Login to FritzBox using Python

fritzbox_login.py

import urllib3
import xml.etree.ElementTree as ET
import hashlib
from requests import Session

username = "xxx"
fritz_pw = "xxx"

def get_md5_hash(challenge, password):
    hash_me = (challenge + "-" + password).encode("UTF-16LE")
    hashed = hashlib.md5(hash_me).hexdigest()
    return challenge + "-" + hashed

def get_session_id():
    session = Session() 

    http = urllib3.PoolManager()
    data = http.request("get", "http://fritz.box/login_sid.lua").data

    tree = ET.fromstring(data)
    sid = tree.findtext("SID")

    if(sid == "0000000000000000"):
        challenge = tree.findtext("Challenge")        
        ret = session.get("http://fritz.box/login_sid.lua", params={
            "username": username,
            "response": get_md5_hash(challenge, fritz_pw)
        })

        tree = ET.fromstring(ret.text)
        sid = tree.findtext("SID")
        if(sid == "0000000000000000"):
            print("Error getting SID")

    return data

I think you wanted to return the sid not the data in the last line:

return sid
instead of
return data