# The Twisted FAQ

## General

### What is Twisted?

Please see Twisted

### Why should I use Twisted?

See The Twisted Advantage

### I have a problem getting Twisted.

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.

### Why is Twisted so big?

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.

If you are distributing Twisted to end-users, you can base your distribution on the Nodocs packages, which are signficantly smaller.

### But won't Twisted bloat my program, since it's so big?

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.

## Stability

### Does the 1.0 release mean that all of Twisted's APIs are stable?

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.

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.

### Which parts of Twisted are stable?

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.

In Twisted 1.1, most of twisted.internet, .cred and .application are completely stable (excepting of course code marked as deprecated).

But as always, the only accurate way of knowing a module's stability is reading the module's docstrings.

## Installation

### I run mktap (from site-packages/twisted/scripts/mktap.py) and nothing happens!

Don't run scripts out of `site-packages`. The Windows installer should install executable scripts to someplace like `C:\Python22\scripts\`, *nix installers put them in `$PREFIX/bin`, which should be in your $PATH.

### Why do the Debian packages for Alphas and Release Candidates have weird versions containing old version numbers?

An example: 1.0.6+1.0.7rc1-1

In Debian versioning, 1.0.7rc1 is greater than 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 less than 1.0.7.

## Core Twisted

### How can I access self.factory from my Protocol's __init__?

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 `connectionMade` method.

Similarly you shouldn't be doing real work, like connecting to databases, in a Factory's `__init__` either. Instead, do that in `startFactory`.

See Writing Servers and Writing Clients for more details.

### Where can I find out how to write Twisted servers?

Try Writing Servers.

### When I try to install my reactor, I get errors about a reactor already being installed. What gives?

Here's the rule - installing a reactor should always be the first thing you do, and I do mean first. Importing other stuff before you install the reactor can break your code.

Tkinter and wxPython support, as they do not install a new reactor, can be done at any point, IIRC.

### `twistd` won't load my `.tap` file! What's this Ephemeral nonsense?

When the pickled application state cannot be loaded for some reason, it is common to get a rather opaque error like so:

    % twistd -f test2.tap

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

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 `twistd.log` file. Open it up to see the full exception.

The error might also look like this:

    Failed to load application: <twisted.persisted.styles.Ephemeral instance at
    0x82450a4> is not safe for unpickling

To load a `.tap` 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.

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 `.tap` file as something like `__main__.MyProtocol`. When the application is reloaded, it will look for the class definition in `__main__`, 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.

The way to fix this is to import the class from the same source file that defines it: if your source file is called `myprogram.py` and defines a class called `MyProtocol`, you will need to do a `from myprogram import MyProtocol` 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 `import myprogram` should only define classes, and should not cause any other subroutines to get run. All the code that builds the Application and saves it out to a `.tap` file must be inside an `if __name__ == '__main__'` clause to make sure it is not run twice (or more).

When you import the class from the module using an external name, that name will be recorded in the pickled `.tap` file. When the `.tap` is reloaded by `twistd`, it will look for `myprogram.py` to provide the definition of `MyProtocol`.

Here is a short example of this technique:

    # file dummy.py
    from twisted.internet import protocol
    class Dummy(protocol.Protocol): pass
    if __name__ == '__main__':
        from twisted.application import service, internet
        a = service.Application("dummy")
        import dummy
        f = protocol.Factory()
        f.protocol = dummy.Dummy # Note! Not "Dummy"
        internet.TCPServer(2000, f).setServiceParent(service.IServiceCollect(a))
        a.save()

### I get Interrupted system call errors when I use `os.popen2`. How do I read results from a sub-process in Twisted?

You should be using `reactor.spawnProcess` (see `interfaces.IReactorProcess.spawnProcess`). There's also a convenience function, `getProcessOutput`, in `twisted.internet.utils`.

### Why don't my spawnProcess programs see my environment variables?

`spawnProcess` defaults to clearing the environment of child processes as a security feature. You can either provide a dictionary with exactly the name-value pairs you want the child to use, or you can simply pass in `os.environ` to inherit the complete environment.

### My Deferred or DeferredList never fires, so my program just mysteriously hangs! What's wrong?

It really depends on what your program is doing, but the most common cause is this: it is firing -- but it's an error, not a success, and you have forgotten to add an errback, so nothing happens. Always add errbacks!

The reason this happens is that unhandled errors in Deferreds get printed when the Deferred is garbage collected. Make sure your Deferred is garbage collected by deleting all references to it when you are done with it, e.g. after `callback()` is called.

### My exceptions and tracebacks aren't getting printed!

See previous question.

### How do I use Deferreds to make my blocking code non-blocking?

You don't. Deferreds don't magically turn a blocking function call into a non-blocking one. A Deferred is just a simple object that represents a deferred result, with methods to allow convenient adding of callbacks. (This is a common misunderstanding; suggestions on how to make this clearer in the Deferred Execution howto are welcome!)

If you have blocking code that you want to use non-blockingly in Twisted, either rewrite it to be non-blocking, or run it in a thread. There is a convenience function, `deferToThread`, to help you with the threaded approach -- but be sure to read Using Threads in Twisted.

### I get exceptions.ValueError: signal only works in main thread when I try to run my Twisted program! What's wrong?

The default reactor, by default, will install signal handlers to catch events like Ctrl-C, SIGTERM, and so on. However, you can't install signal handlers from non-main threads in Python, which means that `reactor.run()` will cause an error. Pass the `installSignalHandlers=0` keyword argument to `reactor.run` (or `Application.run`) to work around this.

