Column disappearing after .apply – Pandas (Python)

I’m new to pandas and I’m trying to merge the following 2 dataframes into 1 :

                     nopat
0 2021-12-31  3.580000e+09
1 2020-12-31  6.250000e+08
2 2019-12-31 -1.367000e+09
3 2018-12-31  2.028000e+09

              capital_employed
0 2021-12-31      5.924000e+10
1 2020-12-31      6.062400e+10
2 2019-12-31      5.203500e+10
3 2018-12-31      5.441200e+10

When I try to apply a function to my new datframe, all columns disappear. Here is my code :

roce_by_year = pd.merge(nopat, capital_employed) \
        .rename(columns={"": "date"}) \
        .sort_values(by='date') \
        .apply(lambda row: compute_roce(row['nopat'], row['capital_employed']), axis=1) \
        .reset_index(name='roce')

Here is the result :

   index      roce
0      3  3.727119
1      2 -2.627078
2      1  1.030945
3      0  6.043214

I would like to have the following result :

   date       roce
0   2018  3.727119
1   2019 -2.627078
2   2020  1.030945
3   2021  6.043214

Do you have an explanation ?

>Solution :

If you want a method-chained solution, you could use something like this:

import pandas as pd


roce_by_year = (
    pd.merge(nopat, capital_employed)
    .rename(columns={"": "date"})
    .assign(
        date=lambda xdf: pd.to_datetime(
            xdf["date"], errors="coerce"
        ).dt.year
    )
    .assign(
        roce=lambda xdf: xdf.apply(
            lambda row: compute roce(
                row["nopat"], row["capital_employed"]
            ), axis=1
        )
    )
    .sort_values("date", ascending=True)
)[["date", "roce"]]