Checksum the passed in dictionary

def checksum(symbol, doc):
    """
    Checksum the passed in dictionary
    """
    sha = hashlib.sha1()
    sha.update(symbol.encode('ascii'))
    for k in sorted(iter(doc.keys()), reverse=True):
        v = doc[k]
        if isinstance(v, six.binary_type):
            sha.update(doc[k])
        else:
            sha.update(str(doc[k]).encode('ascii'))
    return Binary(sha.digest())