Python – Pandas: Create dictionaries from hierarchical data

Let’s say I have the following data frame df:

      A            B       
0     mother1      NaN
1     NaN          child1
2     NaN          child2
3     mother2      NaN
4     NaN          child1
5     mother3      NaN
6     NaN          child1
7     NaN          child2
8     NaN          child3

How can you turn it into a resulting dictionary:

results={'mother1':['child1','child2'],'mother2':['child1'],'mother3':['child1','child2','child3'] }

My opinion:

import pandas as pd
import numpy as np

results={}

for index1,row1 in df.iterrows():
    if row1['A'] is not np.nan:
        children=[]
        for index2,row2 in df.iterrows():
            if row2['B'] is not np.nan:
                children.append(row2['B'])
        results[row1['A']]=children

However, the result is wrong:

In[1]: results
Out[1]: 
{'mother1': ['child1', 'child2', 'child1', 'child1', 'child2', 'child3'],
 'mother2': ['child1', 'child2', 'child1', 'child1', 'child2', 'child3'],
 'mother3': ['child1', 'child2', 'child1', 'child1', 'child2', 'child3']}

Here’s one way:

df['A'].fillna(method='ffill', inplace=True)

Give:

         A       B
0  mother1     NaN
1  mother1  child1
2  mother1  child2
3  mother2     NaN
4  mother2  child1
5  mother3     NaN
6  mother3  child1
7  mother3  child2
8  mother3  child3

Then delete the child NA:

df.dropna(subset=['B'], inplace=True)

Give:

         A       B
1  mother1  child1
2  mother1  child2
4  mother2  child1
6  mother3  child1
7  mother3  child2
8  mother3  child3

Then you can use groupby and dictionary comprehension to get the final result:

results = {k: v['B'].tolist() for k, v in df.groupby('A')}

Result:

{'mother1': ['child1', 'child2'],
 'mother2': ['child1'],
 'mother3': ['child1', 'child2', 'child3']}