Day 11: Python Programming

Exception Handling

What is an Exception?

An exception is an error that occurs during the execution of a program.

Examples of common exceptions:

• ZeroDivisionError
• ValueError
• TypeError
• IndexError
• FileNotFoundError
• KeyError

Why Exception Handling?

Without handling → program crashes.
With handling → program continues safely.

try–except Block (Basic Syntax)
try:
# Code that may cause error
except:
# Code to run when error happens

Example
try:
x = 10 / 0
except:
print("Error occurred!")

Handling Specific Exceptions
try:
a = int("hello")
except ValueError:
print("Invalid number")

Multiple except Blocks
try:
num = int(input("Enter number: "))
print(10 / num)
except ZeroDivisionError:
print("Cannot divide by zero")
except ValueError:
print("Please enter digits only")

else Block
Runs only when no exception occurs.
try:
print("No error")
except:
print("Error found")
else:
print("Else block executed")

finally Block
Runs always, even if an error happens.

Useful for:

• Closing files
• Closing database connection
• Releasing resources try: f = open("sample.txt", "r") print(f.read()) except: print("File not found") finally: print("Closing file")

try–except–else–finally (All Together)

python
try:
    n = int(input("Enter number: "))
except ValueError:
    print("Invalid Input")
else:
    print("Square:", n*n)
finally:
    print("End of program")

**Raise Your Own Exception**

Use `raise` to throw a custom error.
age = 15
if age < 18:
    raise Exception("You must be 18+")

**Custom Exception Class**
class LowBalanceError(Exception):
    pass
balance = 300
try:
    if balance < 500:
        raise LowBalanceError("Balance too low")
except LowBalanceError as e:
    print(e)

**Common Python Exceptions Table**

| Exception         | Meaning                 |
| ----------------- | ----------------------- |
| ZeroDivisionError | Division by zero        |
| ValueError        | Wrong value type        |
| TypeError         | Wrong data type         |
| IndexError        | List index out of range |
| KeyError          | Dictionary key missing  |
| FileNotFoundError | File does not exist     |
| ImportError       | Module not found        |


**Practical Mini Programs (Day 18 Practice)**

**1️⃣ Safe Division Program**
try:
    a = int(input("A: "))
    b = int(input("B: "))
    print(a / b)
except ZeroDivisionError:
    print("Cannot divide by zero")
except ValueError:
    print("Enter numbers only")

**2.File Reader with Exception Handling**
try:
    f = open("demo.txt", "r")
    print(f.read())
except FileNotFoundError:
    print("File doesn't exist")

**3.Convert String to Integer Safely**
def convert(s):
    try:
        return int(s)
    except:
        return "Conversion Failed"
print(convert("123"))
print(convert("abc"))

**4.Custom Exception **
**Example**
class InvalidAge(Exception):
    pass

try:
    age = int(input("Enter age: "))
    if age < 18:
        raise InvalidAge("Age is below 18")
    print("Eligible")
except InvalidAge as e:
    print(e)