Hierarchical data: efficiently build a list of every descendant for each node

Question

I have a two column data set depicting multiple child-parent relationships that form a large tree. I would like to use this to build an updated list of every descendant for

Answer 1

Here is one way using isin() and map

df_new = df.append(df[df['parent'].isin(df['child'].values.tolist())])\
.reset_index(drop = True)

df_new.loc[df_new.duplicated(), 'parent'] = df_new.loc[df_new.duplicated(), 'parent']\
.map(df.set_index('child')['parent'])

df_new = df_new.sort_values('parent').reset_index(drop=True)
df_new.columns = [' descendant' , 'ancestor']

You get

    descendant  ancestor
0   2010    1000
1   2100    1000
2   2110    1000
3   3000    1000
4   3011    1000
5   3033    1000
6   3102    1000
7   3111    1000
8   3011    2010
9   3102    2010
10  3033    2100
11  3000    2110
12  3111    2110