Build a ranking on child items based on a parent item in a pandas DataFrame

build_rank.py

import pandas as pd

# Consider a list of movies with unranked actors that you want to rank based on the source list order below.

# [movie_id, actor_id]
movies_list = [
    [123, 54],
    [123, 21],
    [123, 66],
    [45, 22],
    [45, 54],
    [61, 87],
    [61, 21],
    [88, 21],
]

movies_df = pd.DataFrame(movies_list, columns=["movie_id", "actor_id"])

# Building the logical rank for each row first
movies_df["ranking"] = range(len(movies_df))

# Building the sub-ranking grouped by movie_id based on the logical rank defined above
movies_df["ranking"] = movies_df.groupby("movie_id")["ranking"].rank()

# Converting the rank value from float to int
movies_df["ranking"] = movies_df["ranking"].convert_dtypes(int)

print(movies_df)

   movie_id  actor_id  ranking
0       123        54        1
1       123        21        2
2       123        66        3
3        45        22        1
4        45        54        2
5        61        87        1
6        61        21        2
7        88        21        1