DataFrameの重複行の削除

DataFarmeを結合して重複があった場合の削除方法

import pandas as pd
df1 = pd.DataFrame([['2022/1/6', '3000', '2400', '240'],
                    ['2022/1/7', '1000', '800', '80'],
                    ['2022/1/8', '3000', '2400', '240'],
                    ['2022/1/9', '1500', '1200', '120']],
                   columns=['date', 'value', 'cost', 'tax'])
df1['date'] = pd.to_datetime(df1['date']).dt.date
df2 = pd.DataFrame([['2022/1/8', '3000', '2400', '240'],
                    ['2022/1/9', '1500', '1200', '120']],
                   columns=['date', 'value', 'cost', 'tax'])
df2['date'] = pd.to_datetime(df2['date']).dt.date
df3 = pd.DataFrame([['2022/1/9', '1500', '1200', '120', '15'],
                    ['2022/1/10', '500', '400', '40', '5'],
                    ['2022/1/11', '1000', '800', '80', '10']],
                   columns=['date', 'value', 'cost', 'tax', 'tax2'])
df3['date'] = pd.to_datetime(df3['date']).dt.date

df = pd.concat([df1, df2, df3], ignore_index=True)
df

	date	value	cost	tax	tax2
0	2022-01-06	3000	2400	240	NaN
1	2022-01-07	1000	800	80	NaN
2	2022-01-08	3000	2400	240	NaN
3	2022-01-09	1500	1200	120	NaN
4	2022-01-08	3000	2400	240	NaN
5	2022-01-09	1500	1200	120	NaN
6	2022-01-09	1500	1200	120	15
7	2022-01-10	500	400	40	5
8	2022-01-11	1000	800	80	10

index「2, 4」と「3, 5, 6」にdateの重複があります。dateの重複を判定したいのでsubset='date'を引数にします。
subsetの引数がない場合は行のすべてが重複す場合に削除となります。
また、最初を残す(default)か、最後を残すかを選択できます。
最初を残す( keep='first')と、index「2」と「3」が残ります。

df.drop_duplicates(subset='date', keep='first')

	date	value	cost	tax	tax2
0	2022-01-06	3000	2400	240	NaN
1	2022-01-07	1000	800	80	NaN
2	2022-01-08	3000	2400	240	NaN
3	2022-01-09	1500	1200	120	NaN
7	2022-01-10	500	400	40	5
8	2022-01-11	1000	800	80	10

最後を残す( keep='last')と、index「4」と「6」が残ります。

df.drop_duplicates(subset='date', keep='last')

	date	value	cost	tax	tax2
0	2022-01-06	3000	2400	240	NaN
1	2022-01-07	1000	800	80	NaN
4	2022-01-08	3000	2400	240	NaN
6	2022-01-09	1500	1200	120	15
7	2022-01-10	500	400	40	5
8	2022-01-11	1000	800	80	10

複数のcolumnで判定した場合はsubsetに複数のcloumnをセットする

df.drop_duplicates(subset=['date', 'tax2'])

	date	value	cost	tax	tax2
0	2022-01-06	3000	2400	240	NaN
1	2022-01-07	1000	800	80	NaN
2	2022-01-08	3000	2400	240	NaN
3	2022-01-09	1500	1200	120	NaN
6	2022-01-09	1500	1200	120	15
7	2022-01-10	500	400	40	5
8	2022-01-11	1000	800	80	10

重複している行すべてを削除する場合はkeep=False

df.drop_duplicates(subset='date', keep=False)

	date	value	cost	tax	tax2
0	2022-01-06	3000	2400	240	NaN
1	2022-01-07	1000	800	80	NaN
7	2022-01-10	500	400	40	5
8	2022-01-11	1000	800	80	10

indexの振り直しもできます。ignore_index=True

df.drop_duplicates(subset='date', keep=False, ignore_index=True)

	date	value	cost	tax	tax2
0	2022-01-06	3000	2400	240	NaN
1	2022-01-07	1000	800	80	NaN
2	2022-01-10	500	400	40	5
3	2022-01-11	1000	800	80	10

重複行を判定する場合はduplicatedを使います。引数のsubset、keepの使い方は同じです。

df.duplicated(subset='date', keep=False)

0    False
1    False
2     True
3     True
4     True
5     True
6     True
7    False
8    False
dtype: bool

重複行を判定した結果を使っての削除

df[~df.duplicated(subset='date', keep=False)]

	date	value	cost	tax	tax2
0	2022-01-06	3000	2400	240	NaN
1	2022-01-07	1000	800	80	NaN
7	2022-01-10	500	400	40	5
8	2022-01-11	1000	800	80	10