poloniex api query - python

import urllib
import urllib.request
import hmac, hashlib

key = "89QK21JO-8O72SN4U-ZUR4CGE5-TSGNT63S"
secret = b"50c94254737f8a47364eb6825055fd93c5cf9b2e927d366e7acdf22e58e89e3fa2c32955d1feddc2f395a2d44507c36167be348dbd44b3eae6d1019d938994cd"
poloniex_url = "https://poloniex.com/tradingApi"
post_data = b"nonce=" + b"1399116375497" + b"&command=returnBalances" #int(time.time()*1000)
signature = hmac.new(secret, post_data, hashlib.sha512).hexdigest()
print(poloniex_url)
print("sha512:", signature)
headers = { 'Sign': signature, 'Key': key }
urlreq = urllib.request.Request(poloniex_url, post_data, headers)
print(urllib.request.urlopen(urlreq).read())