How to Group Data by Key in Python with Type Hints

from typing import Any, Dict, List, TypeVar, Union

T = TypeVar("T")

def group_by(data: List[Dict[str, Any]], key: str) -> Dict[Any, List[Dict[str, Any]]]:
    """Group a list of dictionaries by a given key."""
    grouped: Dict[Any, List[Dict[str, Any]]] = {}
    for item in data:
        value = item.get(key)
        grouped.setdefault(value, []).append(item)
    return grouped

def summarize(grouped: Dict[Any, List[Dict[str, Any]]]) -> None:
    """Print a summary of grouped data."""
    for group_key, items in grouped.items():
        print(f"{group_key}: {len(items)} item(s) -> {items}")

if __name__ == "__main__":
    people = [
        {"name": "Alice", "age": 30, "city": "NYC"},
        {"name": "Bob", "age": 25, "city": "LA"},
        {"name": "Charlie", "age": 30, "city": "NYC"},
        {"name": "Diana", "age": 25, "city": "LA"},
    ]
    
    by_city = group_by(people, "city")
    summarize(by_city)
    
    print()
    
    by_age = group_by(people, "age")
    summarize(by_age)