Test functions in PyTest

Introduction

Test functions help check if parts of your code work correctly. They make sure your program does what you expect.

When you want to check if a function returns the right result.

When you fix a bug and want to confirm it is really fixed.

When you add new features and want to make sure old parts still work.

When you want to run quick checks automatically without manual testing.

Syntax

PyTest

def test_function_name():
    assert expression

Test functions start with test_ so pytest can find them.

Use assert to check if something is true.

Examples

This test checks if 2 plus 2 equals 4.

PyTest

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

This test checks if the length of 'hello' is 5.

PyTest

def test_string_length():
    assert len('hello') == 5

This test checks if the number 3 is in the list.

PyTest

def test_list_contains():
    assert 3 in [1, 2, 3]

Sample Program

This program defines a simple add function and tests it with three cases. Each assert checks if the result is correct.

PyTest

def add(a, b):
    return a + b

def test_add():
    assert add(3, 4) == 7
    assert add(-1, 1) == 0
    assert add(0, 0) == 0

Important Notes

Keep test functions small and focused on one thing.

Use clear names starting with test_ so pytest finds them easily.

Run tests often to catch problems early.

Summary

Test functions check if code works as expected.

They start with test_ and use assert statements.

Running tests helps keep your code reliable and bug-free.

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