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pythontwisted
anijhaw
anijhaw

Reputation: 9392

Python twisted Reactor class

What is the significance of the decorators

 @reactor.callWhenRunning,
 @results_deferred.addCallback
 @results_deferred.addErrback.

Also what are deferred strings, for example in the 

 twisted.internet.utils.getProcessOutput()

returns a deferred string what exactly is happening here?

I am new to twisted hence this might be a very simple question but reading twisted documentation did not help me much

Upvotes: 3

Views: 2201

Answers (3)

Jeffrey Jose
Jeffrey Jose

Reputation: 2052

In the normal programming practice you'd do

db = Database.connect()
result = db.getResult()
processResult(result)

Now depending on your Database and network, these 3 statements can take anywhere from a millisecond to a few seconds.

We've all been programming this way for decades now, and for the most part we're fine with 'waiting'.

But there comes a time when your program cant just wait for results. You'd start to think, gee I could do a lot of other things while I wait for the result. Maybe print an output, or process a function, or just quickly check the socket etc.

Enter Twisted and Deferred.

Instead of waiting for result, in Twisted when invoked the special methods you'll get a Deferred. You'll add a callback function to this deferred which means, call this function when you have the result/answer.

deferredResult = db.nonBlockingGetResult()
deferredResult.addCallback(processOutput)

As soon as the first statement is executed, it returns the 'something' back. And that something is Deferred. There's no blocking there, there no waiting. And to this Deferred you add the callback processOutput which is called when deferred is 'fired' - ie result is ready.

HTH

Upvotes: 4

Yogesh Arora
Yogesh Arora

Reputation: 2256

I am not sure about python , but this looks a like Active object pattern, and Futures. Futures is going to be standard in next c++ version. If you read through Active object and Futures you will get an idea

Upvotes: 1

msalib
msalib

Reputation: 450

A deferred is a like a promise to return output in the future. You really should read the documentation on Deferreds here and here. Also, you should read up on Python decorators in general. One introduction is here.

More specifically, what is happening is that when you call getProcessOutput(), the result is not quite ready. It might be ready in an instant or in an hour. But you probably don't care: whenever it is ready, you probably want to take the output and pass it to a function. So instead of returning the output (which is not going to be ready right away), getProcessOutput returns a deferred object. When the output is finally ready, the deferred object will notice and call whatever processing function you supply, passing along the actual process output data. You really should read up on deferreds though.

Upvotes: 3

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