Merge two csvs

merge.py

import csv
from pathlib import Path

ROOT = Path(__file__).resolve().parent
INPUT_OLD = ROOT / "old.csv"
INPUT_NEW = ROOT / "new.csv"
OUTPUT = ROOT / "final.csv"

# csv data format:
# email,zip_code
# lorem@ipsum,12345

def merge():
    old = {}
    final = {}
    with open(INPUT_OLD, "r") as inputold:
        with open(INPUT_NEW, "r") as inputnew:
            oldreader = csv.reader(inputold)
            newreader = csv.reader(inputnew)

            next(oldreader, None) # skip the headers
            next(newreader, None)

            for row in oldreader:
                if row[1] and len(row[1]) in (4, 5) and row[1].isdigit(): # cp is valid
                    old[row[0]] = row[1].zfill(5)

            for row in newreader:
                if row[1] and len(row[1]) in (4, 5) and row[1].isdigit(): # cp is valid
                    continue

                stored_cp = old.get(row[0])
                if stored_cp:
                    final[row[0]] = stored_cp

    with open(OUTPUT, "w") as output:
        writer = csv.writer(output)
        writer.writerow(["email", "zip_code"])
        for k, v in final.items():
            writer.writerow([k, v])


if __name__ == "__main__":
    merge()
    print("Done")