i have flask application with buildout environment

./bin/pip show flask | grep Version
Version: 0.10.1
./bin/pip show flask-assets | grep Version
Version: 0.10

in src folder src/setup.py have following strings

setup(
    name = 'spf',
    install_requires = [
        'flask',
        'flask-assets',
    ],
    entry_points = {
        'console_scripts': [
            'spf_dev = spf.manage:dev', /* see manage.py dev function */
        ],
    },
}

for generated bin/spf_dev i have src/spf/manage.py with following code

from flask.ext import assets
from . import env

def init (app):
    manager = script.Manager(app)

    manager.add_command(
        'assets',
        assets.ManageAssets(app.assets),
    )
    return manager

def dev ():
    init(env.dev.app).run()

for flask environment initialization i use src/spf/env/dev.py

from spf import init
app = init({
    'ASSETS_DIR': 'src/spf/static/assets',
    'ASSETS_URL': '/assets/',

    'SASS_STYLE': 'compressed',

    'UGLIFYJS_EXTRA_ARGS': (
        '-c',
        '--screw-ie8',
    ),

})

and i implement init function for return wsgi app in src/spf/init.py

import flask
from . import assets

def init (env_config=None):
    app = flask.Flask(
        'spf',
        static_url_path='',
    )

    app.config.update(evn_config)
    app.assets = assets.Assets(app)
    return app

assets module with Bundle registration src/spf/assets.py

from flask.ext.assets import (
    Environment,
    Bundle,
)

class Assets (Environment):
    def __init__ (self, app):
        super(Assets, self).__init__(app)

        if 'ASSETS_DIR' in app.config:
            self.directory = app.config['ASSETS_DIR']

        if 'ASSETS_URL' in app.config:
            self.url = app.config['ASSETS_URL']

        if 'SASS_STYLE' in app.config:
            self.config['sass_style'] =  app.config['SASS_STYLE']

        if 'UGLIFYJS_EXTRA_ARGS' in app.config:
            self.config['UGLIFYJS_EXTRA_ARGS'] = \
                app.config['UGLIFYJS_EXTRA_ARGS']

        self.register('theme.css', Bundle(
            'scss/theme.scss',
            filters='scss',
            output='theme.css',
        ))

        self.append_path('src/assets')

src/assets/scss/theme.scss

@import 'btn-o';

src/assets/scss/_btn-o.scss exists, not empty and have 0777 access right

but when i run

buildout -c src/buildout.cfg
./bin/spf_dev assets -v build

i have error

Building bundle: theme.css
Failed, error was: sass: subprocess had error: stderr=Error: File to import not found or unreadable: ./btn-o. on line 1 of standard input
  Use --trace for backtrace. , stdout=, returncode=65

i have read https://github.com/miracle2k/webassets/blob/master/src/webassets/filter/sass.py#L36

but i don't understand ;-( Why sass don't use src/assets for resolve relative path in @import directive from stdin after i used self.append_path('src/assets') ?

You need to add a load path to your sass filter for it to work with 3.4.14

sass = get_filter('scss')
sass.load_paths = [os.path.join(app.static_folder, 'scss')]

Then, when you register your bundle:

self.register('theme.css', Bundle(
    'scss/theme.scss',
    filters=(sass,),
    output='theme.css',
))