df = pd.DataFrame({'Categories' : ['comedy | action | romcom', 'horror | thriller', 'comedy | coming-of-age'],
                   'Score' : [7.8, 5.2, 3.4]})


#Now how to get the cat list

cat_list = []

for row in df.itertuples(index=True, name='Pandas'):
    categories = getattr(row, 'Categories').split('|')
    categories = [x.strip() for x in categories]
    for cat in categories:
        cat_list.append(cat)

cat_list = list(set(cat_list)) #lazily remove dups


#Below will get an average of something for every cat


meanslist = []

for cat in cat_list:
    mean = []
    for row in df.itertuples(index=True, name='Pandas'):
        categories = getattr(row, 'Categories')
        if re.search(cat, (categories + ' \||\| ' + categories)):
            mean.append(getattr(row, "Score"))

    mean = sum(mean)/len(mean)
    meanslist.append([cat, mean])

print(meanslist)