How can I parse signed_request from facebook graph api in django 1.7

Facebook sends a POST request to access a custom tab in a page and that request comes with a parameter signed_request which is encoded.

I tried to use this code to decode it:

def base64_url_decode(inp):
    padding_factor = (4 - len(inp) % 4) % 4
    inp += "="*padding_factor
    return base64.b64decode(unicode(inp).translate(dict(zip(map(ord, u'-_'),
                                                        u'+/'))))


def parse_signed_request(signed_request, secret):

    l = signed_request.split('.', 2)
    encoded_sig = l[0]
    payload = l[1]

    sig = base64_url_decode(encoded_sig)
    data = json.loads(base64_url_decode(payload))

    if data.get('algorithm').upper() != 'HMAC-SHA256':
        print('Unknown algorithm')
        return None
    else:
        expected_sig = hmac.new(secret, msg=payload, digestmod=hashlib.sha256).digest()

    if sig != expected_sig:
        return None
    else:
        print('valid signed request received..')
        return data

It is not working for me. Just returns None.

How to fix this? or is there any other way to decode the signed_request?