><a href="#auto21">How do I use Deferreds to make my blocking code non-blocking?</a></li><li><a href="#auto22">I get exceptions.ValueError: signal only works in main thread when I

try to run my Twisted program! What's wrong?</a></li><li><a href="#auto23">I'm trying to stop my program with sys.exit(), but Twisted

seems to catch it! How do I exit my program?</a></li><li><a href="#auto24">How do I find out the IP address of the other end of my connection?</a></li><li><a href="#auto25">Why don't Twisted's network methods support Unicode objects as well as

strings?</a></li></ul><li><a href="#auto26">Perspective Broker</a></li><ul><li><a href="#auto27">How can I get the reference to a client from a Perspective?</a></li></ul><li><a href="#auto28">Requests and Contributing</a></li><ul><li><a href="#auto29">Twisted is cool, but I need to add more functionality.</a></li><li><a href="#auto30">I have a patch. How do I maximize the chances the Twisted developers

will include it?</a></li><li><a href="#auto31">And to whom do I send it?</a></li><li><a href="#auto32">My company would love to use Twisted, but it's missing feature X, can you implement it?</a></li></ul><li><a href="#auto33">Documentation</a></li><ul><li><a href="#auto34">Twisted really needs documentation for X, Y or Z - how come it's not documented?.</a></li><li><a href="#auto35">Wow the Twisted documentation is nice! I want my docs to look like that

too!</a></li></ul><li><a href="#auto36">Communicating with us</a></li><ul><li><a href="#auto37">There's a bug in Twisted. Where do I report it?</a></li><li><a href="#auto38">Where do I go for help?</a></li><li><a href="#auto39">How do I e-mail a Twisted developer?</a></li></ul></ol></div><div class="content"><span></span><h2>General<a name="auto0"></a></h2><h3>What is <q>Twisted</q>?<a name="auto1"></a></h3><p>Please see

<a href="http://twistedmatrix.com/products/twisted">Twisted</a></p><h3>Why should I use Twisted?<a name="auto2"></a></h3><p>See <a href="http://twistedmatrix.com/services/twisted-advantage">The

Twisted Advantage</a></p><h3>I have a problem <q>getting</q> Twisted.<a name="auto3"></a></h3><p>Did you check the HOWTO collection? There are so many documents there

that they might overwhelm you... try starting from the index, reading through

the overviews and seeing if there seems to be a chapter which explains what

you need to. You can try reading the PostScript or PDF formatted books,

inside the distribution. And, remember, the source will be with

you... always.</p><h3>Why is Twisted so big?<a name="auto4"></a></h3><p>Twisted is a lot of things, rolled into one big package. We're not sure if

it'll stay this way, yet, but for now, if you have only specific needs, we

recommend grabbing the big Twisted tarball, and if you want, you can run the

'setup.py' script with a modified config file to generate a package with only

certain Twisted sub-packages. Twisted as a whole makes it into many operating

system distributions (FreeBSD, Debian and Gentoo, at least) so size shouldn't

be an issue for the end developer or user. In addition, packaging Twisted

as a whole makes sure the end users do not have to worry about versioning

parts of Twisted and inter-version compatibility.</p><p>If you are distributing Twisted to end-users, you can base your distribution

on the <q>Nodocs</q> packages, which are signficantly smaller.</p><h3>But won't Twisted bloat my program, since it's so big?<a name="auto5"></a></h3><p>No. You only need to import the sub-packages which you want to use, meaning

only those will be loaded into memory. So if you write a low-level network

protocol, you'd only import twisted.internet, leaving out extraneous things

like twisted.web, etc. Twisted itself is very careful with internal

dependancies, so importing one subpackage is not likely to import the whole

twisted package.</p><h2>Stability<a name="auto6"></a></h2><h3>Does the 1.0 release mean that all of Twisted's APIs are stable?<a name="auto7"></a></h3><p>No, only specific parts of Twisted are stable, i.e. we only promise

backwards compatibility for some parts of Twisted. While these APIs may be

extended, they will not change in ways that break existing code that uses

them. </p><p>While other parts of Twisted are not stable, we will however do

our best to make sure that there is backwards compatibility for these parts

as well. In general, the more the module or package are used, and the closer they

are to being feature complete, the more we will concentrate on providing backwards

compatibility when API changes take place.</p><h3>Which parts of Twisted are stable?<a name="auto8"></a></h3><p>Only modules explictily marked as such can be considered stable. Semi-stable

modules may change, but not in a large way and some sort of backwards-compatibily

will probably be provided. If no comment about API stability is present, assume

the module is unstable.</p><p>In Twisted 1.1, <em>most of twisted.internet, .cred and

