Understanding Python's finally statement

Python’s finally statement is a control flow tool that should be added to your toolbelt.

This statement is normally combined with a try/except block to execute code regardless if an exception is thrown or not.

Using finally in a try/except block

Normally, your try/except statements may look something like this:

try:
    file = open('example.txt', 'r')
    data = file.readlines()
except FileNotFoundError:
    print("File not found!")

In the snippet above, if the file doesn’t exist we’ll throw an error. However, we may want to do some additional steps.

When we combine the above snippet with a finally statement, we can perform additional steps, such as closing the file:

try:
    file = open('example.txt', 'r')
    data = file.readlines()
except FileNotFoundError:
    print("File not found!")
finally:
    file.close()
    print('File has been closed.')

When a finally statement is paired with a try/except statement, the finally statement will always execute whether an exception is thrown or not.

Similarly, we don’t need an except statement (but it’s good practice to have this for logging/debugging purposes):

try:
    file = open('example.txt', 'r')
    data = file.readlines()
finally:
    file.close()
    print('File has been closed.')

Use cases for the finally statement

There are many reasons why you may want to leverage this. Various real-world applications may include:

1. Resource Cleanup (e.g., File Handling): Ensuring that resources like files, network connections, or database connections are properly closed or released, even if an error occurs during their use.

2. Releasing Locks or Synchronization Primitives: Ensuring that locks, semaphores, or other synchronization primitives are released, preventing deadlocks and ensuring that other threads or processes can continue to execute.

3. Cleaning Up After Loop Execution: Executing cleanup code after a loop, whether it completes normally, is interrupted by a break statement, or exits due to an exception.

4. Ensuring Completion of Critical Steps: Guaranteeing that certain critical steps, like committing a transaction or writing final data, are completed regardless of what happens during the operation.

5. Logging and Monitoring: Ensuring that logs or monitoring actions are taken, such as recording the end of a process or capturing error information, regardless of whether an error occurred.