class_docstr: Calculate the difference between two columns.
  
  This calculation can be applied to :class:`.Number` columns to calculate
  numbers. It can also be applied to :class:`.Date`, :class:`.DateTime`, and
  :class:`.TimeDelta` columns to calculate time deltas.
  
  :param before_column_name:
      The name of a column containing the "before" values.
  :param after_column_name:
      The name of a column containing the "after" values.
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code: def __init__(self, before_column_name, after_column_name):
      self._before_column_name = before_column_name
      self._after_column_name = after_column_name
  
file_path: agate/agate/computations/change.py
imports: from agate.aggregations.has_nulls import HasNulls
  from agate.computations.base import Computation
  from agate.data_types import Date, DateTime, Number, TimeDelta
  from agate.exceptions import DataTypeError
  from agate.warns import warn_null_calculation
label_desc: Use image node_id 1 to create a new Change object from inherited base classes: Computation with example: obj = Change(before_column_name, after_column_name)