Working with CSV files in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Reading CSV and counting rows

What is the output of this code that reads a CSV file and counts its rows?

Python

import csv

with open('data.csv', newline='') as csvfile:
    reader = csv.reader(csvfile)
    count = 0
    for row in reader:
        count += 1
print(count)

A 4

B 5

C 6

D Error: FileNotFoundError

💡 Hint

Assume 'data.csv' contains 5 rows including the header.

The code counts all rows including the header. If the file has 5 rows total, the output is 5.

❓ Predict Output

intermediate

Writing CSV with DictWriter

What will be the content of 'output.csv' after running this code?

Python

import csv

with open('output.csv', 'w', newline='') as csvfile:
    fieldnames = ['name', 'age']
    writer = csv.DictWriter(csvfile, fieldnames=fieldnames)
    writer.writeheader()
    writer.writerow({'name': 'Alice', 'age': 30})
    writer.writerow({'name': 'Bob', 'age': 25})

A

name,age
30,Alice
25,Bob

B

name;age
Alice;30
Bob;25

C

name,age
Alice,30
Bob,25

D Error: TypeError

💡 Hint

DictWriter writes CSV with commas by default and writes header first.

The DictWriter writes the header and rows with commas separating fields in the order of fieldnames.

🔧 Debug

advanced

Fix the error in CSV reading code

This code tries to read a CSV file but raises an error. What is the cause?

Python

import csv

with open('data.csv') as f:
    reader = csv.reader(f)
    for row in reader:
        print(row[2])

A IndexError because some rows have fewer than 3 columns

B FileNotFoundError because 'data.csv' does not exist

C TypeError because csv.reader returns a dict, not a list

D No error, prints third column of each row

💡 Hint

Check if all rows have at least 3 columns.

If any row has fewer than 3 columns, accessing row[2] causes IndexError.

❓ Predict Output

advanced

Using csv.DictReader to access data

What is the output of this code snippet?

Python

import csv

with open('people.csv', newline='') as csvfile:
    reader = csv.DictReader(csvfile)
    ages = [int(row['age']) for row in reader]
print(sum(ages))

A 75

B TypeError

C 0

D ValueError

💡 Hint

Assume 'people.csv' has three rows with ages 20, 25, and 30.

The code sums the ages 20 + 25 + 30 = 75.

🧠 Conceptual

expert

CSV Dialect and Delimiter Behavior

Which option correctly creates a CSV writer that uses a semicolon (;) as delimiter and quotes all fields?

A csv.writer(file, delimiter=';', quoting=True)

B csv.writer(file, delimiter=';', quotechar='"')

C csv.writer(file, delimiter=',', quoting=csv.QUOTE_ALL)

D csv.writer(file, delimiter=';', quoting=csv.QUOTE_ALL)

💡 Hint

Check the parameters for csv.writer regarding delimiter and quoting.

Option D correctly sets delimiter to ';' and quoting to csv.QUOTE_ALL to quote all fields.