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from collections import defaultdict
output = defaultdict(list)
for item in data:
    output[item["owner"]].append(item["pet"])

# output is =>
defaultdict(list,
            {'Kent': ['Shiner'],
             'Mary': ['Pumpkin', 'Tasha'],
             'Paige': ['Sushi'],
             'Sarah': ['Shiner', 'Mr. Wigglesworth', 'Pupparoo', 'Lucy']})