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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def observed_vis(self, context):
 """ Observed visibility data source """

        lrow, urow = MS.row_extents(context)

        data = self._manager.ordered_main_table.getcol(
            self._vis_column, startrow=lrow, nrow=urow-lrow)

        return data.reshape(context.shape).astype(context.dtype)