The threading is dirty but it works.

readArduinoHTTPServer.py

import threading
import serial
import json
from BaseHTTPServer import BaseHTTPRequestHandler, HTTPServer

value = 0
ser = serial.Serial('/dev/tty.usbmodemXYZ', 9600)


def read_value():
    global value

    while True:
        value = ser.readline().rstrip()

t = threading.Thread(target=read_value)
t.start()


class Handler(BaseHTTPRequestHandler):
    def do_GET(self):
        self.request.sendall(json.dumps({'led': value}))
        return

httpd = HTTPServer(('127.0.0.1', 5000), Handler)
print('HTTPServer is running')
httpd.serve_forever()