The Full Handler: try-except-else-finally

15 min · Beginner

Python offers a comprehensive 4-part exception handling structure. The else block runs when no exception occurs, and the finally block is guaranteed to run no matter what.

The else block: running on success

Else clause is a block after the except blocks that runs only when the try block finished without an error.

The else block executes only if the try block completed with zero exceptions.

Python

1try:2    num = int("42")3except ValueError:4    print("Conversion error")5else:6    print(f"Success! Parsed number: {num}")

Example 1 · Using else after a good conversion

Python

1text = "250"2
3try:4    amount = int(text)5except ValueError:6    print("Invalid amount")7else:8    print(f"Amount accepted: {amount}")

Output

Amount accepted: 250

What this code does

The try holds only the risky line, int(text).
It succeeds, so except is skipped and else runs.

Keeping code that depends on the try block succeeding inside else avoids accidentally catching exceptions from subsequent statements.

The finally block: guaranteed cleanup

Finally clause is a block that runs every time, whether the try block succeeded, failed or returned. It is used for clean-up.

The finally block always executes, regardless of whether an exception occurred, was caught, or was re-raised.

Python

1try:2    print("Opening connection")3    res = 10 / 24except ZeroDivisionError:5    print("Math error")6finally:7    print("Closing connection")

Example 2 · finally runs even with a return

Python

1def read_stock(stock, item):2    try:3        return stock[item]4    except KeyError:5        return 06    finally:7        print(f"Checked {item}")8
9stock = {"pen": 40}10print(read_stock(stock, "pen"))11print(read_stock(stock, "bag"))

Output

Checked pen
40
Checked bag
0

What this code does

return inside try or except waits until finally has run.
Checked ... prints for both calls, found or not.
The second call returns 0 from except after the finally block.

Even if the try block hit a return or raised an error, finally executes before control leaves the scope.

The complete execution flow

Try statement is the full structure of try, except, else and finally. Each part has one job: run, recover, continue or clean up.

The 4 parts work together with a clear lifecycle:

Which blocks run in a try statement. With no error: try runs fully, except is skipped, else runs, finally runs. With an error that except catches: try stops at the error, except runs, else is skipped, finally runs.

Example 3 · A bank transfer with all four parts

Python

1def transfer_funds(amount):2    print("Begin transaction")3    try:4        if amount <= 0:5            raise ValueError("Amount must be positive")6        balance = 500 - amount7    except ValueError as err:8        print(f"Transaction failed: {err}")9    else:10        print(f"Transfer successful. New balance: {balance}")11    finally:12        print("Transaction session closed")13
14transfer_funds(150)

Output

Begin transaction
Transfer successful. New balance: 350
Transaction session closed

What this code does

Runs try block with valid positive amount (150).
Executes else block because no exception occurred.
Executes finally block unconditionally to close session.

try: runs the guarded code. except: runs only if a matching exception occurs in try.

else: runs only if try completed without any exceptions. finally: runs always, unconditionally, before exiting.

Next step

Check your understanding

8 quick questions

1def read_stock(stock, item):2 try:3 return stock[item]4 except KeyError:5 return 06 finally:7 print(f"Checked {item}")8 9stock = {"pen": 40}10print(read_stock(stock, "pen"))11print(read_stock(stock, "bag"))

1def transfer_funds(amount):2 print("Begin transaction")3 try:4 if amount <= 0:5 raise ValueError("Amount must be positive")6 balance = 500 - amount7 except ValueError as err:8 print(f"Transaction failed: {err}")9 else:10 print(f"Transfer successful. New balance: {balance}")11 finally:12 print("Transaction session closed")13 14transfer_funds(150)