class_docstr: Calculate the mean of a column.
  
  :param column_name:
      The name of a column containing :class:`.Number` data.
code: def run(self, table):
      column = table.columns[self._column_name]
      data = column.values_without_nulls()
      if data:
          sum_total = self._sum.run(table)
          return sum_total / len(data)
  
file_path: agate/agate/aggregations/mean.py
imports: from agate.aggregations.base import Aggregation
  from agate.aggregations.has_nulls import HasNulls
  from agate.aggregations.sum import Sum
  from agate.data_types import Number, TimeDelta
  from agate.exceptions import DataTypeError
  from agate.warns import warn_null_calculation
label_desc: Use image node_id 4 for calling the Mean obj's underlying member method code with example usage: obj.run(table) and returns: unknown