Basic assert statement in PyTest - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this pytest assertion?

Consider the following pytest test function. What will be the test result when running it?

PyTest

def test_sum():
    result = sum([1, 2, 3])
    assert result == 6

A Test passes successfully

B Test fails with AssertionError

C SyntaxError during test execution

D TypeError during test execution

💡 Hint

Check if the sum of the list matches the asserted value.

The sum of [1, 2, 3] is 6, which matches the assertion, so the test passes.

❓ assertion

intermediate

Which assertion correctly checks if a string contains a substring?

You want to assert that the string 'hello world' contains 'world'. Which assertion is correct?

A assert 'world' not in 'hello world'

B assert 'hello world'.contains('world')

C assert 'world' == 'hello world'

D assert 'world' in 'hello world'

💡 Hint

Use Python's membership operator for substring checks.

Option D uses the correct syntax to check substring presence with 'in'. Option D is invalid syntax, C checks equality incorrectly, and D asserts the opposite.

🔧 Debug

advanced

Why does this pytest assertion fail?

Given the test below, why does it fail?

PyTest

def test_list_length():
    items = [1, 2, 3]
    assert len(items) == 3

A SyntaxError due to wrong assignment operator in assertion

B AssertionError because length is not 3

C TypeError because len() returns a string

D Test passes successfully

💡 Hint

Check the operator used in the assert statement.

The assertion uses '=' which is assignment, not comparison. This causes a SyntaxError.

🧠 Conceptual

advanced

What happens if an assertion fails in pytest?

In pytest, when an assert statement fails inside a test function, what is the outcome?

A The test is skipped without any error

B The test is marked as failed and pytest reports AssertionError

C The test passes but with a warning

D The test runner crashes and stops all tests

💡 Hint

Think about how pytest handles failed assertions.

Pytest marks the test as failed and shows the AssertionError details when an assert fails.

❓ framework

expert

Which pytest feature helps to show detailed info on assert failures automatically?

You want pytest to show detailed values in assert failure messages without writing custom messages. Which feature enables this?

A pytest.mark.parametrize decorator

B Using assert with custom message parameter

C pytest's assert rewriting feature

D pytest.skip marker

💡 Hint

This feature modifies assert statements to provide better failure details.

Pytest rewrites assert statements to show detailed info on failure automatically, called assert rewriting.

Practice

1. What does the assert statement do in a pytest test?

easy

A. Skips the test without running it

B. Runs the test multiple times automatically

C. Prints a message to the console

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

Solution

1. Step 1: Understand the purpose of assert

   The assert statement checks if a condition is true during a test.

2. Step 2: Know the effect of assert failing

   If the condition is false, the test stops and reports failure.

3. Final Answer:

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

4. Quick Check:

   Assert = condition check [OK]

Hint: Assert stops test on false condition [OK]

Common Mistakes:

• Thinking assert runs tests multiple times
• Confusing assert with print statements
• Believing assert skips tests

2. Which of the following is the correct syntax for an assert statement in pytest?

easy

A. assert(x == 5; "x should be 5")

B. assert x == 5, "x should be 5"

C. assert x = 5, "x should be 5"

D. assert x == 5 then "x should be 5"

Solution

1. Step 1: Review correct assert syntax

   In Python, assert is followed by a condition, optionally a comma and a message string.

2. Step 2: Identify syntax errors in options

   assert x == 5, "x should be 5" uses correct syntax: assert condition, message. Others have wrong punctuation or assignment.

3. Final Answer:

   assert x == 5, "x should be 5" -> Option B

4. Quick Check:

   assert condition, message [OK]

Hint: Use comma after condition for message [OK]

Common Mistakes:

• Using assignment (=) instead of comparison (==)
• Adding parentheses incorrectly
• Using wrong keywords like 'then'

3. What will be the result of this pytest assertion?

def test_value():
    x = 10
    assert x < 5, "x is not less than 5"

medium

A. Test passes silently

B. Test is skipped

C. Test fails with message 'x is not less than 5'

D. SyntaxError occurs

Solution

1. Step 1: Evaluate the condition in assert

   The condition is x < 5, but x is 10, so 10 < 5 is false.

2. Step 2: Understand assert failure behavior

   Since condition is false, the test fails and shows the message after the comma.

3. Final Answer:

   Test fails with message 'x is not less than 5' -> Option C

4. Quick Check:

   False condition triggers failure [OK]

Hint: False assert condition fails test with message [OK]

Common Mistakes:

• Assuming test passes when condition is false
• Expecting syntax error for valid assert
• Thinking test skips on failure

4. Identify the error in this pytest assertion:

def test_sum():
    total = 3 + 4
    assert total = 7, "Sum should be 7"

medium

A. Using assignment (=) instead of comparison (==) in assert

B. Missing parentheses around assert condition

C. Incorrect indentation of assert statement

D. Wrong message format after assert

Solution

1. Step 1: Check assert condition syntax

   The assert condition must use comparison (==), not assignment (=).

2. Step 2: Identify the error in the code

   Here, 'assert total = 7' uses assignment, which is invalid syntax.

3. Final Answer:

   Using assignment (=) instead of comparison (==) in assert -> Option A

4. Quick Check:

   Use '==' for comparison in assert [OK]

Hint: Use '==' not '=' in assert condition [OK]

Common Mistakes:

• Confusing assignment with comparison
• Adding unnecessary parentheses
• Misplacing the error in indentation

5. You want to check if a list items contains exactly 3 elements using pytest assert. Which is the best assert statement?

hard

A. assert len(items) == 3, "List does not have 3 elements"

B. assert items == 3, "List length check failed"

C. assert items.length == 3, "Wrong length"

D. assert len(items) = 3, "Length must be 3"

Solution

1. Step 1: Understand how to get list length in Python

   Use the built-in function len() to get the number of elements in a list.

2. Step 2: Check correct assert syntax for length comparison

   Compare len(items) with 3 using '==', and provide a clear message after a comma.

3. Final Answer:

   assert len(items) == 3, "List does not have 3 elements" -> Option A

4. Quick Check:

   len(list) == number [OK]

Hint: Use len() and '==' to check list size [OK]

Common Mistakes:

• Comparing list directly to number
• Using JavaScript style .length property
• Using assignment '=' instead of '=='