class ISIN(satypes.TypeDecorator):
    """Validate ISIN on input and convert ISIN to string on ingress, string to ISIN on egress"""
    impl = satypes.String
    def __init__(self, *args, **kargs):
        satypes.TypeDecorator.__init__(self, *args, **kargs)
    
    def convert_bind_param(self, value, engine):
        return self.impl.convert_bind_param(value, engine)
    
    def convert_result_value(self, value, engine):
        return self.impl.convert_result_value(value, engine)

Skeleton sqlalchemy typedecorator

textmate, python

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