Python - Finally Block

It is not recommended to maintain clean-up code (Resource Deallocating Code or Resource Releasing Code) inside the try block, because there is no guarantee that every statement inside the try block will always be executed.

Similarly, it is not recommended to maintain clean-up code inside the except block, because if there is no exception, then the except block will not be executed.

Hence, we require some place to maintain clean-up code that should be executed always, irrespective of:

• whether an exception is raised or not,
• whether the exception is handled or not handled.

Such a place is the finally block.

• Maintain clean-up code.
• Release resources.
• Close files.
• Close database connections.
• Release network resources.
• Execute important statements before program termination.

The speciality of the finally block is:

• It will be executed whether an exception is raised or not raised.
• It will be executed whether the exception is handled or not handled.

If no exception occurs inside the try block, the finally block is still executed.

If an exception occurs inside the try block and a matching except block is available, the exception is handled first, and after that the finally block executes.

If an exception occurs inside the try block but there is no matching except block, Python still executes the finally block before terminating the program.

• In Case 1, no exception occurs, so the except block is skipped and only try and finally run.
• In Case 2, 10/0 raises a ZeroDivisionError, the matching except block handles it, and then finally runs.
• In Case 3, 10/0 raises a ZeroDivisionError but only a NameError handler exists. The finally block still runs, and only after it does Python terminate the program abnormally.

Program Starts
      │
      ▼
Execute try Block
      │
      ▼
Exception ?
 ┌────┴────┐
 │         │
No         Yes
 │         │
 ▼         ▼
Skip or   Matching
Run       except?
 except      │
 │      ┌────┴────┐
 │      │         │
 │      ▼         ▼
 │   Handled   Not Handled
 │      │         │
 └──────┼─────────┘
        │
        ▼
Execute finally Block
        │
        ▼
Program Ends
(After finally, an unhandled
exception still terminates the
program abnormally)
      

1. The finally block is mainly used for clean-up code.
2. It executes even if no exception occurs.
3. It also executes after a handled exception.
4. It even executes before an unhandled exception terminates the program.
5. Resource releasing code should normally be written inside the finally block.

There is only one situation where the finally block will not be executed.

It is when we use:

os._exit(0)

Whenever os._exit(0) is executed, the Python Virtual Machine (PVM) itself is shut down, so the finally block will not execute.

Here, 0 represents the status code, which indicates normal termination.

Because the PVM is shut down directly, finally is not printed.

We can use the else block with try-except-finally blocks.

The else block will be executed if and only if there are no exceptions inside the try block.

• Execute code only when the try block completes successfully.
• Separate normal execution code from exception handling code.
• Improve readability of the program.
• Avoid writing normal statements inside the try block unnecessarily.

If we comment line-1 (print(10/0)), then there is no exception inside the try block:

try
else
finally

If we do not comment line-1 (print(10/0)), then an exception occurs inside the try block:

try
except
finally

1. Whenever we write a try block, we must write either an except block or a finally block.
2. An except block must always be associated with a try block.
3. A finally block must always be associated with a try block.
4. We can write multiple except blocks for one try block, but not multiple finally blocks.
5. Whenever we write an else block, an except block must also be present.
6. The order of the blocks is important: try → except → else → finally.
7. We can define these blocks inside try, except, else, or finally — nesting is always possible.