MODULE 8: Exception Handling & Debugging

Exception handling helps a program handle runtime errors gracefully instead of crashing. Debugging helps identify and fix issues efficiently.

1. Errors vs Exceptions

Brief

• Errors → Problems that stop the program from running
• Exceptions → Runtime issues that can be handled using code

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Errors (Cannot be Handled Easily)

Occur due to syntax or logical mistakes.

if True
    print("Error")   # SyntaxError

---

Exceptions (Can Be Handled)

Occur during execution.

print(10 / 0)   # ZeroDivisionError

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Key Difference

Errors	Exceptions
Compile-time / logical	Runtime
Program crashes	Can be handled
SyntaxError	ZeroDivisionError

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2. try, except, else, finally

Brief

Used to catch and handle exceptions.

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Syntax

try:
    risky_code
except ExceptionType:
    handling_code
else:
    executes_if_no_exception
finally:
    always_executes

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Basic Example

try:
    num = int(input("Enter number: "))
    print(10 / num)
except ZeroDivisionError:
    print("Cannot divide by zero")
except ValueError:
    print("Invalid input")

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else Block

Runs only when no exception occurs.

try:
    print("Hello")
except:
    print("Error")
else:
    print("No error occurred")

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finally Block

Always executes (used for cleanup).

try:
    file = open("data.txt", "r")
except FileNotFoundError:
    print("File not found")
finally:
    print("Closing resources")

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3. Multiple Exceptions

Brief

Handling multiple exception types separately or together.

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Multiple except Blocks

try:
    a = int(input("Enter number: "))
    b = int(input("Enter number: "))
    print(a / b)
except ZeroDivisionError:
    print("Division by zero")
except ValueError:
    print("Invalid input")

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Single except for Multiple Exceptions

try:
    print(int("abc") / 2)
except (ValueError, ZeroDivisionError):
    print("Error occurred")

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4. Custom Exceptions

Brief

User-defined exceptions are created for business logic validation.

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Creating Custom Exception

class AgeError(Exception):
    pass

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Raising Custom Exception

def check_age(age):
    if age < 18:
        raise AgeError("Age must be 18 or above")
    print("Access granted")

try:
    check_age(16)
except AgeError as e:
    print(e)

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Real-World Use Cases

• Banking systems
• Login validation
• E-commerce checkout rules

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5. Logging

Brief

Logging records program execution details for debugging and monitoring.

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Why Logging Over print()?

• Stores logs permanently
• Different severity levels
• Used in production systems

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Logging Levels

• DEBUG
• INFO
• WARNING
• ERROR
• CRITICAL

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Basic Logging Example

import logging

logging.basicConfig(level=logging.INFO)
logging.info("Program started")
logging.error("An error occurred")

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Logging to File

logging.basicConfig(
    filename="app.log",
    level=logging.DEBUG,
    format="%(asctime)s - %(levelname)s - %(message)s"
)

logging.debug("Debugging details")

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6. Debugging Techniques

1. Using print()

print("Value of x:", x)

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2. Using debugger (pdb)

import pdb
pdb.set_trace()

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3. IDE Debugger

• VS Code / PyCharm
• Breakpoints
• Step into / step over

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4. Reading Traceback

• Last line shows the actual error
• Follow call stack top to bottom

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7. Common Python Errors

SyntaxError

if True
    print("Error")

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IndentationError

if True:
print("Error")

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TypeError

"10" + 5

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ValueError

int("abc")

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NameError

print(x)

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IndexError

lst = [1, 2]
print(lst[5])

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KeyError

d = {"a": 1}
print(d["b"])

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Real-World Example: ATM Program

try:
    balance = 5000
    amount = int(input("Enter amount: "))
    if amount > balance:
        raise Exception("Insufficient balance")
    print("Withdrawal successful")
except Exception as e:
    print("Error:", e)
finally:
    print("Thank you for using ATM")