Python try, except, else, finally Blocks

Introduction

Exception handling in Python is a crucial mechanism that ensures the smooth functioning of programs by managing errors and exceptions that may occur during execution. Python provides the try, except, else, and finally blocks to handle exceptions and execute specific code based on whether an exception occurs.

Key Concepts

• try Block: The code that may raise an exception is placed inside the try block.
• except Block: The code that runs if an exception occurs in the try block.
• else Block: The code that runs if no exception occurs in the try block.
• finally Block: The code that always runs, regardless of whether an exception occurs or not.

Basic Syntax

try:
    # Code that may raise an exception
    risky_code()
except SomeException:
    # Code that runs if the specified exception occurs
    handle_exception()
else:
    # Code that runs if no exception occurs
    run_if_no_exception()
finally:
    # Code that always runs
    cleanup_code()

Examples

Basic Exception Handling with try and except

try:
    result = 10 / 0
except ZeroDivisionError:
    print("You can't divide by zero!")

Output

You can't divide by zero!

Multiple except Blocks

try:
    value = int("abc")
except ValueError:
    print("Invalid literal for int() with base 10.")
except TypeError:
    print("Type error occurred.")

Output

Invalid literal for int() with base 10.

Using else Block

The else block runs if no exception occurs in the try block.

try:
    result = 10 / 2
except ZeroDivisionError:
    print("You can't divide by zero!")
else:
    print("Division successful!")

Output

Division successful!

Using finally Block

The finally block runs regardless of whether an exception occurs or not.

try:
    result = 10 / 0
except ZeroDivisionError:
    print("You can't divide by zero!")
finally:
    print("Execution completed.")

Output

You can't divide by zero!
Execution completed.

Combining try, except, else, and finally

You can combine all four blocks to handle exceptions comprehensively.

try:
    result = 10 / 2
except ZeroDivisionError:
    print("You can't divide by zero!")
else:
    print("Division successful!")
finally:
    print("Execution completed.")

Output

Division successful!
Execution completed.

Raising Exceptions

You can raise exceptions explicitly using the raise keyword.

def divide(a, b):
    if b == 0:
        raise ValueError("The divisor cannot be zero.")
    return a / b

try:
    result = divide(10, 0)
except ValueError as e:
    print(e)
finally:
    print("Execution completed.")

Output

The divisor cannot be zero.
Execution completed.

Real-World Example: File Handling

Handling file operations often involves exceptions, such as when trying to open a file that doesn’t exist.

try:
    file = open("example.txt", "r")
    content = file.read()
except FileNotFoundError:
    print("File not found.")
else:
    print("File content read successfully.")
    print(content)
finally:
    if 'file' in locals():
        file.close()
    print("File handling completed.")

Output (if the file exists)

File content read successfully.
... (file content) ...
File handling completed.

Output (if the file does not exist)

File not found.
File handling completed.

Conclusion

The try, except, else, and finally blocks in Python provide a robust framework for handling exceptions and ensuring the smooth execution of programs. By understanding and utilizing these blocks, you can write more resilient and error-resistant code, effectively managing potential errors and ensuring proper resource management.