.application are completely stable</em> (excepting of course code

marked as deprecated).</p><p>But as always, the only accurate way of knowing a module's stability is reading

the module's docstrings.</p><h2>Installation<a name="auto9"></a></h2><h3>I run mktap (from site-packages/twisted/scripts/mktap.py)

and nothing happens!<a name="auto10"></a></h3><p>Don't run scripts out of <code>site-packages</code>. The Windows

installer should install executable scripts to someplace like

<code>C:\Python22\scripts\</code>, *nix installers put them in

<code>$PREFIX/bin</code>, which should be in your $PATH.</p><h3>Why do the Debian packages for Alphas and Release Candidates have weird versions containing old version numbers?<a name="auto11"></a></h3><p>

An example: 1.0.6+1.0.7rc1-1

</p><p>

In Debian versioning, 1.0.7rc1 is <em>greater than</em> 1.0.7. This

means that if you install a package with Version: 1.0.7rc1, and then

that package gets a new version 1.0.7, apt will not upgrade it for

you, because 1.0.7 looks like an older version. So, we prefix the

previous version to the actual version. 1.0.6+1.0.7rc1 is <em>less

than</em> 1.0.7.

</p><h2>Core Twisted<a name="auto12"></a></h2><h3>How can I access self.factory from my Protocol's __init__?<a name="auto13"></a></h3><p>You can't. A Protocol doesn't have a Factory when it is created. Instead,

you should probably be doing that in your Protocol's

<code>connectionMade</code> method.</p><p>Similarly you shouldn't be doing <q>real</q> work, like connecting to

databases, in a Factory's <code>__init__</code> either. Instead, do that in

<code>startFactory</code>.</p><p>See <a href="servers.html">Writing Servers</a> and

<a href="clients.html">Writing Clients</a> for more details.</p><h3>Where can I find out how to write Twisted servers?<a name="auto14"></a></h3><p>Try <a href="servers.html">Writing Servers</a>.</p><h3>When I try to install my reactor, I get errors about a reactor

already being installed. What gives?<a name="auto15"></a></h3><p>Here's the rule - installing a reactor should always be the

<strong>first</strong> thing you do, and I do mean first. Importing other stuff

before you install the reactor can break your code.</p><p>Tkinter and wxPython support, as they do not install a new reactor, can be

done at any point, IIRC.</p><h3><code>twistd</code> won't load my <code>.tap</code> file! What's this

Ephemeral nonsense?<a name="auto16"></a></h3><p>When the pickled application state cannot be loaded for some reason, it

is common to get a rather opaque error like so:</p><pre class="shell">

% twistd -f test2.tap

Failed to load application: global name 'initRun' is not defined

</pre><p>The rest of the error will try to explain how to solve this problem,

but a short comment first: this error is indeed terse -- but there is

probably more data available elsewhere -- namely, the <code>twistd.log</code>

file. Open it up to see the full exception.</p><p>The error might also look like this:</p><pre class="shell">

Failed to load application: <twisted.persisted.styles.Ephemeral instance at

0x82450a4> is not safe for unpickling

</pre><p>To load a <code>.tap</code> file, as with any unpickling operation, all

the classes used by all the objects inside it must be accessible at the time

of the reload. This may require the PYTHONPATH variable to have the same

directories as were available when the application was first pickled.</p><p>A common problem occurs in single-file programs which define a few

classes, then create instances of those classes for use in a server of some

sort. If the class is used directly, the name of the class will be recorded

in the <code>.tap</code> file as something like

<code>__main__.MyProtocol</code>. When the application is reloaded, it will

look for the class definition in <code>__main__</code>, which probably won't

have it. The unpickling routines need to know the module name, and therefore

the source file, from which the class definition can be loaded.</p><p>The way to fix this is to import the class from the same source file that

defines it: if your source file is called <code>myprogram.py</code> and

defines a class called <code>MyProtocol</code>, you will need to do a

<code>from myprogram import MyProtocol</code> before (and in the same

namespace as) the code that references the MyProtocol class. This makes it

important to write the module cleanly: doing an <code>import

myprogram</code> should only define classes, and should not cause any other

subroutines to get run. All the code that builds the Application and saves

100

it out to a <code>.tap</code> file must be inside an <code>if __name__ ==

101

'__main__'</code> clause to make sure it is not run twice (or more).</p><p>When you import the class from the module using an <q>external</q> name,

