Pandas Dataframe Groupby join string whilst preserving order of strings

Question

Similar questions have been asked i.e Concatenate strings from multiple rows using Pandas groupby and remove duplicates from the comma separated cell

I would like to concatenate string values in a pandas groupby lambda function, however whilst maintaining the order of the strings, solutions use the set function which does not preserve the order when multiple values are passed in.

df = df.sort_values(
        ['id', 'order_column']
    ).groupby('id').agg(
        {
            'channel': lambda x: ' > '.join(set(x)),
            'value': np.sum
        }
    )

How do I do this whilst maintaining the order of values passed to the sequence? In my example, the data is

1             Email
2         Affiliate
3    Organic Search
4             Email
5    Branded Social
6            Direct
7    Branded Social
8            Direct
9    Branded Social
10            Email
11        Affiliate
12            Email
13           Direct
14            Email
15           Direct
16            Email
17   Branded Social
18           Direct
19   Branded Social

What I get: 'Affiliate > Email > Organic Search > Branded Social > Direct'

What I expect 'Email > Affiliate > Organic Search > Branded Social > Direct'

Answer 1

Use the sort=False parameter in groupby and drop_duplicates instead set:

df = df.sort_values(
        ['id', 'order_column']
    ).groupby('id', sort=False).agg(
        {
            'channel': lambda x: ' > '.join(x.drop_duplicates()),
            'value': np.sum
        }
    )