OSError: [Errno 78] Function not implemented Flask-Assets

Question

I get the following error when accessing a URL with Flask-Assets which is supposed to render and minify css.

ERROR    2016-09-01 02:45:00,096 app.py:1587] Exception on /SomeFile [GET]
Traceback (most recent call last):
  File "/Users/vinay/App-Engine/zion-alpha/lib/flask/app.py", line 1988, in wsgi_app
    response = self.full_dispatch_request()
  File "/Users/vinay/App-Engine/zion-alpha/lib/flask/app.py", line 1641, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/Users/vinay/App-Engine/zion-alpha/lib/flask/app.py", line 1544, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/vinay/App-Engine/zion-alpha/lib/flask/app.py", line 1639, in full_dispatch_request
    rv = self.dispatch_request()
  File "/Users/vinay/App-Engine/zion-alpha/lib/flask/app.py", line 1625, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/Users/vinay/App-Engine/zion-alpha/app/routes/home_routes.py", line 14, in show_file
    return render_template('main.html')
  File "/Users/vinay/App-Engine/zion-alpha/lib/flask/templating.py", line 134, in render_template
    context, ctx.app)
  File "/Users/vinay/App-Engine/zion-alpha/lib/flask/templating.py", line 116, in _render
    rv = template.render(context)
  File "/Users/vinay/App-Engine/zion-alpha/lib/jinja2/environment.py", line 989, in render
    return self.environment.handle_exception(exc_info, True)
  File "/Users/vinay/App-Engine/zion-alpha/lib/jinja2/environment.py", line 754, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/vinay/App-Engine/zion-alpha/app/templates/main.html", line 6, in top-level template code
    {% assets "css_all" %}
OSError: [Errno 78] Function not implemented
INFO     2016-09-01 02:45:00,116 module.py:788] default: "GET /SomeFile HTTP/1.1" 500 291
INFO     2016-09-01 02:45:00,853 module.py:402] [default] Detected file changes:
  /Users/vinay/App-Engine/zion-alpha/app

The following is the code containing Flask-Assets

def create_app():
    """Create the Flask App"""
    app = Flask(__name__)
    configure_blueprints(app)
    css = Bundle('css/main.css',
                 'css/main2.css',
                 filters="cssmin",
                 output="static/css/min.css"
                 )
    assets = Environment(app)
    assets.register('css_all', css)
    return app

HTML

{% assets "css_all" %}
    <link rel="stylesheet" href="{{ ASSET_URL }}"/>
{% endassets %}

Folder structure

.
├── admin
│   ├── __init__.py
│   ├── __init__.pyc
│   ├── routes
│   │   ├── __init__.py
│   │   ├── __init__.pyc
│   │   ├── admin_routes.py
│   │   └── admin_routes.pyc
│   ├── static
│   │   └── css
│   │       ├── admin-2.css
│   │       └── admin.css
│   └── templates
│       └── adminindex.html
├── app
│   ├── __init__.py
│   ├── __init__.pyc
│   ├── routes
│   │   ├── __init__.py
│   │   ├── __init__.pyc
│   │   ├── home_routes.py
│   │   └── home_routes.pyc
│   ├── static
│   │   └── css
│   │       ├── main.css
│   │       └── main2.css
│   └── templates
│       └── main.html
├── app.yaml
├── appengine_config.py
├── appengine_config.pyc

4 directories, 9 files

Answer 1

This seems to be a problem with flask-assets and Google App Engine because GAE disallows file creation at run-time.

Possible duplicate of: GAE: Flask/webassets throws an expection on {% extends "base.html" %}

Either minify the css before deploying to GAE, or find a different method of serving the minified css either at runtime (not preferred) or buildtime (preferred).

One possibility is to create the minified js on the first run (buildtime), which can then be uploaded to a google storage bucket.

Another option is to create the minified js at runtime and keep it stored in a memcache.