Add support for Sass compilation via libsass-python

Removes a hard dependency on installing Ruby for GMusicProcurator (unless
you're developing it, in which case it's still required because of scss-lint).

INSTALL.rst

@@ -15,13 +15,15 @@ Backend
 Frontend
 ~~~~~~~~
 
-* Sass_
+* Either libsass-python_ or the reference implementation of Sass_ (which
+  requires Ruby)
 * Node.js + NPM
 * Bower (``npm install -g bower``)
 * CoffeeScript (``npm install -g coffee-script``)
 * UglifyJS2 (``npm install -g uglify-js``)
 * importer (``npm install -g importer``)
 
+.. _libsass-python: http://dahlia.kr/libsass-python/
 .. _Sass: http://sass-lang.com/
 
 Browser
@@ -64,6 +66,9 @@ Then run the following (lines that start with ``#`` are comments, not commands):
    user@host:gmusicprocurator$ virtualenv venv
    user@host:gmusicprocurator$ source venv/bin/activate
    (venv)user@host:gmusicprocurator$ pip install -r requirements.txt
+   # Only run the next line if you wish to use libsass-python instead of the
+   # Ruby version of Sass:
+   (venv)user@host:gmusicprocurator$ pip install libsass
    (venv)user@host:gmusicprocurator$ python -m gmusicprocurator list_devices --no-desktop
 
 The last command will print out a list of mobile devices that are registered

gmusicprocurator/assets.py

@@ -17,10 +17,31 @@
 """webassets-related code for GMusicProcurator."""
 
 from flask.ext.assets import Bundle
+try:
+    import sass
+except ImportError:
+    sass_filter = 'scss'
+else:
+    import os.path
+    from webassets.filter import Filter
+    sass_filter = 'libsass'
 from webassets.filter import ExternalTool, register_filter
 
 from .app import assets
 
+if sass_filter == 'libsass':
+
+    class LibSassFilter(Filter):
+        name = 'libsass'
+
+        def input(self, _in, out, **kw):
+            source_path = kw['source_path']
+            include_paths = [os.path.dirname(source_path)]
+            out.write(sass.compile(string=_in.read(),
+                                   include_paths=include_paths))
+
+    register_filter(LibSassFilter)
+
 
 class ImporterFilter(ExternalTool):
 
@@ -58,7 +79,7 @@ def bundlify(fmt, modules, **kwargs):
 normalize = 'vendor/normalize-css/normalize.css'
 
 # typography is in a separate bundle so it can be placed before pure
-typography = Bundle('scss/typography.scss', filters='scss',
+typography = Bundle('scss/typography.scss', filters=sass_filter,
                     output='scss/typography.out.css')
 
 pure_modules = [
@@ -68,7 +89,7 @@ def bundlify(fmt, modules, **kwargs):
 
 pure = bundlify('vendor/pure/{0}.css', pure_modules)
 
-main_css = Bundle('scss/main.scss', filters='scss',
+main_css = Bundle('scss/main.scss', filters=sass_filter,
                   output='scss/main.out.css')
 
 css = Bundle(normalize, typography, pure, main_css, filters='cssmin',

setup.py

@@ -109,6 +109,9 @@ def requires_from_req_txt(filename):
         ],
     },
     'install_requires': requires,
+    'extras_requires': {
+        'libsass': ['libsass'],
+    }
     'classifiers': CLASSIFIERS,
 }