Try–except execution flow in Python - Step-by-Step Execution

Concept Flow - Try–except execution flow

Start try block

↓

Execute code in try

↓

Error?

No→Continue after try-except

Yes↓

Go to except block

↓

Handle error

↓

Continue after try-except

The program tries to run code inside the try block. If an error happens, it jumps to the except block to handle it, then continues.

Execution Sample

Python

try:
    x = 5 / y
except ZeroDivisionError:
    x = 0
print(x)

This code tries to divide 5 by y. If y is zero, it catches the error and sets x to 0, then prints x.

Execution Table

Step	Action	Evaluation	Result
1	Enter try block	Execute 'x = 5 / y'	Error: NameError (y not defined)
2	Error caught?	No, NameError not caught by except ZeroDivisionError	Program crashes (no except for NameError)
3	If y=0 and defined, execute 'x = 5 / 0'	ZeroDivisionError raised	Jump to except block
4	In except ZeroDivisionError block	Set x = 0	x is now 0
5	After try-except	Print x	Output: 0

💡 Execution stops if error is not caught; otherwise continues after except block.

Variable Tracker

Variable	Start	After Step 1	After Step 4	Final
x	undefined	error (not set)	0	0
y	undefined	undefined	undefined	undefined or 0 if defined

Key Moments - 3 Insights

Why does the program crash when y is not defined?

Because the except block only catches ZeroDivisionError, but the error is NameError, so it is not caught (see execution_table step 2).

What happens if y is zero?

A ZeroDivisionError occurs, which is caught by the except block, so x is set to 0 and the program continues (see execution_table steps 3 and 4).

Does the except block run if no error happens?

No, the except block only runs if an error occurs in the try block (see execution_table step 1 no error path).

Visual Quiz - 3 Questions

Look at the execution table, what error occurs at step 1 if y is not defined?

A NameError

B ZeroDivisionError

C SyntaxError

D No error

💡 Hint

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

At which step does the except block set x to 0?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Action' and 'Result' columns in step 4 of the execution_table.

If y is 2, what will be printed after the try-except?

A 0

B 2

C 2.5

D An error occurs

💡 Hint

If no error occurs, the try block runs fully; check what 'x' is assigned in the code sample.

Concept Snapshot

try:
  # code that might cause error
except ErrorType:
  # code to handle error

- Runs try block first
- If error matches except, runs except block
- Otherwise error stops program
- Continues after try-except if handled

Full Transcript

This visual shows how Python runs code inside a try block. If an error happens, it checks if the except block matches the error type. If yes, it runs the except block to handle it and continues. If no, the program stops with an error. For example, dividing by zero raises ZeroDivisionError, which can be caught. But if a variable is not defined, NameError occurs and is not caught by except ZeroDivisionError, so the program crashes. The variable x changes only if the except block runs. This helps keep programs running even when errors happen.