    import quickfix

    class MyStringField(quickfix.StringField):
    	def __init__(self, data = None)
    		if data == None:
    			quickfix.StringField.__init__(self, 6123)
    		else:
    			quickfix.StringField.__init__(self, 6123, data)

    class MyPriceField(quickfix.DoubleField):
    	def __init__(self, data = None)
    		if data == None:
    			quickfix.DoubleField.__init__(self, 8756)
    		else:
    			quickfix.DoubleField.__init__(self, 8756, data)