def group_by_key(items, key_func):
    grouped_data = {}
    for item in items:
        key = key_func(item)
        if key not in grouped_data:
            grouped_data[key] = []
        grouped_data[key].append(item)
    return grouped_data

# Example Usage (grouping people by their age):
# people = [
#     {'name': 'Alice', 'age': 30},
#     {'name': 'Bob', 'age': 25},
#     {'name': 'Charlie', 'age': 30},
#     {'name': 'David', 'age': 25}
# ]
# grouped_by_age = group_by_key(people, lambda p: p['age'])
# print(grouped_by_age)
# # Expected: {30: [{'name': 'Alice', 'age': 30}, {'name': 'Charlie', 'age': 30}], 25: [{'name': 'Bob', 'age': 25}, {'name': 'David', 'age': 25}]}