Common exception types in Python - Step-by-Step Execution

Concept Flow - Common exception types

Start

↓

Run code

↓

Error occurs?

No→End

Yes↓

Identify exception type

↓

Handle or show error

↓

End

The program runs code, checks if an error happens, identifies the type of error, then handles or shows it before ending.

Execution Sample

Python

try:
    x = int('abc')
except ValueError as e:
    print('ValueError:', e)

This code tries to convert a string to an integer and catches a ValueError if it fails.

Execution Table

Step	Action	Evaluation	Result
1	Try to convert 'abc' to int	int('abc')	Raises ValueError
2	Catch ValueError exception	except ValueError as e	Exception caught
3	Print error message	print('ValueError:', e)	Output: ValueError: invalid literal for int() with base 10: 'abc'
4	End of try-except block	No more code	Program continues or ends

💡 Exception caught and handled, program does not crash

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
x	undefined	Exception raised, no value assigned	undefined	undefined
e	undefined	undefined	ValueError instance	ValueError instance

Key Moments - 3 Insights

Why does the program not crash when int('abc') fails?

Because the ValueError exception is caught in the except block (see execution_table step 2), so the error is handled gracefully.

What happens if the exception type in except does not match the error raised?

The exception is not caught and the program crashes. In the example, if except caught TypeError instead of ValueError, the ValueError would not be handled.

Can multiple exception types be caught in one except block?

Yes, by listing them in parentheses like except (TypeError, ValueError): to handle multiple error types together.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of variable 'x' after step 1?

A An integer value

B Undefined because exception was raised

C A string 'abc'

D None

💡 Hint

Check execution_table row 1 and variable_tracker for 'x' after step 1

At which step does the program print the error message?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at execution_table row 3 for the print action

If the except block was changed to except TypeError, what would happen?

A The ValueError would still be caught

B The program would print a TypeError message

C The program would crash with ValueError

D The program would ignore the error and continue silently

💡 Hint

Refer to key_moments about matching exception types

Concept Snapshot

Common exceptions in Python include ValueError, TypeError, ZeroDivisionError, and KeyError.
Use try-except blocks to catch and handle exceptions.
The except block must match the exception type to catch it.
Catching exceptions prevents program crashes and allows graceful error handling.

Full Transcript

This visual execution shows how Python handles common exceptions. The program tries to convert a string 'abc' to an integer, which raises a ValueError. The try-except block catches this error, preventing the program from crashing. The variable 'x' never gets a value because the exception stops the assignment. The exception object 'e' holds the error details and is printed. If the except block did not match the error type, the program would crash. This example teaches how to catch and handle common exceptions safely.

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