Basic assert statement in PyTest - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the assert statement in pytest?

The assert statement checks if a condition is true. If it is false, the test fails and pytest shows an error.

beginner

How does pytest report a failed assert statement?

Pytest shows the exact expression that failed and the values involved, helping you understand why the test failed.

beginner

Write a simple pytest assert statement to check if 5 is greater than 3.

def test_greater():
    assert 5 > 3

This test will pass because 5 is indeed greater than 3.

intermediate

Can assert statements include messages? How?

Yes, you can add a message after a comma to explain the failure.

assert x == 10, "x should be 10"

beginner

Why is using assert better than printing values for testing?

Because assert automatically fails the test if the condition is false, making tests clear and automated without manual checks.

What happens if an assert condition is false in pytest?

A The program crashes immediately

B The test passes silently

C The test fails and pytest shows an error

D Nothing happens, it continues

If the assert condition is false, pytest marks the test as failed and shows the error details.

Which of these is a valid assert statement in pytest?

A assert 2 + 2 == 4

B assert(2 + 2 = 4)

C assert 2 + 2 equals 4

D assert 2 + 2 == '4'

Option A uses correct syntax and logic. Option B has wrong syntax, A is not valid Python, D compares int to string.

How can you add a custom message to an assert statement?

A assert x == 5 then print "x should be 5"

B assert x == 5: "x should be 5"

C assert(x == 5 message "x should be 5")

D assert x == 5, "x should be 5"

You add a message after a comma in the assert statement to explain failure.

What does pytest show when an assert fails?

A A success message

B The failed expression and its values

C No output at all

D Only 'Test failed' message

Pytest shows the exact expression that failed and the values involved to help debugging.

Why is using assert statements important in automated testing?

A They automatically check conditions and fail tests if needed

B They print values for manual checking

C They slow down the test execution

D They are only for debugging, not testing

Assert statements make tests automatic by checking conditions and failing tests when conditions are false.

Explain how the assert statement works in pytest and why it is useful.

Think about how assert helps catch errors automatically.

Write a simple pytest test function using assert to check if a number is positive.

Use assert with a condition like x > 0 inside a function starting with test_

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 '=='