Error Handling

Exceptions

Exceptions are errors detected during execution. Common built-in exceptions:

Exception	Meaning
SyntaxError	Invalid Python syntax
NameError	Variable not defined
TypeError	Operation on wrong type
ValueError	Value is inappropriate
IndexError	List index out of range
KeyError	Dictionary key not found
FileNotFoundError	File doesn’t exist
ZeroDivisionError	Division by zero
AttributeError	Attribute doesn’t exist on object

try / except

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

Catching Multiple Exceptions

try:
    value = risky_operation()
except (ValueError, TypeError, RuntimeError) as e:
    print(f"Caught: {e}")

try / except / else / finally

try:
    file = open("data.txt", "r")
    content = file.read()
except FileNotFoundError:
    print("File not found.")
else:
    print(f"Read {len(content)} characters.")
    # else runs only if no exception occurred
finally:
    file.close()
    # finally always runs — even if return/break occurs

Raising Exceptions

def withdraw(balance, amount):
    if amount < 0:
        raise ValueError("Amount cannot be negative")
    if amount > balance:
        raise ValueError("Insufficient funds")
    return balance - amount

Creating Custom Exceptions

class InsufficientFundsError(Exception):
    """Raised when account balance is too low."""
    def __init__(self, balance, amount):
        self.balance = balance
        self.amount = amount
        super().__init__(f"Insufficient funds: ${balance} < ${amount}")

def withdraw(balance, amount):
    if amount > balance:
        raise InsufficientFundsError(balance, amount)
    return balance - amount

The Assert Statement

def divide(a, b):
    assert b != 0, "Division by zero!"
    return a / b

Asserts are for debugging. They can be disabled with -O flag.

Debugging Techniques

Print Debugging

print(f"DEBUG: x={x}, y={y}")  # quick and effective

Using pdb

import pdb

def buggy_function():
    x = 10
    pdb.set_trace()   # execution pauses here — interactive debugger
    y = x / 0

Common Debugger Commands

Command	Action
n	Next line
s	Step into function
c	Continue until next breakpoint
p var	Print variable value
l	Show source code context
q	Quit debugger

---

Next: Chapter 10 — Working with Libraries & Next Steps