Python — files, CSV and JSON

Data is stored in files. Three formats you will encounter frequently in professional work:

• Text file (.txt): simple lines.
• CSV: tabular format, one row per record, columns separated by commas or semicolons.
• JSON: nested structures (lists and dictionaries) that correspond to Python's list and dict.

Always open files with with open(...) as f: so that the file is closed automatically, even if an error occurs. Specify encoding="utf-8" to avoid issues with special characters such as å, ä and ö.

import csv, json

with open("anstallda.csv", encoding="utf-8") as f:
    rader = list(csv.DictReader(f, delimiter=";"))
print(rader[0])                 # {'namn': 'Anna', 'timmar': '14'}
total = sum(int(r["timmar"]) for r in rader)

data = {"avdelning": "IT", "anstallda": rader}
with open("avdelning.json", "w", encoding="utf-8") as f:
    json.dump(data, f, ensure_ascii=False, indent=2)

try:
    with open("saknas.csv") as f:
        ...
except FileNotFoundError:
    print("Filen hittades inte — kontrollera sökvägen")

The CSV reader always returns strings — you must convert them to numbers yourself. JSON preserves data types (numbers, lists, boolean values).