Programming

Part 10: Poetry Transformed

This continues the introduction started here. You can find an index to the entire series here.

Client 5.0

Now we’re going to add some transformation logic to our poetry client, along the lines suggested in Part 9. But first, I have a shameful and humbling confession to make: I don’t know how to write the Byronification Engine. It is beyond my programming abilities. So instead, I’m going to implement a simpler transformation, the Cummingsifier. The Cummingsifier is an algorithm that takes a poem and returns a new poem like the original but written in the style of e.e. cummings. Here is the Cummingsifier algorithm in its entirety:

Part 9: A Second Interlude, Deferred

This continues the introduction started here. You can find an index to the entire series here.

More Consequence of Callbacks

We’re going to pause for a moment to think about callbacks again. Although we now know enough about deferreds to write simple asynchronous programs in the Twisted style, the Deferred class provides more features that only come into play in more complex settings. So we’re going to think up some more complex settings and see what sort of challenges they pose when programming with callbacks. Then we’ll investigate how deferreds address those challenges.

Part 8: Deferred Poetry

This continues the introduction started here. You can find an index to the entire series here.

Client 4.0

Now that we have know something about deferreds, we can rewrite our Twisted poetry client to use them. You can find client 4.0 in twisted-client-4/get-poetry.py.

Our get_poetry function no longer needs callback or errback arguments. Instead, it returns a new deferred to which the user may attach callbacks and errbacks as needed.

Part 7: An Interlude,  Deferred

This continues the introduction started here. You can find an index to the entire series here.

Callbacks and Their Consequences

In Part 6 we came face-to-face with this fact: callbacks are a fundamental aspect of asynchronous programming with Twisted. Rather than just a way of interfacing with the reactor, callbacks will be woven into the structure of any Twisted program we write. So using Twisted, or any reactor-based asynchronous system, means organizing our code in a particular way, as a series of “callback chains” invoked by a reactor loop.

Part 6: And Then We Took It Higher

This continues the introduction started here. You can find an index to the entire series here.

Poetry for Everyone

We’ve made a lot of progress with our poetry client. Our last version (2.0) is using Transports, Protocols, and Protocol Factories, the workhorses of Twisted networking. But there are more improvements to make. Client 2.0 (and also 2.1) can only be used for downloading poetry at the command line. This is because the PoetryClientFactory is not only in charge of getting poetry, but also in charge of shutting down the program when it’s finished. That’s an odd job for something called “PoetryClientFactory”, it really ought to do nothing beyond making PoetryProtocols and collecting finished poems.

Part 5: Twistier Poetry

This continues the introduction started here. You can find an index to the entire series here.

Abstract Expressionism

In Part 4 we made our first poetry client that uses Twisted. It works pretty well, but there is definitely room for improvement.

First of all, the client includes code for mundane details like creating network sockets and receiving data from those sockets. Twisted provides support for these sorts of things so we don’t have to implement them ourselves every time we write a new program. This is especially helpful because asynchronous I/O requires a few tricky bits involving exception handling as you can see in the client code. And there are even more tricky bits if you want your code to work on multiple platforms. If you have a free afternoon, search the Twisted sources for “win32” to see how many corner cases that platform introduces.

Part 4: Twisted Poetry

This continues the introduction started here. You can find an index to the entire series here.

Our First Twisted Client

Although Twisted is probably more often used to write servers, clients are simpler than servers and we’re starting out as simply as possible. Let’s try out our first poetry client written with Twisted. The source code is in twisted-client-1/get-poetry.py. Start up some poetry servers as before:

python blocking-server/slowpoetry.py --port 10000 poetry/ecstasy.txt --num-bytes 30
python blocking-server/slowpoetry.py --port 10001 poetry/fascination.txt
python blocking-server/slowpoetry.py --port 10002 poetry/science.txt

And then run the client like this:

Part 3: Our Eye-beams Begin to Twist

This continues the introduction started here. You can find an index to the entire series here.

Doing Nothing, the Twisted Way

Eventually we are going to re-implement our asynchronous poetry client using Twisted. But first let’s write a few really simple Twisted programs just to get the flavor of things. As I mentioned in Part 2, I developed these examples using Twisted 8.2.0. Twisted APIs do change, but the core APIs we are going to use will likely change slowly, if at all, so I expect these examples to work for many future releases. If you don’t have Twisted installed you can obtain it here.

Part 2: Slow Poetry and the Apocalypse

This continues the introduction started here. And if you read it, welcome back. Now we’re going to get our hands dirty and write some code. But first, let’s get some assumptions out of the way.

My Assumptions About You

I will proceed as if you have a basic working knowledge of writing synchronous programs in Python, and know at least a little bit about Python socket programming. If you have never used sockets before, you might read the socket module documentation now, especially the example code towards the end. If you’ve never used Python before, then the rest of this introduction is probably going to be rather opaque.

Part 1: In Which We Begin at the Beginning

Preface

Someone recently posted to the Twisted mailing list asking for something like the “Twisted introduction for people on a deadline”. Full disclosure: this isn’t it. On the spectrum of introductions to Twisted and asynchronous programming in Python, it may be on the exact opposite end. So if you don’t have any time, or any patience, this isn’t the introduction you are looking for.