Pandas, for each unique value in one column, get unique values in another column

Question

I have a dataframe where each row contains various meta-data pertaining to a single Reddit comment (e.g. author, subreddit, comment text).

I want to do the following

Answer 1

By using sacul's sample data

df['subreddit'].groupby(df['author']).unique().apply(pd.Series)
Out[370]: 
          0    1
author          
a       sr1  sr2
b       sr2  NaN