Basic assert statement in PyTest - Build an Automation Script

Verify that the sum of two numbers is correct

Preconditions (2)

Step 1: Create a function that adds two numbers

Step 2: Call the function with inputs 2 and 3

Step 3: Check if the result equals 5 using an assert statement

✅ Expected Result: The assert statement passes confirming the sum is correct

Automation Requirements - pytest

Assertions Needed:

Assert that the sum of 2 and 3 equals 5

Best Practices:

Use clear and simple assert statements

Keep test functions small and focused

Name test functions starting with 'test_'

Automated Solution

PyTest

def add_numbers(a: int, b: int) -> int:
    return a + b

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

The add_numbers function adds two numbers and returns the result.

The test function test_add_numbers calls this function with 2 and 3, then uses an assert statement to check if the result is 5.

If the result is not 5, the assert will fail and show a clear message.

This simple test shows how to use basic assert statements in pytest.

Common Mistakes - 3 Pitfalls

Not using assert statement and just printing the result

{'mistake': "Using assert with wrong comparison operator like '=' instead of '=='", 'why_bad': "Single '=' is assignment, not comparison, causing syntax errors.", 'correct_approach': "Use '==' to compare values in assert statements."}

Not providing a helpful message in assert

Bonus Challenge

Now add data-driven testing with 3 different pairs of numbers and their expected sums

Show Hint

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 '=='