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Python - Exception Handling

What is Exception Handling?

Exception handling in Python is a mechanism that allows you to manage runtime errors gracefully using try, except, finally, and raise. It helps prevent programs from crashing abruptly and allows recovery from unexpected conditions.

Why Use Exception Handling?

  • To prevent application crashes due to runtime errors
  • To handle user input errors, file errors, network errors, etc.
  • To ensure resources (like files or connections) are released properly

Basic try-except Block

python 
try:
    x = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero!")

Handling Multiple Exceptions

python 
try:
    num = int("abc")
except ValueError:
    print("Invalid number format.")
except TypeError:
    print("Type error occurred.")

Using finally Block

The finally block is always executed, whether or not an exception occurs.

python 
try:
    f = open("sample.txt", "r")
    content = f.read()
except FileNotFoundError:
    print("File not found.")
finally:
    print("Executing finally block.")
    f.close()

Using the raise Keyword

You can raise exceptions manually using raise.

python 
age = -1
if age < 0:
    raise ValueError("Age cannot be negative")

Custom Exceptions

You can create your own exception classes by inheriting from the built-in Exception class.

python 
class NegativeNumberError(Exception):
    pass

def check_number(n):
    if n < 0:
        raise NegativeNumberError("Negative numbers are not allowed")
    return n

try:
    check_number(-5)
except NegativeNumberError as e:
    print("Caught custom exception:", e)

Best Practices

  • Catch specific exceptions instead of using a generic except
  • Use finally to release resources
  • Avoid using exceptions for control flow logic
  • Document custom exceptions clearly

Conclusion

Exception handling is vital for building robust and user-friendly Python applications. Mastering try-except-finally and custom exceptions ensures your code handles unexpected issues gracefully.