Why exceptions occur in Python - Quick Recap

Recall & Review

beginner

What is an exception in Python?

An exception is an error that happens during the running of a program, which stops the normal flow of the program.

beginner

Name a common reason why exceptions occur.

Exceptions often occur because the program tries to do something invalid, like dividing by zero or accessing a missing file.

beginner

What happens if an exception is not handled?

If an exception is not handled, the program will stop running and show an error message.

beginner

Why might a 'FileNotFoundError' exception occur?

This exception happens when the program tries to open a file that does not exist or the path is wrong.

beginner

How does Python signal that an exception has occurred?

Python raises an exception object that interrupts the normal flow and looks for code to handle it.

What causes a ZeroDivisionError in Python?

A Using a variable before defining it

B Dividing a number by zero

C Trying to open a missing file

D Writing invalid syntax

Dividing by zero is not allowed and causes a ZeroDivisionError.

What happens if you do not handle an exception?

A The program stops and shows an error

B The program continues normally

C The program ignores the error silently

D The program restarts automatically

Uncaught exceptions stop the program and display an error message.

Which of these is NOT a reason for exceptions?

A Correctly writing a print statement

B Dividing by zero

C Using a variable that is not defined

D Trying to access a missing file

Correct code like a print statement does not cause exceptions.

What does Python do when an exception occurs?

A Ignores it

B Automatically fixes the error

C Raises an exception object

D Restarts the program

Python raises an exception object to signal the error.

Why might a FileNotFoundError happen?

A File has wrong permissions

B File is too large

C File is open in another program

D File path is incorrect or file is missing

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

Explain in your own words why exceptions occur in Python programs.

Think about what happens when the program tries to do something it cannot.

List some common examples of exceptions and why they happen.

Recall errors like dividing by zero or missing files.

Practice

1. Why do exceptions occur in a Python program?

easy

A. Because the program encounters an unexpected error during execution

B. Because the program runs perfectly without any errors

C. Because the program finishes all tasks successfully

D. Because the program has no code to execute

Solution

1. Step 1: Understand what exceptions mean

   Exceptions happen when the program faces an unexpected problem it cannot handle normally.

2. Step 2: Identify the cause of exceptions

   Unexpected errors like dividing by zero or accessing missing files cause exceptions.

3. Final Answer:

   Because the program encounters an unexpected error during execution -> Option A

4. Quick Check:

   Unexpected error = Exception occurs [OK]

Hint: Exceptions happen only when errors occur during running [OK]

Common Mistakes:

• Thinking exceptions occur when program runs fine
• Confusing exceptions with normal program flow
• Believing exceptions happen without any error

2. Which of the following is the correct way to catch an exception in Python?

easy

A. try:
# code
except Exception:
# handle error

B. catch:
# code
try Exception:
# handle error

C. handle:
# code
catch Exception:
# handle error

D. try:
# code
catch Exception:
# handle error

Solution

1. Step 1: Recall Python syntax for exception handling

   Python uses try to run code and except to catch errors.

2. Step 2: Match the correct syntax

   try: # code except Exception: # handle error uses try and except Exception, which is correct.

3. Final Answer:

   try:\n # code\nexcept Exception:\n # handle error -> Option A

4. Quick Check:

   Use try and except keywords [OK]

Hint: Remember: try and except catch errors in Python [OK]

Common Mistakes:

• Using catch instead of except
• Swapping try and except keywords
• Incorrect keyword order or spelling

3. What will be the output of this code?

try:
    x = 5 / 0
except ZeroDivisionError:
    print('Cannot divide by zero')

medium

A. No output

B. 5

C. ZeroDivisionError

D. Cannot divide by zero

Solution

1. Step 1: Analyze the code inside try block

   The code tries to divide 5 by 0, which causes a ZeroDivisionError.

2. Step 2: Check the except block

   The except block catches ZeroDivisionError and prints 'Cannot divide by zero'.

3. Final Answer:

   Cannot divide by zero -> Option D

4. Quick Check:

   ZeroDivisionError caught prints message [OK]

Hint: Division by zero triggers ZeroDivisionError [OK]

Common Mistakes:

• Expecting program to crash without output
• Confusing error name with printed message
• Ignoring except block handling

4. Find the error in this code snippet:

try:
    print(10 / 2)
except ZeroDivisionError
    print('Error')

medium

A. No error in the code

B. Wrong exception type used

C. Missing colon after except statement

D. Missing try keyword

Solution

1. Step 1: Check syntax of except statement

   The except line lacks a colon at the end, which is required in Python.

2. Step 2: Confirm other parts are correct

   try keyword and exception type are correct; only colon is missing.

3. Final Answer:

   Missing colon after except statement -> Option C

4. Quick Check:

   except line must end with colon [OK]

Hint: Always put colon after except statement [OK]

Common Mistakes:

• Forgetting colon after except
• Assuming wrong exception type causes syntax error
• Thinking try keyword is missing

5. You want to open a file and read its content, but the file might not exist. Which code correctly handles this exception?

hard

A. try:
with open('data.txt') as f:
print(f.read())
except:
pass

B. try:
with open('data.txt') as f:
print(f.read())
except FileNotFoundError:
print('File not found')

C. try:
with open('data.txt') as f:
print(f.read())
except ZeroDivisionError:
print('File not found')

D. with open('data.txt') as f:
print(f.read())
except FileNotFoundError:
print('File not found')

Solution

1. Step 1: Understand the problem

   Opening a file that may not exist can cause FileNotFoundError.

2. Step 2: Check which option correctly catches FileNotFoundError

   try: with open('data.txt') as f: print(f.read()) except FileNotFoundError: print('File not found') uses try-except with FileNotFoundError and prints a message, which is correct.

3. Final Answer:

   try:\n with open('data.txt') as f:\n print(f.read())\nexcept FileNotFoundError:\n print('File not found') -> Option B

4. Quick Check:

   Catch FileNotFoundError to handle missing files [OK]

Hint: Catch FileNotFoundError to handle missing files [OK]

Common Mistakes:

• Placing except outside try block
• Catching wrong exception type
• Ignoring exception handling completely