Basic assert statement in PyTest - Test Execution Trace

Test Overview

This test checks if the sum of two numbers is correct using a simple assert statement in pytest.

Test Code - pytest

PyTest

def test_sum():
    result = 2 + 3
    assert result == 5, f"Expected 5 but got {result}"

Execution Trace - 4 Steps

Step	Action	System State	Assertion	Result
1	Test function 'test_sum' starts execution	Python interpreter ready to run the test	—	PASS
2	Calculate sum of 2 and 3, store in variable 'result'	Variable 'result' holds value 5	—	PASS
3	Assert that 'result' equals 5	Check if 5 == 5	Assert statement verifies result == 5	PASS
4	Test completes successfully	No errors raised, test passes	—	PASS

Failure Scenario

Failing Condition: If the sum calculation is incorrect, e.g., result is not 5

Execution Trace Quiz - 3 Questions

What does the assert statement check in this test?

A If the sum of 2 and 3 equals 6

B If the variable 'result' is defined

C If the sum of 2 and 3 equals 5

D If the test function runs without errors

Key Result

Use simple assert statements in pytest to verify expected outcomes clearly and get immediate feedback if something is wrong.

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 '=='