I have a pandas dataframe:
banned_titles = TitleId RelatedTitleId 0 89989 32598 1 89989 3085083 2 95281 3085083 <
banned_titles = TitleId RelatedTitleId 0 89989 32598 1 89989 3085083 2 95281 3085083
Try list one line (no lambda):
dict(df.groupby('TitleId')['RelatedTitleId'].apply(list)) # {89989: [32598, 3085083], 95281: [3085083]}