How do I write a CSV file in Python?

Use csv.DictWriter. Tell it the column names once, write the header, then hand it your rows as dictionaries.

import csv

orders = [
    {"id": "A-1001", "customer": "Ada", "total": 49.99},
    {"id": "A-1002", "customer": "Grace", "total": 120.00},
]

with open("orders.csv", "w", newline="", encoding="utf-8") as f:
    writer = csv.DictWriter(f, fieldnames=["id", "customer", "total"])
    writer.writeheader()
    writer.writerows(orders)

with open("orders.csv", encoding="utf-8") as f:
    print(f.read())

id,customer,total
A-1001,Ada,49.99
A-1002,Grace,120.0

Notice 120.0: the writer calls str() on each value. If the file is going to a person, format it yourself first, for example f"{total:.2f}".

Don't build the line with string joins

",".join(values) works right up until a value contains a comma. The csv module quotes it for you, which is the whole reason to use it.

import csv, io

with io.StringIO() as out:                  # stands in for an open file
    writer = csv.writer(out)
    writer.writerow(["Hopper, Grace", 'She said "hi"', 120])
    print(out.getvalue())

"Hopper, Grace","She said ""hi""",120

Any spreadsheet program reads that back as three cells, exactly as written.

The two things that bite

• A blank line between every row on Windows: you forgot newline="" in open(). The writer already ends each row with \r\n; without newline="", Windows adds another.
• Excel shows Zoë as Zoë: Excel assumes the old Windows encoding. Write with encoding="utf-8-sig" and Excel recognises the file as UTF-8.

To add rows to a file that already exists, open it with "a" instead of "w" and skip writeheader().