Join two CSV files on shared key column in Python

import csv

def join_csv(file1, file2, key, output="joined.csv"):
    # Read first CSV into dict keyed by the join column
    with open(file1, newline="") as f1:
        reader1 = csv.DictReader(f1)
        data1 = {row[key]: row for row in reader1}

    # Read second CSV and merge matching rows
    with open(file2, newline="") as f2:
        reader2 = csv.DictReader(f2)
        fieldnames = list(data1[next(iter(data1))].keys()) + [
            col for col in reader2.fieldnames if col != key
        ]
        with open(output, "w", newline="") as out:
            writer = csv.DictWriter(out, fieldnames=fieldnames)
            writer.writeheader()
            for row in reader2:
                if row[key] in data1:
                    merged = {**data1[row[key]], **{
                        k: v for k, v in row.items() if k != key
                    }}
                    writer.writerow(merged)

if __name__ == "__main__":
    # Create sample CSVs inline for demonstration
    with open("customers.csv", "w", newline="") as f:
        w = csv.writer(f)
        w.writerow(["id", "name"])
        w.writerow(["1", "Alice"])
        w.writerow(["2", "Bob"])
        w.writerow(["3", "Carol"])

    with open("orders.csv", "w", newline="") as f:
        w = csv.writer(f)
        w.writerow(["id", "item"])
        w.writerow(["1", "apple"])
        w.writerow(["1", "banana"])
        w.writerow(["2", "cherry"])

    join_csv("customers.csv", "orders.csv", key="id")
    with open("joined.csv") as f:
        print(f.read())