Test functions in PyTest - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a test function in pytest?

A test function in pytest is a regular Python function whose name starts with test_. It contains code to check if a part of your program works correctly.

beginner

How do you write a simple test function to check if 2 + 2 equals 4?

You write a function starting with test_ and use an assert statement like this:

def test_addition():
    assert 2 + 2 == 4

intermediate

Why should test functions be independent?

Test functions should be independent so that one test's result does not affect another. This makes tests reliable and easier to debug.

beginner

What happens if an assertion in a test function fails?

If an assertion fails, pytest marks the test as failed and shows an error message explaining what went wrong.

beginner

How does pytest find test functions to run?

Pytest automatically finds functions that start with test_ in files named test_*.py or *_test.py.

What prefix should a function name have to be recognized as a test by pytest?

A check_

B test_

C verify_

D run_

Pytest recognizes functions starting with test_ as test functions.

What keyword is used inside a test function to check if a condition is true?

A assert

B check

C verify

D test

The assert keyword is used to verify conditions in test functions.

If a test function's assertion fails, what will pytest report?

A Test passed

B Test skipped

C Test failed

D Test ignored

Pytest reports the test as failed if an assertion inside it fails.

Which file name pattern does pytest use to find test files?

A test_*.py

B run_*.py

C check_*.py

D verify_*.py

Pytest looks for files named test_*.py or *_test.py to find tests.

Why is it important that test functions do not depend on each other?

A To make tests run faster

B To reduce code size

C To use less memory

D To avoid test failures caused by other tests

Independent tests avoid failures caused by side effects from other tests.

Explain how to write a basic test function in pytest and what it should contain.

Think about a simple function that checks if something is true.

Describe why test functions should be independent and how pytest finds them.

Consider reliability and automatic test discovery.

Practice

1. What is the main purpose of a test function in pytest?

easy

A. To create user interfaces

B. To run the program faster

C. To check if the code works as expected

D. To store data permanently

Solution

1. Step 1: Understand the role of test functions

   Test functions are written to verify that the code behaves correctly.

2. Step 2: Identify the main goal

   The main goal is to check if the code works as expected, catching bugs early.

3. Final Answer:

   To check if the code works as expected -> Option C

4. Quick Check:

   Test functions = Verify code correctness [OK]

Hint: Test functions verify code behavior, not speed or UI [OK]

Common Mistakes:

• Thinking test functions improve program speed
• Confusing test functions with UI code
• Assuming test functions store data

2. Which of the following is the correct way to name a test function in pytest?

easy

A. check_function()

B. TestFunction()

C. function_test()

D. test_function()

Solution

1. Step 1: Recall pytest naming rules

   Pytest requires test functions to start with 'test_' to be recognized automatically.

2. Step 2: Identify the correct naming

   Only 'test_function()' starts with 'test_', so it is correct.

3. Final Answer:

   test_function() -> Option D

4. Quick Check:

   Test function names start with 'test_' [OK]

Hint: Test functions must start with 'test_' prefix [OK]

Common Mistakes:

• Using names without 'test_' prefix
• Capitalizing function names incorrectly
• Placing 'test' at the end of the name

3. What will be the result of running this pytest test function?

def test_sum():
    result = 2 + 3
    assert result == 5

medium

A. Test passes successfully

B. Test fails with AssertionError

C. SyntaxError occurs

D. Test is skipped automatically

Solution

1. Step 1: Analyze the test code

   The function calculates 2 + 3 and stores in 'result'.

2. Step 2: Check the assertion

   The assertion checks if 'result' equals 5, which is true.

3. Final Answer:

   Test passes successfully -> Option A

4. Quick Check:

   2 + 3 == 5 means test passes [OK]

Hint: If assert condition is true, test passes [OK]

Common Mistakes:

• Assuming assertion fails when it is true
• Confusing syntax errors with assertion errors
• Thinking tests skip without skip decorators

4. Identify the error in this pytest test function:

def test_multiply():
    result = 4 * 5
    assert result = 20

medium

A. Incorrect assert syntax using '=' instead of '=='

B. Function name does not start with 'test_'

C. Multiplication operator '*' is invalid

D. Missing return statement in test function

Solution

1. Step 1: Check the assert statement syntax

   The assert statement uses '=' which is assignment, not comparison.

2. Step 2: Identify correct syntax

   Assert requires '==' to compare values, so '=' causes a syntax error.

3. Final Answer:

   Incorrect assert syntax using '=' instead of '==' -> Option A

4. Quick Check:

   Use '==' in assert, not '=' [OK]

Hint: Assert needs '==' for comparison, '=' causes error [OK]

Common Mistakes:

• Using '=' instead of '==' in assert
• Ignoring function naming rules
• Thinking return is needed in test functions

5. You want to test a function that returns the maximum number from a list. Which pytest test function below correctly tests this behavior including an edge case for an empty list?

def max_number(numbers):
    return max(numbers)

# Choose the correct test function:

hard

A. def test_max_number():
assert max_number([1, 3, 2]) == 3
assert max_number([]) == None

B. def test_max_number():
assert max_number([1, 3, 2]) == 3
import pytest
with pytest.raises(ValueError):
max_number([])

C. def test_max_number():
assert max_number([1, 3, 2]) == 2
assert max_number([0]) == 0

D. def test_max_number():
assert max_number([1, 3, 2]) == 3
assert max_number([1, 3, 2]) == 1

Solution

1. Step 1: Understand max() behavior on empty list

   Calling max([]) raises ValueError, so test must expect this error.

2. Step 2: Check which test handles empty list correctly

   def test_max_number(): assert max_number([1, 3, 2]) == 3 import pytest with pytest.raises(ValueError): max_number([]) uses pytest.raises to check for ValueError, correctly testing the edge case.

3. Final Answer:

   Test function that asserts max and expects ValueError on empty list -> Option B

4. Quick Check:

   Use pytest.raises for exceptions in tests [OK]

Hint: Use pytest.raises to test expected errors [OK]

Common Mistakes:

• Assuming max([]) returns None instead of error
• Wrong expected max value in assertions
• Testing same input with conflicting expected outputs