Python Programming for Lazy Beginners: A Simple and Easy Tutorial

CHAPTER 13

In programming, try/except blocks are like tightrope walkers. They help you stay in control, even when things get wobbly. When you write code, you're confident that everything will go according to plan, but sometimes unexpected events occur that can make your program wobble, or worse, crash.

That's where try/except blocks come in. With try/except, you can tell Python to attempt to run a certain block of code, and if an exception occurs, instead of crashing, Python will jump to an except block and execute the code there. This way, you can gracefully handle exceptions and keep your program running, even when things don't go according to plan.

Here's an example. Let's say you want to read a file that contains user data, but the file doesn't exist. Normally, your program would crash with a FileNotFoundError, but with try/except, you can handle the exception gracefully and keep moving forward. Here's how:

try: with open('users.txt') as f: data = f.read() except FileNotFoundError: print('The file users.txt does not exist')

In this code, the try block attempts to read the file 'users.txt', but if the file doesn't exist, Python raises a FileNotFoundError, which is caught by the except block. The except block then prints a friendly message to the user, and the program keeps running.

Another example is when you're trying to divide a number by zero. This will raise a ZeroDivisionError. You can use try/except blocks to catch this error and handle it gracefully, like this:

try:
result = 10 / 0
except ZeroDivisionError:
print('You cannot divide by zero')

In this example, the try block attempts to divide 10 by zero, which would raise a ZeroDivisionError. The except block catches the error and prints a message to the user, telling them they cannot divide by zero.

So, with try/except blocks in Python, you can handle exceptions gracefully and keep your program running, just like a tightrope walker can stay in control, even when the wind starts to blow.

Raising and handling custom exceptions

Imagine you're a wizard, casting spells and brewing potions in your magical laboratory. You have a set of rules and procedures that you follow to ensure your spells and potions are safe and effective. But sometimes, you encounter situations that aren't covered by your rules. For example, what happens if a potion gets contaminated by a rare plant that you've never encountered before? Your rules don't cover this situation, so you have to make a judgment call.

In programming, raising and handling custom exceptions is like being a wizard in your magical laboratory. You have a set of rules and procedures that your code follows, but sometimes, you encounter situations that aren't covered by your rules. This is where custom exceptions come in handy. With custom exceptions, you can define your own rules and procedures to handle unexpected situations in your code.

Here's an example. Let's say you're writing a program that calculates the average age of a group of people. Your program requires a list of ages as input, but sometimes, users might enter invalid data, like strings or negative numbers. You could raise a custom exception to handle these situations.

Here's how you can define a custom exception in Python:

class InvalidAgeException(Exception): pass

In this code, we've defined a custom exception called InvalidAgeException. This exception extends the built-in Exception class in Python.