### I'm trying to stop my program with `sys.exit()`, but Twisted seems to catch it! How do I exit my program?

Use `reactor.stop()` instead. This will cleanly shutdown the reactor.

### How do I find out the IP address of the other end of my connection?

The `.transport` object (which implements the `ITransport` interface) offers a pair of methods named `getPeer` and `getHost`. `getPeer` will give you a tuple that describes the address of the system at the other end of the connection. For example:

    class MyProtocol(protocol.Protocol):
        def connectionMade(self):
            print "connection from", self.transport.getPeer()

## Web

### Is the Twisted web server a toy?

No. It is a production grade server. It is running continously on several sites and has been proven quite stable. The server can take loads of up to 3000 users at a time and still keep churning several million requests a day, even on low end hardware. It can serve static files or dynamically rendered pages.

### But can Twisted Web do PHP?

Yes. It works out-of-the-box, so long as you've got the standalone php interpreter installed.

### And can Twisted Web do virtual hosting?

Can it ever!

You can decide to go with one big process for all of them, a front server and a seperate server for each virtual host (for example, for permission reasons), and you can even mix-and-match between Apache and Twisted (for example, put Apache in the front and have Twisted handle some subset of the virtual host).

### How do I use twisted.web to do complex things?

See the Twisted.Web Howto.

### I've been using Woven since before it was called Woven. I just upgraded and now I'm getting a confusing traceback talking about INodeMutator. What gives?

You probably have code that's survived the upgrade from PyXML's `minidom` to Twisted's `microdom`. Try deleting any `.pxp` files that you have lying around and the error will probably go away.

### My Woven pages are sent to the browser with a trailing slash appended to the URL, which breaks all of the relative links. How do I get rid of the trailing slash?

If you are subclassing `Page`, you can add a class attribute `addSlash = 0`, like this:

    class Foo(page.Page):
            addSlash = 0

If you are still subclassing `Controller`, you can put the `addSlash = 0` there. Consider subclassing `Page` instead, as having a `Model`, `View`, `Controller` triad as the base of a `Page` will be deprecated in the near future.

If you're just using the generic `Page` instance, you can set it after creation like this:

    resource = page.Page("foo")
    resource.addSlash = 0

The default behavior of Woven is now to automatically add a slash because it makes creating relative links far easier, ironically ;-)

### Argh! When using Woven, my newlines get mangled inside a <pre>

Use the `RawText` view.

### When trying to use Guard, I get infinite redirects to URLs with long hexadecimal numbers. What's the deal?

Are you using an RPY? Add a single line containing `cache()` as the first line of your RPY.

An RPY file is executed every time it is accessed, so the call to `guard.SessionWrapper` is executed once per request, making a new SessionWrapper for each request. There should be only one SessionWrapper object for your application. Caching the RPY file enforces this.

## Perspective Broker

### How can I get the reference to a client from a Perspective?

Firstly, the client must send a reference when it connects to the perspective broker. This can be done by passing the reference as a parameter to `pb.connect`.

At the server end, you must override the `Perspective.attach`, which is called when a client attaches to a perspective. The first argument of this method is a remote reference to the client object that was passed to `pb.connect`.

Note that a single perspective can have many attached clients. For further information, see Managing Clients of Perspectives HOWTO and the `twisted.spread.pb` API docs.

## Requests and Contributing

### Twisted is cool, but I need to add more functionality.

Great! Read our the docs, and if you're feeling generous, contribute patches.

### I have a patch. How do I maximize the chances the Twisted developers will include it?

Use unified diff. Either use `svn diff` or, better yet, make a clean checkout and use `diff -urN` between them. Make sure your patch applies cleanly. In your post to the mailing list, make sure it is inlined and without any word wrapping.

### And to whom do I send it?

Add it to the bug tracker, and if it's an urgent or important issue you may want to tell the mailing list. about the issue you added

### My company would love to use Twisted, but it's missing feature X, can you implement it?

You have 3 options:

- Pay one of the Twisted developers to implement the feature.
- Implement the feature yourself.
- Add a feature request to our bug tracker. We will try to implement the feature, but there are no guarantees when and if this will happen.

## Documentation

### Twisted really needs documentation for X, Y or Z - how come it's not documented?.

We are doing the best we can, and there is documentation in progress for many parts of Twisted. There is a limit to how much we can do in our free time. See also the answer to the next question.

### Wow the Twisted documentation is nice! I want my docs to look like that too!

Now you can, with `twisted.lore`. See the manual page for `lore`. For source format documentation, see the documentation standard description. For a more comprehensive explanation, see the Lore HOWTO.

## Communicating with us

### There's a bug in Twisted. Where do I report it?

Unless it is a show-stopper bug, we usually won't fix it if it's already fixed in Subversion, so you would do well to look there. Then send any pertinent information about the bug (hopefully as much information needed to reproduce it: OS, Subversion versions of any important files, Python version, code you wrote or things you did to trigger the bug, etc.) to the mailing list. If no one answers immediately, you should add it to the bug tracker.

### Where do I go for help?

Ask for help where the Twisted team hangs out

### How do I e-mail a Twisted developer?

First, note that in many cases this is the wrong thing to do: if you have a question about a part of Twisted, it's usually better to e-mail the mailing list. However, the preferred e-mail addresses for all Twisted developers are listed in the file CREDITS in the Subversion repository.