Serving debian-distributed javascript libraries in Tornado

Debian conveniently distribute JavaScript libraries, and expects packaged software to use them rather than embedding their own copy.

Here is a convenient custom StaticFileHandler for Tornado that looks for the Debian-distributed versions of JavaScript libraries, and falls back to the vendored versions if they are not found:

from tornado import web
import pathlib

class StaticFileHandler(web.StaticFileHandler):
    StaticFileHandler that allows overriding paths in the static directory with
    system provided versions
    SYSTEM_ASSET_PATH = pathlib.Path("/usr/share/javascript")

    def get_absolute_path(self, root, path):
        path = pathlib.PurePath(path)
        if not
            return super().get_absolute_path(root, path)

        system_dir = self.SYSTEM_ASSET_PATH.joinpath([0])
        if system_dir.is_dir():
            # If that asset directory exists in the system, look for things in
            # there
            return self.SYSTEM_ASSET_PATH.joinpath(path)
            # Else go ahead with the default static dir
            return super().get_absolute_path(root, path)

    def validate_absolute_path(self, root, absolute_path):
        Rewrite of tornado's validate_absolute_path not to raise an error for
        paths in /usr/share/javascript/
        root = pathlib.Path(root)
        absolute_path = pathlib.Path(absolute_path)

        is_system_root =[:len(] ==
        is_static_root =[:len(] ==

        if not is_system_root and not is_static_root:
            raise web.HTTPError(403, "%s is not in root static directory or system assets path",

        if absolute_path.is_dir() and self.default_filename is not None:
            # need to look at the request.path here for when path is empty
            # but there is some prefix to the path that was already
            # trimmed by the routing
            if not self.request.path.endswith("/"):
                self.redirect(self.request.path + "/", permanent=True)
            absolute_path = absolute_path.joinpath(self.default_filename)
        if not absolute_path.exists():
            raise web.HTTPError(404)
        if not absolute_path.is_file():
            raise web.HTTPError(403, "%s is not a file", self.path)
        return str(absolute_path)

This is how to use it:

class DebianApplication(tornado.web.Application):
    def __init__(self, *args, **settings):
        from .static import StaticFileHandler
        settings.setdefault("static_handler_class", StaticFileHandler)
        super().__init__(*args, **settings)

And from HTML it's simply a matter of matching the first path component to what is used by Debian's packages under /usr/share/javascript:

    <link rel="stylesheet" href="{{static_url('bootstrap4/css/bootstrap.min.css')}}">
    <script src="{{static_url('jquery/jquery.min.js')}}"></script>
    <script src="{{static_url('popper.js/umd/popper.min.js')}}"></script>
    <script src="{{static_url('bootstrap4/js/bootstrap.min.js')}}"></script>

I find it quite convenient: this way I can start writing prototype code without worrying about fetching javascript libraries to bundle.

I only need to start worrying about it if I need to deploy outside of Debian, or to old stable versions of Debian that don't contain the required JavaScript dependencies. In that case, I just cp -r from a working /usr/share/javascript into Tornado's static directory, and I'm done.