我建议您使用networkx，因为这是一个图形问题。特别是后代函数：
import networkx as nx
import pandas as pd
data = [['A', 'B', 0, 1],
['B', 'C', 1, 2],
['B', 'D', 1, 2],
['X', 'Y', 0, 2],
['X', 'D', 0, 2],
['Y', 'Z', 2, 3]]
df = pd.DataFrame(data=data, columns=['parent', 'child', 'parent_level', 'child_level'])
roots = df.parent[df.parent_level.eq(0)].unique()
dg = nx.from_pandas_edgelist(df, source='parent', target='child', create_using=nx.DiGraph)
result = pd.DataFrame(data=[[root, nx.descendants(dg, root)] for root in roots], columns=['root', 'children'])
print(result)
root children
0 A {D, B, C}
1 X {Z, Y, D}
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