Iterates over values of a given column.

    Args:
      column_name: A nome of the column to retrieve the values for.

    Yields:
      Values of the specified column.

    Raises:
      KeyError: If given column is not present in the table.

def Column(self, column_name):
    """Iterates over values of a given column.

    Args:
      column_name: A nome of the column to retrieve the values for.

    Yields:
      Values of the specified column.

    Raises:
      KeyError: If given column is not present in the table.
    """
    column_idx = None
    for idx, column in enumerate(self.header.columns):
      if column.name == column_name:
        column_idx = idx
        break

    if column_idx is None:
      raise KeyError("Column '{}' not found".format(column_name))

    for row in self.rows:
      yield row.values[column_idx]