[英]Output of a Pandas Merge of two data frames does not produce the expected shape
執行 Join 時必須重復,例如:
import pandas as pd
left_data = {'name':['John','Mark'],'value':[1,5]}
right_data = {'name':['John','Mark','John','Mark'],'children':['Celius','Stingher','Celius','Stingher'],'process_date':['2019-02-05','2019-02-05','2019-03-05','2019-03-05']}
left_df = pd.DataFrame(left_data)
right_df = pd.DataFrame(right_data)
right_df['process_date'] = pd.to_datetime(right_df['process_date'])
它們是這樣的:
print(left_df)
name value
0 John 1
1 Mark 5
print(right_df)
name children process_date
0 John Celius 2019-02-05
1 Mark Stingher 2019-02-05
2 John Celius 2019-03-05
3 Mark Stingher 2019-03-05
即使由於right_df
中有多個process_date
值而left
合並,因此left
dataframe 將被復制,以適合right
dataframe 傳遞的所有值。
df = left_df.merge(right_df,how='left',left_on='name',right_on='name')
print(df)
name value children process_date
0 John 1 Celius 2019-02-05
1 John 1 Celius 2019-03-05
2 Mark 5 Stingher 2019-02-05
3 Mark 5 Stingher 2019-03-05
過濾它的一種方法是.sort_values()
按特定順序,然后.drop_duplicates(subset=list(left_df),keep={'last','first'})
。 通過這種方式,我們消除了重復行並保留了最新的可用信息:
df = df.sort_values('process_date',ascending=True).drop_duplicates(list(left_df),keep='last')
print(df)
name value children process_date
1 John 1 Celius 2019-03-05
3 Mark 5 Stingher 2019-03-05
合並 dataframe 的長度,匹配left_df
的長度。
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