Rivets

Rivets is a Python port of the Rivets Ruby library for compiling and serving web assets. It features declarative dependency management for JavaScript and CSS assets, as well as a powerful preprocessor pipeline that allows you to write assets in languages like CoffeeScript and SCSS.

Installation

Install rivets with pip:

$ pip install rivets

Understanding the Rivets Environment

You'll need an instance of the rivets.Environment class to access and serve assets from your application.

The Rivets Environment has methods for retrieving and serving assets, manipulating the load path, and registering processors. It is also a CherryPy compatible Route Dispatcher application that can be mounted at a URL to serve assets over HTTP.

The Load Path

The load path is an ordered list of directories that Rivets uses to search for assets.

In the simplest case, a Rivets environment's load path will consist of a single directory containing your application's asset source files. When mounted, the environment will serve assets from this directory as if they were static files in your public root.

The power of the load path is that it lets you organize your source files into multiple directories -- even directories that live outside your application -- and combine those directories into a single virtual filesystem. That means you can easily bundle JavaScript, CSS and images into a python library and import them into your application.

Manipulating the Load Path

To add a directory to your environment's load path, use the append_path and prepend_path methods. Directories at the beginning of the load path have precedence over subsequent directories.

environment = rivets.Environment()
environment.append_path('app/assets/javascripts')
environment.append_path('lib/assets/javascripts')
environment.append_path('vendor/assets/jquery')

In general, you should append to the path by default and reserve prepending for cases where you need to override existing assets.

Accessing Assets

Once you've set up your environment's load path, you can mount the environment as a Rack server and request assets via HTTP. You can also access assets programmatically from within your application.

Logical Paths

Assets in Rivets are always referenced by their logical path.

The logical path is the path of the asset source file relative to its containing directory in the load path. For example, if your load path contains the directory app/assets/javascripts:

In this way, all directories in the load path are merged to create a virtual filesystem whose entries are logical paths.

Serving Assets Over HTTP

When you mount an environment, all of its assets are accessible as logical paths underneath the mount point. For example, if you mount your environment at /assets and request the URL /assets/application.js, Rivets will search your load path for the file named application.js and serve it.

To mount the environment in CherryPy you will need to create an instance of the environment and map the route to it's call action:

import rivets

environment = rivets.Environment()
environment.append_path('app/assets/javascripts')
environment.append_path('app/assets/stylesheets')

d = cherrypy.dispatch.RoutesDispatcher()
d.connect('assets','/assets/:path',controller = env, action='run')

Accessing Assets Programmatically

You can use the find_asset method (aliased as []) to retrieve an asset from a Rivets environment. Pass it a logical path and you'll get a BundledAsset instance back:

environment['application.js']
# => #<rivets.assets.bundled_asset.BundledAsset object ...>

Call to_string on the resulting asset or cast to str to access its contents, length to get its length in bytes, mtime to query its last-modified time, and pathname to get its full path on the filesystem.

Using Engines

Asset source files can be written in another language, like SCSS or CoffeeScript, and automatically compiled to CSS or JavaScript by Rivets. Compilers for these languages are called engines.

Engines are specified by additional extensions on the asset source filename. For example, a CSS file written in SCSS might have the name layout.css.scss, while a JavaScript file written in CoffeeScript might have the name dialog.js.coffee.

Styling with SCSS

Sass is a language that compiles to CSS and adds features like nested rules, variables, mixins and selector inheritance.

You will need to install pyScss to use Scss in your application.

Rivets currently only supports the newer Scss syntax. Unfortunately due to differences between pyScss's implementation of Scss and the original Ruby gem's implementation, not all Sass features are properly supported. I'm going to be looking at doing a more consistent port of the original library to enable Rivets to override the importers to handle caching better etc.

Scripting with CoffeeScript

CoffeeScript is a language that compiles to the "good parts" of JavaScript, featuring a cleaner syntax with array comprehensions, classes, and function binding.

