Common exception types in Python

Introduction

Exceptions help us find and handle errors in our programs so they don't crash unexpectedly.

When reading a file that might not exist.

When dividing numbers and the divisor could be zero.

When converting user input to a number that might be invalid.

When accessing a list item that might be out of range.

When working with keys in a dictionary that might not be present.

Syntax

Python

try:
    # code that might cause an error
except ExceptionType:
    # code to handle the error

Replace ExceptionType with the specific error you want to catch.

You can catch multiple exceptions by listing them in parentheses.

Examples

This catches the error when dividing by zero.

Python

try:
    x = 1 / 0
except ZeroDivisionError:
    print("Cannot divide by zero!")

This catches the error when converting a string that is not a number.

Python

try:
    value = int('abc')
except ValueError:
    print("Invalid number!")

This catches the error when accessing a list index that does not exist.

Python

try:
    my_list = [1, 2, 3]
    print(my_list[5])
except IndexError:
    print("Index is out of range!")

Sample Program

This program asks the user for a number, then divides 10 by that number. It handles errors if the input is not a number or if the number is zero.

Python

try:
    number = int(input("Enter a number: "))
    result = 10 / number
    print(f"10 divided by {number} is {result}")
except ValueError:
    print("Oops! That is not a valid number.")
except ZeroDivisionError:
    print("Oops! Cannot divide by zero.")

Important Notes

Always catch specific exceptions instead of a general Exception to avoid hiding bugs.

You can use multiple except blocks to handle different errors separately.

Use try-except to keep your program running smoothly even when errors happen.

Summary

Exceptions help catch and handle errors in your code.

Common exceptions include ValueError, ZeroDivisionError, and IndexError.

Use try-except blocks to manage errors and keep your program running.

Practice

1. Which exception is raised when you try to divide a number by zero in Python?

easy

A. ValueError

B. ZeroDivisionError

C. IndexError

D. TypeError

Solution

1. Step 1: Understand division by zero

   Dividing any number by zero is mathematically undefined and causes an error in Python.

2. Step 2: Identify the exception type

   Python raises a ZeroDivisionError when division by zero occurs.

3. Final Answer:

   ZeroDivisionError -> Option B

4. Quick Check:

   Division by zero = ZeroDivisionError [OK]

Hint: Division by zero always raises ZeroDivisionError [OK]

Common Mistakes:

• Confusing ZeroDivisionError with ValueError
• Thinking IndexError occurs for division
• Assuming TypeError is raised for zero division

2. Which of the following code snippets will raise a ValueError?

easy

A. open('file.txt')

B. 5 / 0

C. my_list[10] where my_list has 5 elements

D. int('abc')

Solution

1. Step 1: Analyze each option for ValueError

   int('abc') tries to convert a non-numeric string to int, which causes ValueError.

2. Step 2: Check other options for different exceptions

   5 / 0 causes ZeroDivisionError, C causes IndexError, A may cause FileNotFoundError.

3. Final Answer:

   int('abc') -> Option D

4. Quick Check:

   Invalid int conversion = ValueError [OK]

Hint: ValueError occurs when conversion or value is invalid [OK]

Common Mistakes:

• Confusing ZeroDivisionError with ValueError
• Assuming file open errors cause ValueError
• Mixing IndexError with ValueError

3. What will be the output of this code?

my_list = [1, 2, 3]
print(my_list[3])

medium

A. IndexError

B. 3

C. None

D. ValueError

Solution

1. Step 1: Understand list indexing

   List indices start at 0, so valid indices for my_list are 0, 1, 2.

2. Step 2: Accessing index 3

   Index 3 is out of range, so Python raises an IndexError.

3. Final Answer:

   IndexError -> Option A

4. Quick Check:

   Out of range index = IndexError [OK]

Hint: Accessing invalid list index raises IndexError [OK]

Common Mistakes:

• Thinking it returns last element
• Assuming None is returned for invalid index
• Confusing IndexError with ValueError

4. Identify the error in this code snippet:

try:
    x = int('hello')
except ZeroDivisionError:
    print('Cannot divide by zero')

medium

A. Wrong exception caught, should catch ValueError

B. SyntaxError due to missing colon

C. No error, code runs fine

D. ZeroDivisionError will be raised

Solution

1. Step 1: Analyze the try block

   int('hello') raises ValueError because 'hello' cannot convert to int.

2. Step 2: Check except block

   Except block catches ZeroDivisionError, which does not handle ValueError, so error is uncaught.

3. Final Answer:

   Wrong exception caught, should catch ValueError -> Option A

4. Quick Check:

   Exception type mismatch = catch correct exception [OK]

Hint: Catch the exact exception your code may raise [OK]

Common Mistakes:

• Catching wrong exception type
• Assuming code runs without error
• Confusing syntax errors with exception handling

5. You want to safely convert user input to an integer and print it. Which code correctly handles invalid input without crashing?

hard

A. try:
num = int(input())
except ZeroDivisionError:
print('Invalid number')

B. num = int(input())
print(num)

C. try:
num = int(input())
except ValueError:
print('Invalid number')

D. num = input()
print(int(num))

Solution

1. Step 1: Understand input conversion risks

   User input may not be a valid integer, causing ValueError on conversion.

2. Step 2: Check exception handling

   try: num = int(input()) except ValueError: print('Invalid number') uses try-except to catch ValueError and print a message, preventing crash.

3. Final Answer:

   try-except catching ValueError with message -> Option C

4. Quick Check:

   Handle invalid input with ValueError catch [OK]

Hint: Use try-except to catch ValueError on int conversion [OK]

Common Mistakes:

• Not catching exceptions causing program crash
• Catching wrong exception type like ZeroDivisionError
• Assuming input is always valid integer