text: #!/usr/bin/env python
  
  import warnings
  
  class NullCalculationWarning(RuntimeWarning):  # pragma: no cover
      """
      Warning raised if a calculation which can not logically
      account for null values is performed on a :class:`.Column` containing
      nulls.
      """
      pass
  
  def warn_null_calculation(operation, column):
      warnings.warn('Column "%s" contains nulls. These will be excluded from %s calculation.' % (
          column.name,
          operation.__class__.__name__
      ), NullCalculationWarning)
  