Assert statement usage in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of assert with a failing condition

What will be the output when running this code?

Python

def check_positive(x):
    assert x > 0, "Value must be positive"
    return x

print(check_positive(-5))

A Value must be positive

B AssertionError: Value must be positive

C -5

D null

💡 Hint

Remember what assert does when the condition is false.

The assert statement checks if the condition is true. If false, it raises an AssertionError with the given message.

❓ Predict Output

intermediate

Assert statement with no message

What is the output of this code snippet?

Python

def test_value(val):
    assert val == 10
    return "Passed"

print(test_value(5))

A 5

B Passed

C AssertionError

D null

💡 Hint

What happens if assert condition is false and no message is given?

Assert raises AssertionError without a message if the condition is false.

❓ Predict Output

advanced

Effect of disabling assert statements

What will be the output of this code when run with python -O (optimize) flag?

Python

def check():
    assert false, "Fail"
    return "Checked"

print(check())

A null

B AssertionError: Fail

C false

D Checked

💡 Hint

The -O flag disables assert statements.

When Python runs with -O, assert statements are ignored, so the function returns "Checked".

🧠 Conceptual

advanced

Purpose of assert statements in code

Which of the following best describes the main purpose of assert statements in Python?

A To check conditions during development and catch bugs early

B To improve program performance by skipping checks

C To replace if-else statements for decision making

D To handle runtime errors gracefully

💡 Hint

Think about when and why developers use assert.

Assert statements are used to verify assumptions during development and testing, helping catch bugs early.

❓ Predict Output

expert

Output with assert inside a loop and else clause

What is the output of this code?

Python

for i in range(3):
    assert i < 2, f"Index {i} too high"
else:
    print("Loop completed")

A AssertionError: Index 2 too high

B Loop completed

C 0\n1\n2

D null

💡 Hint

Check when the assert fails and if the else block runs.

The assert fails when i is 2, raising AssertionError before the else block can run.

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