python custom ErrorMsg inherit form str

error_msg.py

"""Partice to custom string inherit python str
"""

class ErrorMsg(str):
    def __new__(cls, *args, **kw):
        if len(args) > 0:
            message = args[0]
        else:
            message = kw.get('message')
        return str.__new__(cls, message)

    def __init__(self, message, payload=None):
        print "init", message, payload
        self._message = message
        self._payload = payload

    def __str__(self):
        return self._message

    
msg = ErrorMsg("oops", {"a": "A" , "b": "B"})
print msg
print "test: %s" % msg
print "test: " + msg
print msg._payload