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).

Practice

1. What does the assert statement do in Python?

easy

A. Prints a message when a condition is false

B. Runs a loop until a condition is true

C. Defines a function to check conditions

D. Checks if a condition is true and stops the program if false

Solution

1. Step 1: Understand the purpose of assert

   The assert statement tests a condition and raises an error if the condition is false.

2. Step 2: Compare options with assert behavior

   Only Checks if a condition is true and stops the program if false correctly describes assert's behavior of stopping the program when the condition is false.

3. Final Answer:

   Checks if a condition is true and stops the program if false -> Option D

4. Quick Check:

   Assert checks condition and stops if false [OK]

Hint: Assert stops program if condition is false [OK]

Common Mistakes:

• Thinking assert runs loops
• Confusing assert with print
• Believing assert defines functions

2. Which of the following is the correct syntax for an assert statement with a message?

easy

A. assert x > 0, "x must be positive"

B. assert (x > 0) "x must be positive"

C. assert x > 0; "x must be positive"

D. assert x > 0: "x must be positive"

Solution

1. Step 1: Recall assert syntax

   The correct syntax is assert condition, message with a comma separating condition and message.

2. Step 2: Check each option

   Only assert x > 0, "x must be positive" uses a comma correctly between condition and message.

3. Final Answer:

   assert x > 0, "x must be positive" -> Option A

4. Quick Check:

   Assert syntax uses comma before message [OK]

Hint: Use comma between condition and message in assert [OK]

Common Mistakes:

• Using colon or semicolon instead of comma
• Missing comma before message
• Putting message without quotes

3. What will be the output of this code?

def check_age(age):
    assert age >= 18, "Age must be at least 18"
    return "Access granted"

print(check_age(20))
print(check_age(16))

medium

A. Access granted\nAccess granted

B. Access granted\nAssertionError: Age must be at least 18

C. AssertionError: Age must be at least 18\nAccess granted

D. No output, program crashes immediately

Solution

1. Step 1: Analyze first function call

   check_age(20) passes the assert since 20 >= 18, so it returns "Access granted" and prints it.

2. Step 2: Analyze second function call

   check_age(16) fails the assert because 16 < 18, so it raises AssertionError with the message.

3. Final Answer:

   Access granted\nAssertionError: Age must be at least 18 -> Option B

4. Quick Check:

   Assert stops program on false condition with error [OK]

Hint: Assert stops at first false condition with error [OK]

Common Mistakes:

• Thinking both print statements run
• Ignoring the error message
• Assuming assert prints message without error

4. Find the error in this code snippet:

assert x > 10 "x should be greater than 10"

medium

A. Missing comma between condition and message

B. Missing parentheses around condition

C. Message should be a variable, not a string

D. Assert cannot have a message

Solution

1. Step 1: Check assert syntax

   Assert requires a comma between the condition and the message string.

2. Step 2: Identify the error

   The code misses the comma, causing a syntax error.

3. Final Answer:

   Missing comma between condition and message -> Option A

4. Quick Check:

   Comma separates condition and message in assert [OK]

Hint: Always put a comma before the assert message [OK]

Common Mistakes:

• Using colon or space instead of comma
• Thinking parentheses are mandatory
• Believing assert can't have messages

5. You want to check a list of numbers to ensure all are positive using assert. Which code correctly uses assert inside a loop to do this?

nums = [3, 5, -1, 7]
for n in nums:
    ?

hard

A. assert n > 0; f"Number {n} is not positive"

B. assert n > 0: f"Number {n} is not positive"

C. assert n > 0, f"Number {n} is not positive"

D. assert (n > 0), "Number n is not positive"

Solution

1. Step 1: Understand assert in loop context

   We want to check each number and stop if any is not positive, showing which one failed.

2. Step 2: Check syntax correctness

   assert n > 0, f"Number {n} is not positive" uses correct assert syntax with a comma and f-string for message. Others use invalid punctuation.

3. Final Answer:

   assert n > 0, f"Number {n} is not positive" -> Option C

4. Quick Check:

   Assert syntax: condition, message with comma [OK]

Hint: Use comma and f-string for assert message in loops [OK]

Common Mistakes:

• Using colon or semicolon instead of comma
• Not using f-string for variable message
• Putting message outside assert statement