Exception Handling

Introduction to Exceptions

What is an Exception

An exception is an error that occurs during the execution of a program. When an exception occurs, the program stops unless it is handled properly.

Example

print(10 / 0)  # ZeroDivisionError

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Types of Errors

1. Syntax Errors (compile-time errors)

if True
    print("Hello")  # Missing colon

2. Runtime Errors (exceptions)

x = int("abc")  # ValueError

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Common Built-in Exceptions

• ZeroDivisionError
• ValueError
• TypeError
• IndexError
• KeyError

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Importance of Exception Handling

• Prevents program crashes
• Improves user experience
• Helps debugging
• Ensures smooth program flow

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• try, except Block

Structure

try:
    # code that may cause error
except:
    # code to handle error

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//Handling Single Exception

try:
    num = int(input("Enter number: "))
except ValueError:
    print("Invalid input")

//Handling Multiple Exceptions

try:
    a = int(input())
    b = int(input())
    print(a / b)
except ValueError:
    print("Invalid number")
except ZeroDivisionError:
    print("Cannot divide by zero")

//Generic Exception

try:
    x = int("abc")
except Exception as e:
    print("Error:", e)

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Best Practices

• Catch specific exceptions first
• Avoid using only generic except
• Keep try block small
• Provide meaningful error messages

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• else and finally Block

else Block

//Executes if no exception occurs.

try:
    num = int("10")
except ValueError:
    print("Error")
else:
    print("Success:", num)

//finally Block

//Always executes, whether an exception occurs or not.

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

Execution Flow

1. try block executes
2. If exception occurs → except block runs
3. If no exception → else block runs
4. finally block always runs

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Real-World Example

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

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• Custom Exceptions

Custom exceptions allow you to define your own error types.

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//Creating Custom Exception

class MyError(Exception):
    pass

//Raising an Exception

def check_age(age):
    if age < 18:
        raise ValueError("Age must be 18 or above")check_age(16)

//Using Custom Exception

class InvalidAgeError(Exception):
    passdef validate(age):
    if age < 18:
        raise InvalidAgeError("Not eligible")try:
    validate(15)
except InvalidAgeError as e:
    print(e)

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Best Practices

• Use meaningful names
• Extend from Exception class
• Use for specific application logic

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Conclusion

Exception handling is a vital part of Python programming that ensures programs run smoothly even when errors occur. By using try, except, else, and finally blocks, developers can manage errors effectively and improve program reliability.

Custom exceptions further enhance flexibility by allowing developers to define their own error conditions. Mastering exception handling helps in building robust, user-friendly, and professional applications.

Understanding and applying these concepts prepares learners for real-world programming, where handling unexpected situations is essential for success.