Writing Basic Tests in pytest

Now that pytest is set up, it’s time to write and run your first test. In this section, you’ll learn the basics of creating test functions, naming them correctly, and using assert statements.

Writing Your First Test Function

A test function is a regular Python function that checks if part of your code works as expected. pytest identifies test functions automatically based on their names.

Example: First Test

Create a file named test_example.py with the following content:

def test_addition():
    assert 1 + 1 == 2  # This checks if the addition is correct

Steps to Run the Test

1. Save the file in your project directory.
2. Open a terminal or command prompt and navigate to the file’s directory.
3. Run the test using:

   pytest test_example.py
4. You should see output indicating that the test passed:

   ================== test session starts ==================
   collected 1 item
   
   test_example.py .                                     [100%]
   
   ================== 1 passed in 0.01s ==================

Understanding pytest Naming Conventions

To ensure pytest can discover your test functions, follow these simple naming rules:

File Naming

• Test files should be named with a test_ prefix or a _test suffix (e.g., test_example.py, example_test.py).

Function Naming

• Test function names should start with test_ (e.g., test_addition, test_subtraction).

Why Naming Matters

pytest uses these naming conventions to automatically find and run your test cases. Without them, pytest might skip your tests.

Using assert Statements in pytest

The assert statement is a fundamental tool in pytest. It checks if a condition is true. If the condition is false, the test fails.

Examples of Assert Statements

1. Checking Equality

def test_equality():
    assert 3 * 2 == 6  # Passes because 3 times 2 equals 6

2. Checking Conditions

def test_condition():
    assert 5 > 3  # Passes because 5 is greater than 3

3. Checking Membership

def test_membership():
    fruits = ["apple", "banana", "cherry"]
    assert "apple" in fruits  # Passes because 'apple' is in the list

What Happens When an Assertion Fails?

If an assertion fails, pytest stops the test and shows detailed output explaining the failure. For example:

def test_failure():
    assert 2 + 2 == 5  # This will fail

Output:

E       assert 4 == 5
E        +  where 4 = 2 + 2

This output tells you exactly why the test failed.

By following these simple steps and conventions, you can start writing basic tests with pytest. Assertions help ensure your code behaves as expected, making debugging and development more efficient.

Ready to explore more? Let’s dive into running tests and understanding test output next!