'Given a SELECT column expression, return a wrapping SQL expression.
This is typically a SQL function that wraps a column expression
as rendered in the columns clause of a SELECT statement.
It is used for special data types that require
columns to be wrapped in some special database function in order
to coerce the value before being sent back to the application.
It is the SQL analogue of the :meth:`.TypeEngine.result_processor`
method.
The method is evaluated at statement compile time, as opposed
to statement construction time.
See also:
:ref:`types_sql_value_processing`'

def column_expression(self, colexpr):

    return None
