Python Programming Exception Handling

Robust Programming Error Management

Python Exception Handling Complete Guide

Learn Python exception handling - try-except blocks, finally clause, custom exceptions with practical examples, patterns, and real-world applications for robust error management.

try-except

Catch and handle errors

Custom Exceptions

Create your own error types

finally Clause

Cleanup operations

raise Statement

Throw exceptions

What is an Exception?

An exception is an error that occurs during program execution.

Python exceptions overview: runtime errors such as ZeroDivisionError and TypeError, exception objects, and why programs need try except handling — **Exceptions:** When something goes wrong at runtime, Python raises an exception—your code can catch it or let it stop the program.

Example

print(10 / 0)

Output: ZeroDivisionError: division by zero

Without exception handling, the program crashes.

Python exception handling flowchart: try block runs guarded code, except catches matching errors, optional else runs if no error, finally always runs — **Control flow:** Typical path through `try` → success or `except` → optional `else` / `finally` for cleanup.

Why Use Exception Handling?

Prevent program crashes
Display user-friendly error messages
Handle unexpected situations
Improve program reliability

Basic Syntax

try-except syntax

try:
    # risky code
except:
    # code to handle error

Basic example

try:
    num = 10 / 0
except:
    print("Error occurred")

Output: Error occurred

Flow of Exception Handling

1. try and except

The try block contains code that may produce an error. The except block handles the error.

try/except example

try:
    a = int(input("Enter number: "))
    print(100 / a)
except:
    print("Invalid input")

Handling Specific Exceptions

Specific exception blocks

try:
    num = int(input("Enter number: "))
    result = 10 / num
except ZeroDivisionError:
    print("Cannot divide by zero")
except ValueError:
    print("Please enter valid integer")

Common Python Exceptions

Exception	Description
ZeroDivisionError	Division by zero
ValueError	Invalid value
TypeError	Wrong data type
IndexError	Invalid list index
KeyError	Invalid dictionary key
NameError	Variable not defined
FileNotFoundError	File does not exist

Multiple Exceptions in One Block

Tuple except

try:
    x = int(input())
    print(10 / x)
except (ValueError, ZeroDivisionError):
    print("Error occurred")

Using else Block

else example

try:
    num = int(input("Enter number: "))
    print(10 / num)
except ZeroDivisionError:
    print("Cannot divide by zero")
else:
    print("Program executed successfully")

Using finally Block

finally example

try:
    file = open("data.txt")
except FileNotFoundError:
    print("File not found")
finally:
    print("Execution completed")

try-except-else-finally Structure

Full structure

try:
    num = int(input())
except ValueError:
    print("Invalid input")
else:
    print("No exception")
finally:
    print("Always executed")

Raising and Custom Exceptions

Raising Exceptions

raise example

age = -5
if age < 0:
    raise ValueError("Age cannot be negative")

Custom Exceptions

Custom exception class

class InvalidAgeError(Exception):
    pass

age = -1
if age < 0:
    raise InvalidAgeError("Invalid age")

Using Exception Object

as e example

try:
    print(10 / 0)
except ZeroDivisionError as e:
    print("Error:", e)

Output: Error: division by zero

Nested try-except

Nested block example

try:
    try:
        print(10 / 0)
    except ZeroDivisionError:
        print("Inner exception handled")
except:
    print("Outer exception handled")

File Handling with Exceptions

File exception example

try:
    file = open("sample.txt", "r")
    print(file.read())
except FileNotFoundError:
    print("File does not exist")
finally:
    print("File operation completed")

Best Practices

Catch specific exceptions instead of generic except
Use finally for cleanup tasks
Avoid hiding errors unnecessarily
Write meaningful error messages
Use custom exceptions for large projects

Real-Life Examples

ATM Withdrawal

balance = 5000
try:
    amount = int(input("Enter withdrawal amount: "))
    if amount > balance:
        raise ValueError("Insufficient balance")
    print("Withdrawal successful")
except ValueError as e:
    print(e)

List Index Example

try:
    data = [10, 20, 30]
    print(data[5])
except IndexError:
    print("Index out of range")

Interview Questions

1. Difference between Error and Exception?
Errors are serious issues. Exceptions can be handled.

2. Difference between except and finally?
except runs only when exception occurs. finally always runs.

3. Why use custom exceptions?
For better readability and project-specific error handling.

4. Can finally run without except?
Yes.

finally without except

try:
    print("Hello")
finally:
    print("Done")

Mini Practice Programs

Handle divide by zero
Handle invalid user input
Handle file not found
Create custom exception
Handle list index error

            
                Summary
            
            try -> risky code
except -> handles errors
else -> runs if no error
finally -> always runs
raise -> manually create exceptions
Custom exceptions improve program structure

        

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