102

that name will be recorded in the pickled <code>.tap</code> file. When the

103

<code>.tap</code> is reloaded by <code>twistd</code>, it will look for

104

<code>myprogram.py</code> to provide the definition of

105

<code>MyProtocol</code>.</p><p>Here is a short example of this technique:</p><pre class="python">

106

<span class="py-src-comment"># file dummy.py

107

</span><span class="py-src-keyword">from</span> <span class="py-src-variable">twisted</span>.<span class="py-src-variable">internet</span> <span class="py-src-keyword">import</span> <span class="py-src-variable">protocol</span>

108

<span class="py-src-keyword">class</span> <span class="py-src-identifier">Dummy</span>(<span class="py-src-parameter">protocol</span>.<span class="py-src-parameter">Protocol</span>): <span class="py-src-keyword">pass</span>

109

<span class="py-src-keyword">if</span> <span class="py-src-variable">__name__</span> == <span class="py-src-string">'__main__'</span>:

110

<span class="py-src-keyword">from</span> <span class="py-src-variable">twisted</span>.<span class="py-src-variable">application</span> <span class="py-src-keyword">import</span> <span class="py-src-variable">service</span>, <span class="py-src-variable">internet</span>

111

<span class="py-src-variable">a</span> = <span class="py-src-variable">service</span>.<span class="py-src-variable">Application</span>(<span class="py-src-string">"dummy"</span>)

112

<span class="py-src-keyword">import</span> <span class="py-src-variable">dummy</span>

113

<span class="py-src-variable">f</span> = <span class="py-src-variable">protocol</span>.<span class="py-src-variable">Factory</span>()

114

<span class="py-src-variable">f</span>.<span class="py-src-variable">protocol</span> = <span class="py-src-variable">dummy</span>.<span class="py-src-variable">Dummy</span> <span class="py-src-comment"># Note! Not "Dummy"</span>

115

<span class="py-src-variable">internet</span>.<span class="py-src-variable">TCPServer</span>(<span class="py-src-number">2000</span>, <span class="py-src-variable">f</span>).<span class="py-src-variable">setServiceParent</span>(<span class="py-src-variable">a</span>)

116

<span class="py-src-variable">a</span>.<span class="py-src-variable">save</span>()

117

</pre><h3>I get <q>Interrupted system call</q> errors when I use <code class="python">os.popen2</code>. How do I read results from a sub-process in

118

Twisted?<a name="auto17"></a></h3><p>You should be using <code class="python">reactor.spawnProcess</code> (see

119

<code base="twisted.internet" class="API">interfaces.IReactorProcess.spawnProcess</code>).

120

There's also a convenience function, <code base="twisted.internet.utils" class="API">getProcessOutput</code>, in <code class="API">twisted.internet.utils</code>.</p><h3>Why don't my spawnProcess programs see my environment variables?<a name="auto18"></a></h3><p><code base="twisted.internet.interfaces.IReactorProcess" class="API">spawnProcess</code>

121

defaults to clearing the environment of child processes as a security

122

feature. You can either provide a dictionary with exactly the name-value

123

pairs you want the child to use, or you can simply pass in

124

<code>os.environ</code> to inherit the complete environment. </p><h3>My Deferred or DeferredList never fires, so my program just mysteriously

125

hangs! What's wrong?<a name="auto19"></a></h3><p>It really depends on what your program is doing, but the most common cause

126

is this: it <em>is</em> firing -- but it's an error, not a success, and you

127

have forgotten to add an <a href="glossary.html#errback">errback</a>, so

128

nothing happens. Always add errbacks!</p><p>The reason this happens is that unhandled errors in Deferreds get printed

129

when the Deferred is garbage collected. Make sure your Deferred is garbage

130

collected by deleting all references to it when you are done with it, e.g.

