Paste Script

Paste Script is a pluggable command-line script (called paster). Using plugins you can add new commands like paster my-command to it, either adding them globally or framework-local. "Framework" doesn't necessarily mean web framework, it can mean anything, including ad hoc commands specific to a project.

Paste Script provides two commands by default:

• paster create is a command that creates or updates projects according to some layout template. The templates themselves are also pluggable. This is essentially the start-project command already present in many recent frameworks.
• paster serve is a command that loads Paste Deploy configuration files (more about that later) and serves them up.

Paste Deploy

Paste Deploy is a pluggable configuration-file loader for web applications and servers.

The "plugins" that you provide to Paste Deploy are actually factories for applications and servers. The configuration file will indicate what factory you want to use, and then the arguments to send to the factory. This is to say, the application and its configuration.

Factories themselves can be other configuration files, or Python Egg entry points. That's part of setuptools, not Paste, so I won't describe it here. But it's something good to understand. Ben Bangert had a post describing setuptools in the context of Paste, which might be helpful.

Paste Deploy is pretty simple when you get down to it -- it's just factories, passing in configuration data, and returning a WSGI app. What's a WSGI app? Your framework should be able to give you one of those -- if you are just writing an application, you shouldn't have to know about the details of WSGI. If your framework doesn't support WSGI, then you should ask the framework authors to fix that.

Paste Deploy does implicitly encourage some specific practices. One is that you keep your configuration separate from your application. Hopefully the benefits are obvious, but in practice this is not the norm in a lot of current frameworks. Also, there are other tools (in Paste core which I talk about next) which work better if your application is well encapsulated and obeys the WSGI spec and uses WSGI semantics (one common issue is respecting and properly adjusting SCRIPT_NAME and PATH_INFO, though other ones come up). These again are framework issues, but ones that several frameworks do not currently get right.

Another feature of a Paste-Deploy-enabled application is that it will have a simple factory that takes specific arguments. So while it makes the application configurable, it also makes it really easy to invoke the application programmatically. This means that it should be easy to create applications that embed each other, by delegating some incoming requests to the embedded application.

Maybe I'll do a screencast on Paste Deploy sometime soon. It's best feature, IMHO, is that it's actually quite simple, and it maps to simple Python function invocation.

Paste (the rest)

So there's another package called just "Paste". It has a bunch of tools. These tools are "dead", in that they don't magically appear in your environment, you aren't forced to use them, and they mostly aren't required in order to work with each other. The full list is described on the features page.

You don't have to use any of these. Most of them are of more interest to a framework developer than an application programmer, and they are best used when adopted into a framework. If you have an ad hoc framework then you might be interested; if you aren't using some other framework, you've likely built an ad hoc framework whether you meant to or not. It's hard to program directly against CGI, mod_python, or some other low-level system without somehow creating a framework.

All the Paste parts use WSGI, because they are all intended to be framework neutral, and there is no "framework neutral" in Python web programming for much of this functionality except for WSGI.

A number of the tools are "WSGI middleware", which means they wrap WSGI applications. What's a WSGI application? Remember: if you aren't developing a framework you shouldn't need to know, except to know that your framework creates a WSGI application for you. But, back to middleware -- it means that a request goes through the middleware (which can modify it) and the response goes out through the middleware (which can also modify the response). This is how things like exception catching are implemented:

• The exception catcher wraps your app in try:except:
• If everything goes fine, it passed the request through, and passes the response out without modification.
• If your application raises an exception, it turns this into an exception report, and depending on configuration may email it to someone or just display it in the browser.

But, this is a framework concern. If you like the exception catching middleware (because it's cool), then your framework should adopt it, or at least allow for it, and it should pass through configuration to that code. It's probably not something that a modest application developer will concern themselves with. Of course this is open source, so everyone can be a framework contributor. In the end, if you use lots of these Paste tools you can write a framework from scratch with very little code. Your framework will be all about creating an aesthetically cohesive wrapper around the tools; Paste isn't a framework, WSGI isn't a framework, they are just agreements, and tools that utilize those agreements.

There are also some tools in Paste that are useful with Paste Deploy, like something to map requests to subapplications based on URL prefixes. It should be possible to apply these to a well-encapsulated WSGI application without problem, and so these are "deployer" concerns, not application author or framework developer concerns.

Conclusion

So that's what Paste is. Maybe it makes more sense this time.

One thing I'll note is that I think Paste Deploy is the most important of these pieces. The other's are convenient, they might make your development easier, but if they don't then you shouldn't use them. But making your application available through Paste Deploy makes life easier for everything that wants to use your application. If, say, CatWalk was available through Paste Deploy, then I could embed it into any application I write, even if I'm not using TurboGears. If PyFileServer was Paste Deploy enabled, then I could include a WebDAV server with any app, etc. Paste Deploy is what starts to address Python's flaws compared to PHP.