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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 get_column(column_name, node, context):
 """Get a column by name from the selectable. Args: column_name: str, name of the column to retrieve. node: SqlNode, the node the column is being retrieved for. context: CompilationContext, compilation specific metadata. Returns: column, the SQLAlchemy column if found. Raises an AssertionError otherwise. """

    column = try_get_column(column_name, node, context)
    if column is None:
        selectable = get_node_selectable(node, context)
        raise AssertionError(
            u'Column "{}" not found in selectable "{}". Columns present are {}. '
            u'Context is {}.'.format(column_name, selectable.original,
                                     [col.name for col in selectable.c], context))
    return column