Checking Facebook’s signed_request in Python

from django.conf import settings
import hmac
import hashlib
import base64
import json
 
def validate_signed_fb_request(signed_request):
    """Return dictionary with signed request data."""
    try:
      l = signed_request.split('.', 2)
      encoded_sig = str(l[0])
      payload = str(l[1])
    except IndexError:
      raise ValueError("'signed_request' malformed")
 
    sig = base64.urlsafe_b64decode(encoded_sig + "=" * ((4 - len(encoded_sig) % 4) % 4))
    data = base64.urlsafe_b64decode(payload + "=" * ((4 - len(payload) % 4) % 4))
 
    data = json.loads(data)
 
    if data.get('algorithm').upper() != 'HMAC-SHA256':
      raise ValueError("'signed_request' is using an unknown algorithm")
    else:
      expected_sig = hmac.new(settings.FACEBOOK_APP_SECRET, msg=payload, digestmod=hashlib.sha256).digest()
 
    if sig != expected_sig:
      raise ValueError("'signed_request' signature mismatch")
    else:
      return data