Why is there a need for Twisted?

I have been playing around with the twisted framework for about a week now(more because of curiosity rather than having to use it) and its been a lot of fun doing event driven asynchronous network programming.

However, there is something that I fail to understand. The twisted documentation starts off with

Twisted is a framework designed to be very flexible and let you write powerful servers.

My doubt is :- Why do we need such an event-driven library to write powerful servers when there are already very efficient implementat开发者_运维知识库ions of various servers out there?

Surely, there must have been more than a couple of concrete implementations which the twisted developers had in mind while writing this event-driven I\O library. What are those? Why exactly was twisted made?

In a comment on another answer, you say "Every library is supposed to have ...". "Supposed" by whom? Having use-cases is certainly a nice way to nail down your requirements, but it's not the only way. It also doesn't make sense to talk about the use-cases for all of Twisted at once. There is no use case that justifies every single API in Twisted. There are hundreds or thousands of different use cases, each which justifies a lesser or greater subdivision of Twisted. These came and went over the years of Twisted's development, and no attempt has been made to keep a list of them. I can say that I worked on part of Twisted Names so that I would have a topic for a paper I was presenting at the time. I implemented the vt102 parser in Twisted Conch because I am obsessed with terminals and wanted a fun project involving them. And I implemented the IMAP4 support in Twisted Mail because I worked at a company developing a mail server which required tighter control over the mail store than any other IMAP4 server at the time offered.

So, as you can see, different parts of Twisted were written for widely differing reasons (and I've only given examples of my own reasons, not the reasons of any other developers).

The initial reason for a program being written often doesn't matter much in the long run though. Now the code is written: Twisted Names now runs the DNS for many domain names on the internet, the vt102 parser helped me get a job, and the company that drove the IMAP4 development is out of business. What really matters is what useful things you can do with the code now. As MattH points out, the resulting plethora of functionality has resulted in a library that (perhaps uniquely) addresses a wide array of interesting problems.

Why do we need such an event-driven library to write powerful servers when there are already very efficient implementations of various servers out there?

So paraphrasing: you can't imagine why anyone would need a toolkit when dyecast products already exist?

I'm guessing you've never needed to knock up a protocol gateway, e.g.
- write a daemon to md5 local files on demand over a unix socket
- interrogate a piece of software using udp and expose statistics over http.

I wrote a little proof-of-concept for the second example for a question here on SO in a handful of minutes. I couldn't do that without twisted.

Have you looked at: ProjectsUsingTwisted?

More on 'why': (disclaimer: I'm not a developer of Twisted proper), it's necessary to consider Twisted's high age (relative to Python's). When Twisted was written there was no sufficiently powerful non-blocking network/event driven library written around the reactor pattern (almost everyone was using threads back then). Twisted's initial use case was a large multiplayer game, although the specifics of this game seems to be somewhat lost in time.

Since the origins, as @MattH's link suggest, a very large amount of various network servers written in Python is based on Twisted.

This PyCon talk by the creator of Twisted should give you answers.

It has changed my opinion of Twisted. Before I viewed it as a massive piece of software with interfaces and weird names, two things that many developers dislike but that are actually just superficial things, and now that I’ve seen the history behind and the amazing number of use cases I respect it a lot. Life is short, you need Twisted :)