Python – Subtract Date by First Entry?

I’m using polars library in python to manipulate some dataframe.

I’m trying to do the following:
:

For some dataframe:

I want to return the "distance" between the current fighter-pair and the first fight they had, such that:
:

<What I’ve Tried>:

1. polars "first" function: Only returned the head of the dataframe
2. polars "first" function with some combinations of "over"/"group_by"/"rolling" functions: Returned some numbers, but I can’t make sense of why the output was that way

Does anyone have any advice on how to attempt this?

I think I might need to use some combination of "group_by" or "over", "first", and perhaps "sub" (to subtract two dates?), but I’m not sure how to proceed.
The hardest part for me is to try to extract the first entry of a given group (e.g. first date entry of the A-B pair, or the A-C pair, etc.)

>Solution :

Here’s one way to do this:

Group the dataframe by [person, fight_with], select the min on, and then join it with the original dataframe.

Here’s some code:

import polars as pl

df = pl.DataFrame(
    {
        "person": ["A", "A", "A", "A", "A", "A", "A"],
        "fight_with": ["B", "C", "D", "E", "B", "B", "C"],
        "on": [3, 4, 5, 5, 10, 20, 20],
    }
)

first_fight = df.group_by("person", "fight_with").agg(pl.min("on").alias("min_on"))

df = df.join(first_fight, on=["person", "fight_with"]).with_columns(
    distance=pl.col("on") - pl.col("min_on")
)

print(df)

Output:

shape: (7, 5)
┌────────┬────────────┬─────┬────────┬──────────┐
│ person ┆ fight_with ┆ on  ┆ min_on ┆ distance │
│ ---    ┆ ---        ┆ --- ┆ ---    ┆ ---      │
│ str    ┆ str        ┆ i64 ┆ i64    ┆ i64      │
╞════════╪════════════╪═════╪════════╪══════════╡
│ A      ┆ B          ┆ 3   ┆ 3      ┆ 0        │
│ A      ┆ C          ┆ 4   ┆ 4      ┆ 0        │
│ A      ┆ D          ┆ 5   ┆ 5      ┆ 0        │
│ A      ┆ E          ┆ 5   ┆ 5      ┆ 0        │
│ A      ┆ B          ┆ 10  ┆ 3      ┆ 7        │
│ A      ┆ B          ┆ 20  ┆ 3      ┆ 17       │
│ A      ┆ C          ┆ 20  ┆ 4      ┆ 16       │
└────────┴────────────┴─────┴────────┴──────────┘