How to modify dataframe based on column values - pandas

Question

I want to add relationships to column 'relations' based on rel_list. Specifically, for each tuple, i.e. ('a', 'b'), I want to replace the relationships column value '' with 'b' in the first row, but no duplicate, meaning that for the 2nd row, don't replace '' with 'a', since they are considered as duplicated. The following code doesn't work fully correct:
import pandas as pd
data = {
"names": ['a', 'b', 'c', 'd'],
"ages": [50, 40, 45, 20],
"relations": ['', '', '', '']
}
rel_list = [('a', 'b'), ('a', 'c'), ('c', 'd')]
df = pd.DataFrame(data)
for rel_tuple in rel_list:
head = rel_tuple[0]
tail = rel_tuple[1]
df.loc[df.names == head, 'relations'] = tail
print(df)
The current result of df is:
names ages relations
0 a 50 c
1 b 40
2 c 45 d
3 d 20
However, the correct one is:
names ages relations
0 a 50 b
0 a 50 c
1 b 40
2 c 45 d
3 d 20
There are new rows that need to be added. The 2nd row in this case, like above. How to do that?

Answer 1

You can craft a dataframe and merge:
(df.drop('relations', axis=1)
.merge(pd.DataFrame(rel_list, columns=['names', 'relations']),
on='names',
how='outer'
)
# .fillna('') # uncomment to replace NaN with empty string
)
Output:
names ages relations
0 a 50 b
1 a 50 c
2 b 40 NaN
3 c 45 d
4 d 20 NaN

Answer 2

Instead of updating df you can create a new one and add relations row by row:
import pandas as pd
data = {
"names": ['a', 'b', 'c', 'd'],
"ages": [50, 40, 45, 20],
"relations": ['', '', '', '']
}
rel_list = [('a', 'b'), ('a', 'c'), ('c', 'd')]
df = pd.DataFrame(data)
new_df = pd.DataFrame(data)
new_df.loc[:, 'relations'] = ''
for head, tail in rel_list:
new_row = df[df.names == head]
new_row.loc[:,'relations'] = tail
new_df = new_df.append(new_row)
print(new_df)
Output:
names ages relations
0 a 50
1 b 40
2 c 45
3 d 20
0 a 50 b
0 a 50 c
2 c 45 d
Then, if needed, in the end you can delete all rows without value in 'relations':
new_df = new_df[new_df['relations']!='']