Reading and writing CSV data in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Reading CSV with csv.reader

What is the output of this code snippet that reads a CSV string using csv.reader?

Python

import csv
from io import StringIO

csv_data = "name,age\nAlice,30\nBob,25"
file_like = StringIO(csv_data)
reader = csv.reader(file_like)
rows = list(reader)
print(rows)

A ['name,age', 'Alice,30', 'Bob,25']

B [{'name': 'Alice', 'age': '30'}, {'name': 'Bob', 'age': '25'}]

C [['name', 'age'], ['Alice', '30'], ['Bob', '25']]

D SyntaxError

💡 Hint

csv.reader returns each row as a list of strings.

csv.reader reads CSV data line by line and returns each row as a list of strings. So the output is a list of lists representing rows.

❓ Predict Output

intermediate

Writing CSV with csv.DictWriter

What is the output of this code that writes a list of dictionaries to CSV format using csv.DictWriter?

Python

import csv
from io import StringIO

output = StringIO()
fieldnames = ['name', 'age']
writer = csv.DictWriter(output, fieldnames=fieldnames)
writer.writeheader()
writer.writerow({'name': 'Alice', 'age': 30})
writer.writerow({'name': 'Bob', 'age': 25})
print(output.getvalue())

A name,age\nAlice,30\nBob,25\n

B ['name', 'age']\n['Alice', '30']\n['Bob', '25']

C {'name': 'Alice', 'age': 30}\n{'name': 'Bob', 'age': 25}

D TypeError

💡 Hint

csv.DictWriter writes CSV rows as strings with headers.

csv.DictWriter writes the header row first, then each dictionary as a CSV row with values converted to strings.

❓ Predict Output

advanced

Reading CSV with custom delimiter

What is the output of this code that reads CSV data with a semicolon delimiter?

Python

import csv
from io import StringIO

csv_data = "name;age\nAlice;30\nBob;25"
file_like = StringIO(csv_data)
reader = csv.reader(file_like, delimiter=';')
rows = list(reader)
print(rows)

A [['name;age'], ['Alice;30'], ['Bob;25']]

B ValueError

C [{'name': 'Alice', 'age': '30'}, {'name': 'Bob', 'age': '25'}]

D [['name', 'age'], ['Alice', '30'], ['Bob', '25']]

💡 Hint

Specify the delimiter parameter to split columns correctly.

By setting delimiter=';', csv.reader splits each line by semicolon, returning lists of values.

❓ Predict Output

advanced

Writing CSV with quoting

What is the output of this code that writes CSV data with quoting for all fields?

Python

import csv
from io import StringIO

output = StringIO()
fieldnames = ['name', 'note']
writer = csv.DictWriter(output, fieldnames=fieldnames, quoting=csv.QUOTE_ALL)
writer.writeheader()
writer.writerow({'name': 'Alice', 'note': 'Hello, world!'})
writer.writerow({'name': 'Bob', 'note': 'Goodbye'})
print(output.getvalue())

A name,note\nAlice,Hello, world!\nBob,Goodbye\n

B "name","note"\n"Alice","Hello, world!"\n"Bob","Goodbye"\n

C ['name', 'note']\n['Alice', 'Hello, world!']\n['Bob', 'Goodbye']

D SyntaxError

💡 Hint

csv.QUOTE_ALL adds quotes around every field.

With quoting=csv.QUOTE_ALL, every field including headers is enclosed in double quotes.

🧠 Conceptual

expert

csv.DictReader behavior without header row

What is the output of this code when reading CSV data without a header row using csv.DictReader?

Python

import csv
from io import StringIO

csv_data = "Alice,30\nBob,25"
file_like = StringIO(csv_data)
reader = csv.DictReader(file_like)
rows = list(reader)
print(rows)

A [{'Alice': 'Bob', '30': '25'}]

B ValueError

C [{'fieldnames': None}, {'fieldnames': None}]

D [{'Alice': '30'}, {'Bob': '25'}]

💡 Hint

csv.DictReader uses the first row as header by default.

csv.DictReader uses the first row ['Alice', '30'] as fieldnames if none provided. The second row ['Bob', '25'] maps to {'Alice': 'Bob', '30': '25'}.