I followed the instructions from How to serve static files in Flask, but still couldn't get it working.

Here's my project structure:

Project_path  
 |
 +--app
 |  |
 |  +--main.py
 +--static
    |
    +--js
       |
       +--jquery-1.11.2.min.js

Here's main.py:

@app.route('/js/<path:path>')
def serve_static(path):
    root_dir = os.path.dirname(os.getcwd())
    print(os.path.join(root_dir, 'static', 'js', path))
    return app.send_static_file(os.path.join(root_dir, 'static', 'js', path))

Here's index.html:

  ...
  <script type="text/javascript" src="/js/jquery-1.11.2.min.js"></script>
  ...

And when I visit /, I could see the correct path of javascript file printed on the screen
which is Project_path/static/js/jquery-1.11.2.min.js.

But still, I got

127.0.0.1 - - [22/Dec/2014 11:26:30] "GET /js/jquery-1.11.2.min.js HTTP/1.1" 404 -

Any help is appreciated.

EDIT
After stepping through the send_static_file method, I find out what's going on. Basically, I shouldn't use abspath as argument, flask has a judgement in send_static_file:

if os.path.isabs(filename) or \
   filename == '..' or \
   filename.startswith('../'):
    raise NotFound()

And since the filename I passed into is a abspath, flask raise NotFound().
It seems that what it supposed to be passed in is a relative path to self.static_folder(self is <Flask 'main'>), which, in my project, is Project_name/app/static. However, I didn't set static_folder myself which means flask thinks the static folder should be there.

I'm still trying to figure out what to do.

Finally got it working. use flask.send_from_directory

from flask import send_from_directory

@app.route('/js/<path:filename>')
def serve_static(filename):
    root_dir = os.path.dirname(os.getcwd())
    return send_from_directory(os.path.join(root_dir, 'static', 'js'), filename)

It is now clear to me that flask really hate people putting app.py or in my case main.py into a subdirectory. Use send_static_file only if your static folder is what flask thinks to be, i.e. a folder with name static in the same directory with app.py.