Dictionary-based CSV handling in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Reading CSV with DictReader output

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

Python

import csv
from io import StringIO

csv_data = "name,age,city\nAlice,30,New York\nBob,25,Los Angeles"

f = StringIO(csv_data)
reader = csv.DictReader(f)
result = [row for row in reader]
print(result)

A SyntaxError

B [{'name': 'Alice', 'age': 30, 'city': 'New York'}, {'name': 'Bob', 'age': 25, 'city': 'Los Angeles'}]

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

D [{'name': 'Alice', 'age': '30', 'city': 'New York'}, {'name': 'Bob', 'age': '25', 'city': 'Los Angeles'}]

💡 Hint

Remember that csv.DictReader reads all values as strings.

csv.DictReader reads each row as a dictionary with keys from the header row. All values are strings by default.

❓ Predict Output

intermediate

Writing CSV with DictWriter fieldnames order

What will be the content of the CSV string after running this code?

Python

import csv
from io import StringIO

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

A

name,age,city
Alice,30,New York
Bob,25,Los Angeles

B

city,name,age
New York,Alice,30
Los Angeles,Bob,25

C

city,name,age
New York,Alice,30
Los Angeles,Bob,25

D TypeError

💡 Hint

The order of columns in CSV matches the fieldnames list given to DictWriter.

DictWriter writes columns in the order specified by fieldnames. The default line terminator is '\r\n'.

❓ Predict Output

advanced

Handling missing keys in DictWriter

What happens when you run this code that writes a row missing a fieldname key?

Python

import csv
from io import StringIO

output = StringIO()
fieldnames = ['name', 'age', 'city']
writer = csv.DictWriter(output, fieldnames=fieldnames)
writer.writeheader()
writer.writerow({'name': 'Alice', 'city': 'New York'})
result = output.getvalue()
print(result)

A KeyError

B

name,age,city
Alice,0,New York

C

name,age,city
Alice,,New York

D TypeError

💡 Hint

Missing keys in the row dictionary are written as empty strings by default.

DictWriter writes empty strings for missing keys in the row dictionary instead of raising errors.

❓ Predict Output

advanced

Reading CSV with different delimiter using DictReader

What is the output of this code that reads a semicolon-separated CSV string?

Python

import csv
from io import StringIO

csv_data = "name;age;city\nAlice;30;New York\nBob;25;Los Angeles"

f = StringIO(csv_data)
reader = csv.DictReader(f, delimiter=';')
result = [row for row in reader]
print(result)

A [{'name': 'Alice', 'age': '30', 'city': 'New York'}, {'name': 'Bob', 'age': '25', 'city': 'Los Angeles'}]

B [{'name;age;city': 'Alice;30;New York'}, {'name;age;city': 'Bob;25;Los Angeles'}]

C []

D ValueError

💡 Hint

Specify the delimiter parameter to match the CSV format.

DictReader uses the delimiter to split columns correctly. Without it, it treats the whole line as one field.

🧠 Conceptual

expert

Effect of restval in DictWriter on missing keys

Given this code, what is the output CSV content?

Python

import csv
from io import StringIO

output = StringIO()
fieldnames = ['name', 'age', 'city']
writer = csv.DictWriter(output, fieldnames=fieldnames, restval='UNKNOWN')
writer.writeheader()
writer.writerow({'name': 'Alice', 'city': 'New York'})
result = output.getvalue()
print(result)

A

name,age,city
Alice,UNKNOWN,New York

B

name,age,city
Alice,,New York

C KeyError

D TypeError

💡 Hint

restval sets the default value for missing keys when writing rows.

When a key is missing in the row dictionary, DictWriter uses restval as the value instead of empty string.