from collections import defaultdict

data = [
    {'name': 'Alice', 'city': 'New York'},
    {'name': 'Bob', 'city': 'London'},
    {'name': 'Charlie', 'city': 'New York'},
    {'name': 'David', 'city': 'London'},
]

grouped_by_city = defaultdict(list)
for item in data:
    grouped_by_city[item['city']].append(item['name'])

# Result: defaultdict(<class 'list'>, {'New York': ['Alice', 'Charlie'], 'London': ['Bob', 'David']})

# Convert to a regular dict if needed
result_dict = dict(grouped_by_city)
# Result: {'New York': ['Alice', 'Charlie'], 'London': ['Bob', 'David']}