Try–except–finally behavior in Python - Step-by-Step Execution

Concept Flow - Try–except–finally behavior

Start try block

↓

Execute try code

↓

Exception?

No→Execute finally block

↓

Go to except block

↓

Execute except block

↓

Execute finally block

↓

End

The program tries the code in try. If no error, it runs finally then ends. If error, it runs except, then finally, then ends.

Execution Sample

Python

try:
    x = 1 / y
except ZeroDivisionError:
    x = 0
finally:
    print(x)

This code tries to divide 1 by y, handles division errors by setting x to 0, and always prints x at the end.

Execution Table

Step	Action	Evaluation	Result
1	Enter try block	Execute 'x = 1 / y'	Error: NameError (y not defined)
2	Exception caught?	No ZeroDivisionError, but NameError raised	No except block matches, skip except
3	Execute finally block	Print x	Error: x not assigned, NameError raised again
4	Program ends	Unhandled NameError	Program crashes with NameError

💡 No matching except block for NameError, finally runs, then program crashes

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
x	undefined	undefined (error on assignment)	undefined	undefined	undefined (program crashes)
y	undefined	undefined	undefined	undefined	undefined

Key Moments - 3 Insights

Why does the program crash even though there is an except block?

The except block only catches ZeroDivisionError, but the error is NameError because y is not defined. See execution_table step 2.

Does the finally block run if an exception is not caught?

Yes, finally always runs no matter what. See execution_table step 3 where finally runs even though exception is unhandled.

What happens if the except block matches the error?

If except matches, it runs the except code, then finally runs. The program does not crash. This is not shown here but is the normal flow.

Visual Quiz - 3 Questions

Look at the execution table, what error occurs at step 1?

A No error occurs

B ZeroDivisionError because of division by zero

C NameError because y is not defined

D SyntaxError due to wrong code

💡 Hint

Check the 'Evaluation' column in step 1 of the execution_table

At which step does the finally block execute?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look for 'Execute finally block' in the Action column of execution_table

If the except block caught NameError instead, what would change in the execution?

A The except block would run, then finally, and program ends normally

B The program would crash after finally

C The finally block would not run

D The try block would run again

💡 Hint

Refer to key_moments explanation about except block matching the error

Concept Snapshot

try:
  code that might fail
except ErrorType:
  handle error
finally:
  code that always runs

- try runs first
- except runs if matching error
- finally runs always
- program ends after finally

Full Transcript

This example shows how Python runs try-except-finally. The try block runs code that may cause an error. If an error happens and matches the except type, except runs. Finally always runs no matter what. Here, a NameError happens because y is not defined. The except block only catches ZeroDivisionError, so it does not run. Finally runs and tries to print x, but x was never set, so the program crashes with NameError. This shows that finally runs even if except does not catch the error. If except matched the error, it would run, then finally, and the program would end normally.