131

after <code class="python">callback()</code> is called.</p><h3>My exceptions and tracebacks aren't getting printed!<a name="auto20"></a></h3><p>See previous question.</p><h3>How do I use Deferreds to make my blocking code non-blocking?<a name="auto21"></a></h3><a name="deferreds-aren't-magic"></a><p>You don't. Deferreds don't magically turn a blocking function call into a

132

non-blocking one. A Deferred is just a simple object that represents a

133

<em>deferred result</em>, with methods to allow convenient adding of

134

callbacks. (This is a common misunderstanding; suggestions on how to make this

135

clearer in the <a href="defer.html">Deferred Execution</a> howto are

136

welcome!)</p><p>If you have blocking code that you want to use non-blockingly in Twisted,

137

either rewrite it to be non-blocking, or run it in a thread. There is a

138

convenience function, <code base="twisted.internet.threads" class="API">deferToThread</code>, to help you with the

139

threaded approach -- but be sure to read <a href="threading.html">Using

140

Threads in Twisted</a>.</p><h3>I get <q>exceptions.ValueError: signal only works in main thread</q> when I

141

try to run my Twisted program! What's wrong?<a name="auto22"></a></h3><p>The default reactor, by default, will install signal handlers to catch

142

events like Ctrl-C, SIGTERM, and so on. However, you can't install signal

143

handlers from non-main threads in Python, which means that

144

<code>reactor.run()</code> will cause an

145

error. Pass the <code>installSignalHandlers=0</code> keyword argument to

146

<code>reactor.run</code> (or <code>Application.run</code>) to work around

147

this.</p><h3>I'm trying to stop my program with <code>sys.exit()</code>, but Twisted

148

seems to catch it! How do I exit my program?<a name="auto23"></a></h3><p>Use <code>reactor.stop()</code> instead. This will cleanly shutdown the

149

reactor.</p><h3>How do I find out the IP address of the other end of my connection?<a name="auto24"></a></h3><p>The <code>.transport</code> object (which implements the <code base="twisted.internet.interfaces" class="API">ITransport</code> interface) offers a pair

150

of methods named <code base="twisted.internet.interfaces.ITransport" class="API">getPeer</code> and <code base="twisted.internet.interfaces.ITransport" class="API">getHost</code>.

151

<code>getPeer</code> will give you a tuple that describes the address of the

152

system at the other end of the connection. For example:</p><pre class="python">

153

<span class="py-src-keyword">class</span> <span class="py-src-identifier">MyProtocol</span>(<span class="py-src-parameter">protocol</span>.<span class="py-src-parameter">Protocol</span>):

154

<span class="py-src-keyword">def</span> <span class="py-src-identifier">connectionMade</span>(<span class="py-src-parameter">self</span>):

155

<span class="py-src-keyword">print</span> <span class="py-src-string">"connection from"</span>, <span class="py-src-variable">self</span>.<span class="py-src-variable">transport</span>.<span class="py-src-variable">getPeer</span>()

156

</pre><h3>Why don't Twisted's network methods support Unicode objects as well as

157

strings?<a name="auto25"></a></h3><p>In general, such methods (eg <code base="twisted.internet.abstract" class="API">FileDescriptor</code>'s <code>write</code>)

158

are designed to send bytes over the network. These methods use non-Unicode

159

string objects as a container for the bytes that they send and receive.</p><p>Unicode objects are not byte-based and are an abstraction used for

160

representing strings of human readable text. In order to send Unicode strings

161

using these methods, you should explicitly specify a byte-based encoding for

162

them, for example: <code>s.encode("utf-8")</code> and explicitly decode them

163

at the receiving end.</p><p>Twisted cannot choose an encoding for you at this level: your encoding

164

choice will be protocol specific and may need to be specified in the message

165

you send (for example, HTTP headers include a encoding specification).</p><p>For a more complete discussion of the distinction between Unicode strings

166

and specific encodings of Unicode strings, see the following articles:</p><ul><li>Dan Sugalski's <a href="http://www.sidhe.org/~dan/blog/archives/000255.html">What the heck is: A

167

string</a>; and</li><li>Joel Spolsky's <a href="http://www.joelonsoftware.com/articles/Unicode.html">The Absolute

168

Minimum Every Software Developer Absolutely, Positively Must Know

169

About Unicode and Character Sets (No Excuses!)</a>.</li></ul><h2>Perspective Broker<a name="auto26"></a></h2><h3>How can I get the reference to a client from a Perspective?<a name="auto27"></a></h3><p>Firstly, the client must send a reference when it connects to the

