<link rel="stylesheet" href="example.css?{% mercurial_revision "example.css" %}" />

import subprocess

def get_hg_rev(file_path):
    pipe = subprocess.Popen(
        ["hg", "log", "-l", "1", "--template", "{node}", file_path],
        stdout=subprocess.PIPE
        )
    return pipe.stdout.read()

> path_to_file = "/home/jim/workspace/lgr/pinax/projects/lgr/site_media/base.css"
> get_hg_rev(path_to_file)
'0ed525cf38a7b7f4f1321763d964a39327db97c4'

from mercurial import ui, hg
from mercurial.node import hex

repo = hg.repository('/path/to/repo/root', ui.ui())
fctx = repo.filectx('/path/to/file', 'tip')
hexnode = hex(fctx.node())