Simple multithreaded Flask file server with basic auth

server.py

import os.path
from functools import wraps
from flask import Flask, request, Response, redirect
from flask_autoindex import AutoIndex

fileroot = os.path.join(os.path.realpath(os.path.curdir), 'files')
app = Flask(__name__)
idx = AutoIndex(app,
                browse_root=fileroot,
                add_url_rules=False)

def check_auth(username, password):
    """ This function is called to check if a username / password combination is
    valid.
    """
    return username == 'USERNAME' and password == 'PASSWORD'

def authenticate():
    """ Sends a 401 response that enables basic auth """
    return Response(
        'Could not verify your access level for that URL.\n'
        'You have to login with proper credentials', 401,
        {'WWW-Authenticate': 'Basic realm="Auth"'})

def requires_auth(f):
    @wraps(f)
    def decorated(*args, **kwargs):
        auth = request.authorization
        if not auth or not check_auth(auth.username, auth.password):
            return authenticate()
        return f(*args, **kwargs)
    return decorated

@app.route('/')
@app.route('/<path:path>')
@requires_auth
def autoindex(path='.'):
    # For Windows
    relpath = os.path.join(*path.split('%5C'))
    normpath = os.path.normpath(relpath)
    if normpath.startswith(os.path.pardir):
        normpath = '/'
    if normpath != path:
        if normpath == '.':
            return redirect('/')
        return redirect(normpath)
    return idx.render_autoindex(normpath)

if __name__ == '__main__':
    app.run(threaded=True)