分组并在一个条目中将字符串应用于整个组

Question

我需要基于组中的非null值将字符串应用于组。一个例子是：

ID    name    surname  prsn_id
 A    john      smith  prsn_01
 A    john      smith      NaN
 A    john      smith      NaN
 A    john      smith      NaN
 B    mary      jane   prsn_02
 B    mary      jane       NaN
 B    mary      jane       NaN
 B    mary      jane       NaN
 B    mary      jane       NaN
 B    mary      jane       NaN
 B    mary      jane       NaN
 C    Barry   willis   prsn_03
 C    Barry   willis       Nan
 C    Barry   willis       Nan
 C    Barry   willis       Nan
 C    Barry   willis       Nan

输出应为：

ID    name    surname  prsn_id
 A    john      smith  prsn_01
 A    john      smith  prsn_01
 A    john      smith  prsn_01
 A    john      smith  prsn_01
 B    mary      jane   prsn_02
 B    mary      jane   prsn_02
 B    mary      jane   prsn_02
 B    mary      jane   prsn_02
 B    mary      jane   prsn_02
 B    mary      jane   prsn_02
 B    mary      jane   prsn_02
 C    Barry   willis   prsn_03
 C    Barry   willis   prsn_03
 C    Barry   willis   prsn_03
 C    Barry   willis   prsn_03
 C    Barry   willis   prsn_03

或：

ID    name    surname  prsn_id    prsn_id_2
 A    john      smith  prsn_01          NaN
 A    john      smith      NaN      prsn_01
 A    john      smith      NaN      prsn_01
 A    john      smith      NaN      prsn_01
 B    mary      jane   prsn_02          NaN
 B    mary      jane       NaN      prsn_02
 B    mary      jane       NaN      prsn_02
 B    mary      jane       NaN      prsn_02
 B    mary      jane       NaN      prsn_02
 B    mary      jane       NaN      prsn_02
 B    mary      jane       NaN      prsn_02
 C    Barry   willis   prsn_03          NaN
 C    Barry   willis       Nan      prsn_03
 C    Barry   willis       Nan      prsn_03
 C    Barry   willis       Nan      prsn_03
 C    Barry   willis       Nan      prsn_03

我尝试过：

df['prsn_id_2'] = (df
                 .groupby(['ID', 'name', 'surname'])['prsn_id']
                 .fillna(method='ffill'))

这可能会起作用，但是这会花费一些时间，因此以后将不太实用。我需要另一个矢量化且相对较快的解决方案。

Answer 1

使用dropna删除NaN行，然后与merge左联接：

df1 = df.dropna(subset=['prsn_id'])
#if possible duplicates
#df1 = df.dropna(subset=['prsn_id']).drop_duplicates(['ID','name', 'surname'])
df = df.drop('prsn_id', axis=1).merge(df1, on=['ID','name', 'surname'], how='left')
print (df)
   ID   name surname  prsn_id
0   A   john   smith  prsn_01
1   A   john   smith  prsn_01
2   A   john   smith  prsn_01
3   A   john   smith  prsn_01
4   B   mary    jane  prsn_02
5   B   mary    jane  prsn_02
6   B   mary    jane  prsn_02
7   B   mary    jane  prsn_02
8   B   mary    jane  prsn_02
9   B   mary    jane  prsn_02
10  B   mary    jane  prsn_02
11  C  Barry  willis  prsn_03
12  C  Barry  willis  prsn_03
13  C  Barry  willis  prsn_03
14  C  Barry  willis  prsn_03
15  C  Barry  willis  prsn_03

详细信息：

print (df1)
   ID   name surname  prsn_id
0   A   john   smith  prsn_01
4   B   mary    jane  prsn_02
11  C  Barry  willis  prsn_03