Assert statement usage in Python - Step-by-Step Execution

Concept Flow - Assert statement usage

Start Execution

↓

Evaluate Assert Condition

↓

Continue

↓

End or Next Statement

The program checks the assert condition; if true, it continues, if false, it stops with an error.

Execution Sample

Python

x = 5
assert x > 0
print('x is positive')

Checks if x is positive; if yes, prints a message; if not, stops with error.

Execution Table

Step	Action	Condition Evaluated	Result	Next Step
1	Assign x = 5	N/A	x=5	Evaluate assert
2	Evaluate assert x > 0	5 > 0	True	Print message
3	Print message	N/A	'x is positive'	End

💡 Assert condition true, program continues normally.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
x	undefined	5	5	5

Key Moments - 3 Insights

What happens if the assert condition is false?

The program stops immediately and raises an AssertionError, as shown if the condition in step 2 was false.

Does the assert statement run if Python is started with optimization (-O)?

No, assert statements are skipped in optimized mode, so no error or check happens.

Can assert have a message to explain the error?

Yes, you can add a message after the condition, like assert x > 0, 'x must be positive'.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of x after step 1?

A 0

B undefined

C 5

D Error

💡 Hint

Check the 'After Step 1' column in variable_tracker for x.

At which step does the assert condition get evaluated?

A Step 1

B Step 2

C Step 3

D No evaluation

💡 Hint

Look at the 'Condition Evaluated' column in execution_table.

If x was -1, what would happen at step 2?

A AssertionError is raised and program stops

B Assert passes and continues

C Prints 'x is positive'

D x changes to 0

💡 Hint

Recall that assert stops execution if condition is false (see key_moments).

Concept Snapshot

assert condition
Checks if condition is True.
If False, raises AssertionError and stops.
Used for debugging and validating assumptions.
Can include optional error message.
Skipped if Python runs with -O option.

Full Transcript

The assert statement checks a condition during program execution. If the condition is true, the program continues normally. If false, it raises an AssertionError and stops the program. For example, assert x > 0 checks if x is positive. If x is 5, the assert passes and the program prints 'x is positive'. If x was negative, the assert would stop the program with an error. Assert statements help catch bugs early by verifying assumptions. They can include a message to explain the error. Note that assert statements are ignored if Python runs with optimization (-O).