You'll need to have the Python-CoffeeScript module installed on your system to use the CoffeeScript processing.

To write JavaScript assets with CoffeeScript, use the extension .js.coffee.

Invoking Python with Mako

Rivets provides a Mako engine for preprocessing assets using embedded Python code. Append .mako to a CSS or JavaScript asset's filename to enable the Mako engine.

Note: Rivets processes multiple engine extensions in order from right to left, so you can use multiple engines with a single asset. For example, to have a CoffeeScript asset that is first preprocessed with Mako, use the extension .js.coffee.mako.

Python code embedded in an asset is evaluated in the context of a rivets.Context instance for the given asset. Common uses for Mako include:

• embedding another asset as a Base64-encoded data: URI with the asset_data_uri helper
• inserting the URL to another asset, such as with the asset_path helper provided by the Rivets CherryPy plugin
• embedding other application resources, such as a localized string database, in a JavaScript asset via JSON
• embedding version constants loaded from another file

Managing and Bundling Dependencies

You can create asset bundles -- ordered concatenations of asset source files -- by specifying dependencies in a special comment syntax at the top of each source file.

Rivets reads these comments, called directives, and processes them to recursively build a dependency graph. When you request an asset with dependencies, the dependencies will be included in order at the top of the file.

The Directive Processor

Rivets runs the directive processor on each CSS and JavaScript source file. The directive processor scans for comment lines beginning with = in comment blocks at the top of the file.

//= require jquery
//= require jquery-ui
//= require backbone
//= require_tree .

The first word immediately following = specifies the directive name. Any words following the directive name are treated as arguments. Arguments may be placed in single or double quotes if they contain spaces, similar to commands in the Unix shell.

Note: Non-directive comment lines will be preserved in the final asset, but directive comments are stripped after processing. Rivets will not look for directives in comment blocks that occur after the first line of code.

Supported Comment Types

The directive processor understands comment blocks in three formats:

/* Multi-line comment blocks (CSS, SCSS, JavaScript)
*= require foo
*/

// Single-line comment blocks (SCSS, JavaScript)
//= require foo

# Single-line comment blocks (CoffeeScript)
#= require foo

Rivets Directives

You can use the following directives to declare dependencies in asset source files.

For directives that take a path argument, you may specify either a logical path or a relative path. Relative paths begin with ./ and reference files relative to the location of the current file.

The require Directive

require path inserts the contents of the asset source file specified by path. If the file is required multiple times, it will appear in the bundle only once.

The include Directive

include path works like require, but inserts the contents of the specified source file even if it has already been included or required.

The require_directory Directive

require_directory path requires all source files of the same format in the directory specified by path. Files are required in alphabetical order.

The require_tree Directive

require_tree path works like require_directory, but operates recursively to require all files in all subdirectories of the directory specified by path.

The require_self Directive

require_self tells Rivets to insert the body of the current source file before any subsequent require or include directives.

The depend_on Directive

depend_on path declares a dependency on the given path without including it in the bundle. This is useful when you need to expire an asset's cache in response to a change in another file.

The stub Directive

stub path allows dependency to be excluded from the asset bundle. The path must be a valid asset and may or may not already be part of the bundle. Once stubbed, it is blacklisted and can't be brought back by any other require.

Development

Contributing

The Rivets source code is hosted on GitHub. You can check out a copy of the latest code using Git:

$ git clone https://github.com/oinutter/rivets.git

If you've found a bug or have a question, please open an issue on the Rivets issue tracker. Or, clone the Rivets repository, write a failing test case, fix the bug and submit a pull request.

Credits

Huge amounts of credit to Sam Stephenson(@sstephenson) and Josh Peek(@josh) for all their work on the original Sprockets gem. I have basically just rewritten their code and tests in python, tweaking where necessary to make things more 'pythonic' (I hope) and making them work with existing python modules.