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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
class PercentChange(Computation):
"""
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.
"""
def __init__(self, before_column_name, after_column_name):
self._before_column_name = before_column_name
self._after_column_name = after_column_name
def get_computed_data_type(self, table):
return Number()
def validate(self, table):
before_column = table.columns[self._before_column_name]
after_column = table.columns[self._after_column_name]
if not isinstance(before_column.data_type, Number):
raise DataTypeError('PercentChange before column must contain Number data.')
if not isinstance(after_column.data_type, Number):
raise DataTypeError('PercentChange after column must contain Number data.')
if HasNulls(self._before_column_name).run(table):
warn_null_calculation(self, before_column)
if HasNulls(self._after_column_name).run(table):
warn_null_calculation(self, after_column)
def run(self, table):
"""
:returns:
:class:`decimal.Decimal`
"""
new_column = []
for row in table.rows:
before = row[self._before_column_name]
after = row[self._after_column_name]
if before is not None and after is not None:
new_column.append((after - before) / before * 100)
else:
new_column.append(None)
return new_column
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