Section 1.1: Understanding Errors and Exceptions

Before we delve into handling exceptions, it’s crucial to understand their definitions and distinctions.

An error typically refers to a flaw in syntax or logic that prevents a program from executing correctly. For instance, forgetting to close a parenthesis or attempting to divide by zero will trigger an error. The Python interpreter identifies these errors before or during execution, causing the program to halt.

Conversely, an exception occurs during execution and interrupts the normal flow of the program. For example, trying to open a nonexistent file or accessing an out-of-range element in a list generates an exception. These exceptions can be caught and handled using try, except, and finally blocks, which we will discuss shortly.

Errors and exceptions in Python can be classified into various types, including:

• SyntaxError: Raised for syntax issues, such as missing colons.
• ZeroDivisionError: Triggered when dividing by zero.
• NameError: Occurs when referring to an undefined variable.
• TypeError: Raised when an operation is performed on an incompatible type.
• ValueError: Occurs when a function receives an inappropriate value.
• IndexError: Raised for accessing an out-of-bounds index in a sequence.
• KeyError: Triggered when a non-existent key is accessed in a dictionary.
• FileNotFoundError: Raised when attempting to open a file that doesn't exist or lacks permission.

For an extensive overview of built-in errors and exceptions, refer to the official Python documentation.

With this foundational understanding, let’s explore how to explicitly raise exceptions using the raise keyword in the next section.

Section 1.2: Raising Exceptions with the Raise Keyword

In certain scenarios, you may need to intentionally raise an exception in your code. This could be to validate input, enforce specific conditions, or signal an error to another part of your application. The raise keyword in Python allows you to create and raise an exception object.

You can specify the type and message of the exception or utilize an existing exception object. For example:

# Raise a ValueError with a custom message

raise ValueError("Invalid input")

When you raise an exception, the normal flow of execution halts, and Python searches for a handler for the raised exception. If no handler is found, the program will terminate and display the exception message.

Consider the following example where a user is prompted for a positive number:

num = int(input("Enter a positive number: "))

if num < 0:

raise ValueError("The number must be positive")

If the user inputs a negative number, a ValueError will be raised. To handle this exception and prevent a crash, we will use try and except blocks in the next section.

Video Description: Python: Exception Handling with Try Except Finally - This video provides a detailed overview of how to handle exceptions in Python, covering the use of try, except, and finally blocks.

Section 1.3: Handling Exceptions with Try and Except

Now that you understand how to raise exceptions, let’s look at how to handle them using try and except blocks.

The syntax for using these blocks is as follows:

try:

# Code that may raise an exception

except ExceptionType as e:

# Handle the exception

The try block encloses the code that might cause an exception, while the except block contains the code for handling that exception. The ExceptionType specifies which type of exception to catch, and the variable e holds the exception object.

When executing the try block, one of the following outcomes occurs:

• If no exception occurs, the try block completes, and the except block is skipped.
• If an exception matching the specified ExceptionType occurs, the execution of the try block stops and the except block runs.
• If an exception occurs that does not match the specified type, it propagates to the outer scope or the interpreter.

For example, here’s a function that calculates the reciprocal of a number, with handling for ZeroDivisionError:

def reciprocal(num):

try:

result = 1 / num

print(f"The reciprocal of {num} is {result}")

except ZeroDivisionError as e:

print(f"Cannot calculate the reciprocal of {num}")

print(f"Exception: {e}")

When calling this function with a non-zero number, it will return the reciprocal. However, if you call it with zero, the ZeroDivisionError will be managed by the except block.

If an invalid input, such as a string, is passed, a different exception may occur, demonstrating the importance of robust exception handling.

Next, we will learn how to use else and finally blocks alongside try and except.

Section 1.4: Utilizing Else and Finally Blocks

Having explored exception handling with try and except, let’s examine how to incorporate else and finally blocks to execute code based on whether an exception occurs.

The optional else and finally blocks can be added after try and except like this:

try:

# Code that may raise an exception

except ExceptionType as e:

# Handle the exception

else:

# Executes if no exception occurs

finally:

# Executes regardless of whether an exception occurs

The else block contains code that runs only if the try block is successful, while the finally block executes regardless of the outcome. This is particularly useful for cleanup actions.

Consider a function that opens a file, reads its content, and handles the FileNotFoundError:

def read_file(filename):

try:

file = open(filename, "r")

content = file.read()

print(content)

except FileNotFoundError as e:

print(f"Cannot open the file {filename}")

print(f"Exception: {e}")

else:

print(f"Successfully read the file {filename}")

finally:

if file:

file.close()

print(f"Closed the file {filename}")

With this setup, the program properly handles file reading and ensures the file closes whether or not an error occurred.

Now, let's look at how to create custom exceptions using the Exception class.

Section 1.5: Creating Custom Exceptions

While we've covered built-in exceptions, sometimes you may need to define your own custom exceptions for specific scenarios. For example, you might want to handle cases like invalid passwords or insufficient funds.

To create a custom exception, inherit from the Exception class:

class CustomException(Exception):

def __init__(self, message):

super().__init__(message)

self.custom_attribute = "some value"

Now, you can raise and handle this custom exception in your code. For instance, here’s a function that checks user passwords:

class InvalidPasswordException(Exception):

def __init__(self, message):

super().__init__(message)

def check_password(user, password):

correct_password = "secret"

if password == correct_password:

return True

else:

raise InvalidPasswordException("The password is incorrect")

When invoking this function with the correct password, it returns True. However, if the wrong password is provided, it raises the custom exception, which can be handled similarly to built-in exceptions.

Finally, let’s summarize the key points from this tutorial.