An extension of Flask that adds last-modified file time information to static file URLs built by url_for

app.py

import os.path
from flask import Flask
from flask.helpers import safe_join

# Injects an "mt" parameter on the URLs of static files that contains the
# last-modified time of the file.  This allows the use of aggressive cache
# settings on static files, while ensuring that content changes are reflected
# immediately due to the new URLs.  Note that if multiple servers have
# different mod times on files, this can cause static files to be reloaded more
# often than needed.
class AddStaticFileModTimeFlask(Flask):
    def inject_url_defaults(self, endpoint, values):
        super(AddStaticFileModTimeFlask, self).inject_url_defaults(endpoint, values)
        if endpoint == "static" and "filename" in values:
            filepath = safe_join(self.static_folder, values["filename"])
            if os.path.isfile(filepath):
                values["mt"] = os.path.getmtime(filepath)

app = AddStaticFileModTimeFlask(__name__)
# ...

I threw this together for a project that I was working on. It seems to work well, but then I realized that git does not maintain last-modified times on files and I was using git for deployments to multiple web servers. So I'm looking at alternative approaches, but I thought I'd throw this up on gist and maybe it'll be useful to someone, someday.