find the latest task from the dataframe columns based on the task role and replace the column value with the latest column value

I have a dataframe with the following columns

df1:
Task Start Date  |  Task Finish Date  |  Task Role
01-01-2021           01-03-2021            Lead
01-04-2021           02-02-2021            Team member
01-04-2021           02-23-2021            Unknown

I want to create another column 'Origin Role' based on the following conditions:

if the Task Role is lead/anything then skip (fill the same Task Role into the new column ('Origin Role');

else only if Task Role is Unknown then:

• consider the Task Start Date of the record with Task Role 'Unknown' and find the latest record(if there are multiple) with its Task Finish Date <=(less than or equal to) Task Start Date (unknown role).

• Then fill in the Task role of the latest role in place of the unknown role in the new column 'Origin Role'.

My expectation:

df1:
Task Start Date  |  Task Finish Date  |  Task Role       | Origin Role
01-01-2021           01-03-2021            Lead              Lead
01-04-2021           02-02-2021            Team member       Team Member
01-04-2021           02-23-2021            Unknown           Lead

One option is to use a merge_asof:

# ensure datetime
df[['Task Start Date', 'Task Finish Date']] = \
df[['Task Start Date', 'Task Finish Date']].apply(pd.to_datetime, dayfirst=False)


df['Origin Role'] = df['Task Role'].replace({'Unknown': np.nan}).fillna(
    pd.merge_asof(df['Task Start Date'].sort_values().reset_index(),
                  df[['Task Finish Date', 'Task Role']]
                    .loc[lambda d: d['Task Role'].ne('Unknown')]
                    .sort_values(by='Task Finish Date'),
                  left_on='Task Start Date', right_on='Task Finish Date',
                  ).set_index('index')['Task Role']
)

Steps:

• replace "Unknown" by NaN
• sort values by date
• merge on the closest previous date, ignoring "Unknown"
• fillna with this value

Output:

  Task Start Date Task Finish Date    Task Role  Origin Role
0      2021-01-01       2021-01-03         Lead         Lead
1      2021-01-04       2021-02-02  Team member  Team member
2      2021-01-04       2021-02-23      Unknown         Lead