[英]Output of a Pandas Merge of two data frames does not produce the expected shape
执行 Join 时必须重复,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'])
它们是这样的:
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
即使由于right_df
中有多个process_date
值而left
合并,因此left
dataframe 将被复制,以适合right
dataframe 传递的所有值。
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
过滤它的一种方法是.sort_values()
按特定顺序,然后.drop_duplicates(subset=list(left_df),keep={'last','first'})
。 通过这种方式,我们消除了重复行并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
合并 dataframe 的长度,匹配left_df
的长度。
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.