import urllib
import urllib.request
import json
import time
import hmac,hashlib

APIKey = "myAPIKey"
Secret = "mySecret"
post_request = "command=returnBalances"
sign = hmac.new(b'Secret', post_request.encode('utf-8'), hashlib.sha512).hexdigest()

data = urllib.parse.urlencode({
    'nonce': '0008',
})
headers = {
    'Key': APIKey,
    'Sign': sign,
}

ret = urllib.request.Request('https://poloniex.com/tradingApi', headers)
print (ret)

 >>> ret = urllib.request.Request('https://poloniex.com/tradingApi', headers)
>>> print (ret)
<urllib.request.Request object at 0xb6c1f1cc>
>>> print(ret.data)
{'Key': 'myAPIKey', 'Sign': '0b9c2ed1b3a64429ed22c47dbbc0f45866bc4e3e658d57a2486511fcfc59ab18a29463bb6b0f2a2f7906e827e61c453cc50eaeb79f2345a515bb3925d69348fa'}
>>> 

 import urllib.parse
import urllib.request
import json
import time
import hmac,hashlib
APIKey = "APIKey"
Secret = "Secret"
command = "returnBalances"
binary_Secret = Secret.encode('utf-8')
binary_command = command.encode('utf-8')
sign = hmac.new(binary_Secret, binary_command, hashlib.sha512).hexdigest()
header = {
    'Key': APIKey,
    'Sign': sign,
}
req={}
req['command'] = command
req['nonce'] = int(time.time()+5)
data = urllib.parse.urlencode(req)
binary_data = data.encode('utf-8')
f = urllib.request.Request('https://poloniex.com/tradingApi', binary_data, header)
answer = urllib.request.urlopen(f).read().decode('utf-8')
print (json.loads(answer))