text: #!/usr/bin/env python
  
  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
  from agate.exceptions import DataTypeError
  from agate.warns import warn_null_calculation
  
  
  class Mean(Aggregation):
      """
      Calculate the mean of a column.
  
      :param column_name:
          The name of a column containing :class:`.Number` data.
      """
      def __init__(self, column_name):
          self._column_name = column_name
          self._sum = Sum(column_name)
  
      def get_aggregate_data_type(self, table):
          return Number()
  
      def validate(self, table):
          column = table.columns[self._column_name]
  
          if not isinstance(column.data_type, Number):
              raise DataTypeError('Mean can only be applied to columns containing Number data.')
  
          has_nulls = HasNulls(self._column_name).run(table)
  
          if has_nulls:
              warn_null_calculation(self, column)
  
      def run(self, table):
          column = table.columns[self._column_name]
          num_of_values = len(column.values_without_nulls())
          # If there are no non-null columns then return null.
          if num_of_values == 0:
              return None
  
          sum_total = self._sum.run(table)
          return sum_total / num_of_values
  