My code works even though I didn't put the dictionary key / fieldnames in the CSV file

Question

import csv

phone = input("Enter phone name")
os = input("Enter OS")

with open("phos.csv", "a") as phos:
    phos_dict = csv.DictWriter(phos, fieldnames = ["phone_model", "phone_os"])
    phos_dict.writerow({"phone_model": phone, "phone_os": os})

The above code works just fine even though I left the CSV file empty. Then how does the code know how to implement/write the dictionary to the phos.csv file properly?

Answer 1

It works because you set the fieldnames. Then, when you send the dict with the correct keys, the code knows where to put the values.

If you pass a key:value in phos_dict.writerow({"phone_model": phone, "phone_os": os}) that was not implemenet in fieldnames = ["phone_model", "phone_os"] you'll get the following error:

ValueError: dict contains fields not in fieldnames