class_docstr: Calculate the percent difference between two columns.
  
  :param before_column_name:
      The name of a column containing the "before" :class:`.Number` values.
  :param after_column_name:
      The name of a column containing the "after" :class:`.Number` values.
class_docstr_tok: ["Calculate","the","percent","difference","between","two","columns",".",":","param","before_column_name",":","The","name","of","a","column","containing","the","``","before","''",":","class",":","`",".Number","`","values",".",":","param","after_column_name",":","The","name","of","a","column","containing","the","``","after","''",":","class",":","`",".Number","`","values","."]
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/percent_change.py
imports: from agate.aggregations.has_nulls import HasNulls
  from agate.computations.base import Computation
  from agate.data_types import Number
  from agate.exceptions import DataTypeError
  from agate.warns import warn_null_calculation
label_desc: Use image node_id 1 to create a new PercentChange object from inherited base classes: Computation with example: obj = PercentChange(before_column_name, after_column_name)