Running Tests with pytest

Once you’ve written your test functions, the next step is to run them and verify that your code behaves as expected. This section explains how to use the pytest command, interpret the output, and specify specific files or directories to test.

Running Tests with the pytest Command

To run tests, use the pytest command in your terminal or command prompt. By default, pytest discovers and runs all test files and functions in the current directory and its subdirectories.

Basic Command

Run the command:

pytest

This will:

• Automatically find files named test_*.py or *_test.py.
• Execute all functions within those files that start with test_.

Example

Assume you have the following test file:

File: test_math.py

def test_addition():
    assert 2 + 2 == 4
 
def test_subtraction():
    assert 5 - 3 == 2

Run pytest:

pytest

Output:

================== test session starts ==================
collected 2 items

test_math.py ..                                         [100%]

================== 2 passed in 0.01s ==================

Understanding Test Output

pytest provides detailed output to help you understand the status of your tests.

Key Components of Output

1. Test Collection: Shows how many test cases were discovered (e.g., collected 2 items).
2. Test Results:
   • . (dot): Test passed.
   • F: Test failed.
   • E: Error occurred during the test.
3. Summary: Provides a breakdown of passed, failed, and skipped tests.

Example: Failing Test Output

If a test fails, pytest provides detailed information to help debug.

File: test_math.py

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

Run pytest:

pytest

Output:

================== test session starts ==================
collected 1 item

test_math.py F                                         [100%]

================== FAILURES ==================
____________________ test_failure _____________________

def test_failure():
>       assert 2 + 2 == 5
E       assert 4 == 5

test_math.py:2: AssertionError

• Failure Location: Shows the file name and line number where the test failed.
• Assertion Details: Explains the mismatch (4 == 5).

Specifying Test Files or Directories

Sometimes, you might want to run tests in a specific file or folder rather than all tests in the project.

Running a Specific Test File

Use the filename as an argument:

pytest test_math.py

This runs all tests in test_math.py.

Running Tests in a Directory

Provide the directory name:

pytest tests/

This runs all tests in the tests/ directory and its subdirectories.

Running a Specific Test Function

Use the file name followed by :: and the function name:

pytest test_math.py::test_addition

This runs only the test_addition function in test_math.py.

Summary

• Use pytest to discover and run tests automatically.
• Interpret test output to identify passed, failed, and error cases.
• Specify files, directories, or individual test functions for targeted testing.

This foundational understanding will help you efficiently run tests as you continue exploring pytest!