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If all you want is to specify the appEncoding through an
environment variable: this should be really easy. Instead of

class Unicode(varchar):
    def __init__(self, columnName=None,
appEncoding="iso-8859-1"):
        datatype.__init__(self, columnName)
        self._appEncoding = appEncoding

do this instead

class Unicode(varchar):
    def __init__(self, columnName=None, 
                      appEncoding=os.environ.get("ORM_ENCODING",
                                                           
    "iso-8859-1")):
        datatype.__init__(self, columnName)
        self._appEncoding = appEncoding

Then set ORM_ENCODING as you'ld like. No other changes to
your source are required.