CSV files, board style

Part of the free LogicWiz Python course, Pack A: CBSE Class 12 Board Pack. The examples and practice run in the app.

Raj runs a medical store and keeps sales in Sales.csv. On his Windows PC every row was followed by a blank row, and his total crashed with ValueError. Two small things fix it: newline="" and next().

You'll learn to

• Write rows with csv.writer(), writerow() and writerows(), using newline=""
• Read rows with csv.reader() and convert the strings to numbers
• Solve the board's Accept()/Total() and user-id/password programs

Learn

CSV in four calls

A CSV (comma-separated values) file is plain text: one record per line, fields separated by commas. Excel opens it as a table.

• w = csv.writer(f) makes a writer for an open file.
• w.writerow(list) writes one row.
• w.writerows(list_of_lists) writes many rows.
• csv.reader(f) gives each row back as a list of strings.

Open CSV files with newline="", for writing and reading.

Write a table, read it back

import csv
rows = [["P01", "Paracetamol", 10, 25.5], ["P02", "Cough syrup", 2, 90]]
with open("Sales.csv", "w", newline="") as f:
    w = csv.writer(f)
    w.writerow(["Product_ID", "Product_Name", "Quantity_Sold", "Price_Per_Unit"])
    w.writerows(rows)

with open("Sales.csv", "r", newline="") as f:
    for row in csv.reader(f):
        print(row)

Look at the output: 10 came back as '10'. Everything a reader gives you is a string.

Two classic CSV mistakes.

1. Forgetting newline="". The writer already ends each row with \r\n; on Windows, text mode adds another \r, so you see a blank line after every row.
2. Doing maths on strings. row[2] + row[3] joins text. Convert first: int(row[2]) * float(row[3]).

writerow vs writerows

Board pattern: Accept() and a total

import csv

def Accept():
    pid = input("Product ID: ")
    name = input("Product name: ")
    qty = int(input("Quantity sold: "))
    price = float(input("Price per unit: "))
    with open("Sales.csv", "a", newline="") as f:     # a = add, keep old rows
        csv.writer(f).writerow([pid, name, qty, price])

def CalculateTotalSales():
    total = 0
    with open("Sales.csv", "r", newline="") as f:
        r = csv.reader(f)
        next(r)                       # skip the header row
        for row in r:
            total += int(row[2]) * float(row[3])
    return total

Accept()
print()
print("Total sales: Rs", CalculateTotalSales())

next(r) reads one row and throws it away. Without it, int("Quantity_Sold") raises ValueError.

Practical-list program: user-id and password

import csv
with open("users.csv", "w", newline="") as f:
    csv.writer(f).writerows([["aarav12", "chai@123"], ["meher07", "samosa#9"], ["riya", "metro2025"]])

def find_password(uid):
    with open("users.csv", "r", newline="") as f:
        for row in csv.reader(f):
            if row[0] == uid:
                return row[1]
    return None

uid = input("User id: ")
pw = find_password(uid)
if pw is None:
    print("User id not found")
else:
    print("Password:", pw)

This exact task (create a CSV of user-ids and passwords, then search) is on the CBSE suggested practical list.

4-mark CSV question: one function adds a record typed by the user, the other reads the file and computes something. Marks for: import csv · open(..., "a", newline="") · writer() + writerow() · reader() · skipping the header · converting with int()/float() · return. Use "a" in Accept(); "w" would wipe earlier sales.

In one line

Open CSV files with newline=""; writerow/writerows write, reader reads lists of strings; skip the header with next() and convert before maths.