Reading files line by line in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of reading lines with strip()

What is the output of this Python code that reads a file line by line and strips whitespace?

Python

with open('test.txt', 'w') as f:
    f.write('  line1  \nline2\n  line3  \n')

with open('test.txt', 'r') as file:
    lines = [line.strip() for line in file]
print(lines)

A ['line1', 'line2', 'line3']

B [' line1 ', 'line2', ' line3 ']

C ['line1 ', 'line2', 'line3 ']

D [' line1', 'line2', ' line3']

💡 Hint

The strip() method removes spaces at the start and end of each line.

Using line.strip() removes all leading and trailing spaces from each line, so the list contains clean strings without extra spaces.

❓ Predict Output

intermediate

Reading file lines with readlines()

What will be the output of this code that reads lines from a file using readlines()?

Python

with open('test2.txt', 'w') as f:
    f.write('apple\nbanana\ncherry\n')

with open('test2.txt', 'r') as f:
    fruits = f.readlines()
print(fruits)

A ['apple banana cherry']

B ['apple\n', 'banana\n', 'cherry\n']

C ['apple', 'banana', 'cherry']

D ['apple\r\n', 'banana\r\n', 'cherry\r\n']

💡 Hint

readlines() keeps the newline characters at the end of each line.

The readlines() method returns a list of lines including the newline characters '\n' at the end of each line.

🔧 Debug

advanced

Why does this code raise an error?

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

Python

file = open('missing.txt', 'r')
for line in file:
    print(line)
file.close()

A TypeError because file is not iterable

B SyntaxError because the for loop is missing a colon

C FileNotFoundError because the file 'missing.txt' does not exist

D IndentationError because print is not indented

💡 Hint

Check if the file exists before opening it.

Trying to open a file that does not exist in read mode causes a FileNotFoundError.

❓ Predict Output

advanced

Output of reading file with a while loop

What is the output of this code that reads a file line by line using a while loop?

Python

with open('test3.txt', 'w') as f:
    f.write('one\ntwo\nthree\n')

with open('test3.txt', 'r') as f:
    line = f.readline()
    lines = []
    while line:
        lines.append(line.strip())
        line = f.readline()
print(lines)

A ['one', 'two', 'three']

B ['one\ntwo\nthree\n']

C ['one', 'two', 'three', '']

D ['one', 'two', 'three', None]

💡 Hint

readline() returns an empty string when the file ends.

The loop reads each line until readline() returns an empty string, stripping spaces and newlines, so the list contains clean lines.

❓ Predict Output

expert

Number of lines read with a generator expression

What is the number of lines read by this code using a generator expression?

Python

with open('test4.txt', 'w') as f:
    f.write('a\nb\nc\nd\n')

with open('test4.txt', 'r') as f:
    count = sum(1 for _ in f if _.strip() != 'b')
print(count)

A 1

B 4

C 2

D 3

💡 Hint

Count lines except those equal to 'b' after stripping.

The generator counts lines where the stripped line is not 'b'. Lines are 'a', 'b', 'c', 'd', so count is 3.