170

perspective broker. This can be done by passing the reference as a parameter to

171

<code base="twisted.spread" class="API">pb.connect</code>.</p><p>At the server end, you must override the <code base="twisted.spread.pb" class="API">Perspective.attach</code>, which is called when a client attaches

172

to a perspective. The first argument of this method is a remote reference to

173

the client object that was passed to <code base="twisted.spread" class="API">pb.connect</code>. </p><p>Note that a single perspective can have many attached clients. For further

174

information, see <a href="pclients.html">Managing Clients of Perspectives</a>

175

HOWTO and the <code class="API">twisted.spread.pb</code> API docs.</p><h2>Requests and Contributing<a name="auto28"></a></h2><h3>Twisted is cool, but I need to add more functionality.<a name="auto29"></a></h3><p>Great! Read our the docs, and if you're feeling generous, contribute

176

patches.</p><h3>I have a patch. How do I maximize the chances the Twisted developers

177

will include it?<a name="auto30"></a></h3><p>Use unified diff. Either use <code class="shell">svn diff</code>

178

or, better yet, make a clean checkout and use <code class="shell">diff

179

-urN</code> between them. Make sure your patch applies cleanly. In

180

your post to the mailing list, make sure it is inlined and without any

181

word wrapping.</p><h3>And to whom do I send it?<a name="auto31"></a></h3><p>Add it to the <a href="http://twistedmatrix.com/bugs/">bug tracker</a>, and if it's an urgent or important issue you may want to tell the <a href="http://twistedmatrix.com/cgi-bin/mailman/listinfo/twisted-python">mailing list</a>. about the issue you added</p><h3>My company would love to use Twisted, but it's missing feature X, can you implement it?<a name="auto32"></a></h3><p>You have 3 options:</p><ul><li>Pay one of the Twisted developers to implement the feature.</li><li>Implement the feature yourself.</li><li>Add a feature request to our bug tracker. We will try to implement the

182

feature, but there are no guarantees when and if this will happen.</li></ul><h2>Documentation<a name="auto33"></a></h2><h3>Twisted really needs documentation for X, Y or Z - how come it's not documented?.<a name="auto34"></a></h3><p>Twisted's documentation is a work in progress, and one that we would

183

appreciate assistance with. If you notice a gap or flaw in the documentation,

184

please file a bug in <a href="http://twistedmatrix.com/bugs/">the Twisted bug

185

tracker</a> and mark it as having topic 'documentation'. Patches

186

appreciated.</p><h3>Wow the Twisted documentation is nice! I want my docs to look like that

187

too!<a name="auto35"></a></h3><p>Now you can, with <a href="http://twistedmatrix.com/projects/lore">Lore</a>. </p><h2>Communicating with us<a name="auto36"></a></h2><h3>There's a bug in Twisted. Where do I report it?<a name="auto37"></a></h3><p>Unless it is a show-stopper bug, we usually won't fix it if it's already

188

fixed in <a href="http://twistedmatrix.com/developers/cvs">Subversion</a>, so

189

check if it is fixed there. If it is not fixed in Subversion, you should add

190

it to the <a href="http://twistedmatrix.com/bugs/">bug tracker</a>, including

191

pertinent information about the bug (hopefully as much information needed to

192

reproduce it: OS, Subversion versions of any important files, Python version,

193

code you wrote or things you did to trigger the bug, etc. If the bug appears

194

to be severe, you should also raise it on the <a href="http://twistedmatrix.com/cgi-bin/mailman/listinfo/twisted-python">mailing

195

list</a>, with a pointer to the issue already filed in the bug tracker.</p><h3>Where do I go for help?<a name="auto38"></a></h3><p>Ask for help <a href="http://twistedmatrix.com/services/online-help">where

196

the Twisted team hangs out</a></p><h3>How do I e-mail a Twisted developer?<a name="auto39"></a></h3><p>First, note that in many cases this is the wrong thing to do: if

197

you have a question about a part of Twisted, it's usually better to

198

e-mail the mailing list. However, the preferred e-mail addresses for

199

all Twisted developers are listed in the file <q>CREDITS</q> in the

200

Subversion repository.</p></div><p><a href="../howto/index.html">Index</a></p><span class="version">Version: 2.0.1</span></body></html>

b'\\ No newline at end of file'

Older »