File system interaction basics in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Reading a file's content

What is the output of this code if the file example.txt contains the text Hello World?

Python

with open('example.txt', 'r') as f:
    content = f.read()
print(content)

A "Hello World"

B ['Hello World']

C b'Hello World'

D None

💡 Hint

The read() method returns the whole file content as a string.

The open function with mode 'r' opens the file for reading as text. The read() method reads the entire content as a string. So the output is the exact text inside the file.

❓ Predict Output

intermediate

Writing and reading back a file

What will be printed after running this code?

Python

with open('testfile.txt', 'w') as f:
    f.write('12345')
with open('testfile.txt', 'r') as f:
    print(f.readline())

A 12345

B ['12345']

C None

D 1

💡 Hint

The write method writes text, and readline reads one line as a string.

The file is written with the string 12345. Then reading one line returns the whole string since there is no newline. So it prints 12345.

❓ Predict Output

advanced

Using os.path to check file existence

What does this code print if the file data.txt does NOT exist in the current folder?

Python

import os
if os.path.exists('data.txt'):
    print('Found')
else:
    print('Not Found')

A Found

B Not Found

C Raises FileNotFoundError

D None

💡 Hint

The os.path.exists function returns True if the file exists, otherwise False.

Since the file does not exist, os.path.exists returns False, so the else branch prints Not Found.

❓ Predict Output

advanced

Understanding file modes and errors

What error does this code raise if readonly.txt exists but is read-only?

Python

with open('readonly.txt', 'w') as f:
    f.write('Trying to write')

A No error, writes successfully

B TypeError

C PermissionError

D FileNotFoundError

💡 Hint

Opening a read-only file for writing usually causes a permission error.

Trying to open a file with write mode 'w' when the file is read-only causes a PermissionError because the program lacks permission to modify the file.

🧠 Conceptual

expert

File pointer position after reading

After running this code, what is the value of pos?

Python

with open('numbers.txt', 'w') as f:
    f.write('1234567890')
with open('numbers.txt', 'r') as f:
    f.read(4)
    pos = f.tell()
print(pos)

A 10

B 0

C Raises an error

D 4

💡 Hint

The tell() method returns the current position of the file pointer.

The read(4) reads 4 characters, moving the file pointer 4 positions forward. So tell() returns 4.

Practice

1. What does the mode 'r' mean when opening a file with open() in Python?

easy

A. Open the file for reading only

B. Open the file for writing only

C. Open the file for appending data

D. Create a new file or overwrite existing

Solution

1. Step 1: Understand file modes in Python

   The mode 'r' stands for reading the file only, meaning you can read data but not change it.

2. Step 2: Compare with other modes

   Modes like 'w' are for writing (which overwrites), and 'a' is for appending. 'r' does not allow writing.

3. Final Answer:

   Open the file for reading only -> Option A

4. Quick Check:

   Mode 'r' = read only [OK]

Hint: Remember 'r' means read, 'w' means write, 'a' means append [OK]

Common Mistakes:

• Confusing 'r' with 'w' or 'a'
• Thinking 'r' creates a new file
• Trying to write to a file opened with 'r'

2. Which of the following is the correct syntax to open a file named 'data.txt' for writing in Python?

easy

A. open('data.txt', 'r')

B. open('data.txt', 'w')

C. open('data.txt', 'rw')

D. open('data.txt', 'a+')

Solution

1. Step 1: Identify the mode for writing

   The mode 'w' opens a file for writing and creates it if it doesn't exist or overwrites if it does.

2. Step 2: Check syntax correctness

   open('data.txt', 'w') is the correct syntax. 'r' is for reading, 'rw' is invalid, 'a+' is for appending and reading.

3. Final Answer:

   open('data.txt', 'w') -> Option B

4. Quick Check:

   Write mode = 'w' [OK]

Hint: Use 'w' to write or overwrite files [OK]

Common Mistakes:

• Using 'r' when intending to write
• Using invalid mode 'rw'
• Confusing 'a+' with 'w'

3. What will be the output of this code?

with open('test.txt', 'w') as f:
    f.write('Hello')

with open('test.txt', 'a') as f:
    f.write(' World')

with open('test.txt', 'r') as f:
    print(f.read())

medium

A. Error: file not found

B. Hello

C. Hello World

D. World

Solution

1. Step 1: Write 'Hello' to the file

   The first block opens 'test.txt' in write mode, which creates or clears the file, then writes 'Hello'.

2. Step 2: Append ' World' to the file

   The second block opens the file in append mode and adds ' World' after 'Hello'.

3. Step 3: Read and print the file content

   The last block reads the full content, which is 'Hello World', and prints it.

4. Final Answer:

   Hello World -> Option C

5. Quick Check:

   Write + append = 'Hello World' [OK]

Hint: Write clears file, append adds to end [OK]

Common Mistakes:

• Expecting append to overwrite
• Not closing files before reading
• Confusing write and append modes

4. What is wrong with this code snippet?

f = open('log.txt', 'r')
print(f.read())
f.write('New entry')
f.close()

medium

A. File is opened in read mode but write is attempted

B. File is not closed properly

C. Missing mode argument in open()

D. File path is incorrect

Solution

1. Step 1: Check file mode and operations

   The file is opened with mode 'r' which allows reading only.

2. Step 2: Identify invalid operation

   Calling f.write() on a file opened in read mode causes an error because writing is not allowed.

3. Final Answer:

   File is opened in read mode but write is attempted -> Option A

4. Quick Check:

   Write not allowed in 'r' mode [OK]

Hint: Don't write to files opened with 'r' mode [OK]

Common Mistakes:

• Trying to write without 'w' or 'a' mode
• Forgetting to close files
• Assuming 'r' mode allows writing

5. You want to read a file line by line and print only lines that contain the word 'error'. Which is the best way to do this in Python?

hard

A. open('log.txt', 'r').read().split('error')

B. f = open('log.txt', 'r')
lines = f.readlines()
for line in lines:
if line == 'error':
print(line)
f.close()

C. with open('log.txt', 'w') as f:
for line in f:
if 'error' in line:
print(line)

D. with open('log.txt', 'r') as f:
for line in f:
if 'error' in line:
print(line.strip())

Solution

1. Step 1: Use 'with' and read line by line

   with open('log.txt', 'r') as f: for line in f: if 'error' in line: print(line.strip()) uses 'with' to open the file safely and iterates line by line, which is memory efficient.

2. Step 2: Check condition and print matching lines

   It checks if 'error' is in each line and prints the line without extra spaces using strip().

3. Final Answer:

   with open('log.txt', 'r') as f: for line in f: if 'error' in line: print(line.strip()) -> Option D

4. Quick Check:

   Use 'with' + for line in file + condition [OK]

Hint: Use 'with' and loop lines to filter content [OK]

Common Mistakes:

• Opening file in 'w' mode when reading
• Comparing whole line to 'error' instead of substring
• Not closing file properly
• Using split incorrectly for line filtering