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📘 Python Beginner Course · Lesson 7 of 12

Most office data arrives as files: CSV exports from Tally, SAP or a website. Python reads them, does the work, and writes a result file, without opening Excel at all.

Reading a CSV

import csv
from collections import defaultdict

totals = defaultdict(float)                 # a dict that starts every new key at 0.0
with open("sales.csv", newline="", encoding="utf-8") as f:
    for row in csv.DictReader(f):           # each row is a dict: {"region": "North", "qty": "3", ...}
        totals[row["region"]] += int(row["qty"]) * float(row["rate"])

• with open(...) as f: opens the file and closes it automatically afterwards.
• DictReader uses the header row, so you refer to columns by name.
• Everything read from CSV is text: convert with int() and float().

Writing a CSV

with open("region_totals.csv", "w", newline="", encoding="utf-8") as f:
    w = csv.writer(f)
    w.writerow(["region", "sales"])
    for region in sorted(totals):
        w.writerow([region, round(totals[region])])

"w" creates or overwrites; "a" appends. newline="" stops blank lines appearing between rows on Windows.

Encodings

Files with ₹, Hindi text or names with accents need the right encoding. Use encoding="utf-8"; for CSVs saved by Excel as “CSV UTF-8”, use "utf-8-sig" to drop the invisible marker at the start. Old exports may be "cp1252".

Paths

from pathlib import Path
folder = Path(r"C:\Reports\2026")       # r"..." keeps backslashes literal
for f in folder.glob("*.csv"):
    print(f.name)

Relative names like “sales.csv” are looked up in the folder you run the script from.

Where beginners go wrong

Practice

Download the script (and data file, if listed) below into one folder. Open a terminal in that folder and run python lesson-NN.py. Compare with the expected output, then change something and run it again: that’s how Python is learned.