Testing with pytest

Your GitHub Classroom assignment is graded in part by an autograder that runs pytest against your repository. You should learn to run tests locally so you catch failures before you push.

Install pytest locally

On your machine (using the same Python you use for the assignment—see the virtual environments chapter at the end of the book if you use a venv):

pip install pytest

Then run from your project root (where your code and the autograder’s layout expect to run):

pytest

Your template may already list pytest as a dependency; use whatever pip install -r / uv sync instructions you were given.

A function you can test

Prefer testing functions that return values—easy to assert:

# greeting.py
def greeting(name: str) -> str:
    return f"Hello, {name}!"

# test_greeting.py  (LOCAL ONLY — see warning below)
from greeting import greeting

def test_greeting_includes_name():
    assert greeting("World") == "Hello, World!"

Run: pytest test_greeting.py -v

Testing print output with capsys

To check print("Hello, World!"), capture stdout with pytest’s capsys fixture:

# hello.py
def say_hello() -> None:
    print("Hello, World!")

# test_hello.py  (LOCAL ONLY)
from hello import say_hello

def test_say_hello_prints(capsys):
    say_hello()
    captured = capsys.readouterr()
    assert captured.out.strip() == "Hello, World!"

captured.out is everything printed to standard output during the test.

Parametrize (optional)

Run one test logic with many inputs:

import pytest

@pytest.mark.parametrize("n,expected", [(1, 1), (2, 4), (3, 9)])
def test_square(n, expected):
    assert n * n == expected

Warning

Do not push your own tests
The course autograder supplies the official pytest tests on GitHub Classroom. They run in CI when you push; you do not see those files as something to submit.

Locally, you may create test_*.py files or a tests/ folder to practice. Never git add / commit / push your personal test files unless your instructor explicitly tells you to. Pushing extra tests can interfere with grading, collide with hidden tests, or violate the assignment rules.

Do not push __pycache__, .pytest_cache/, or local virtualenv folders either—use .gitignore as your template provides.

Final Example

Here is one final example complete with test output. For this one, the main program is quite simple, just like what you have seen already.

def main() -> None:
    # intentionally missing the comma "," after Hello
    print("Hello World!")    


if __name__ == "__main__":
    main()

When you run it, you should hope to see this:

➜  hello git:(main) ✗ python3 hello.py 
Hello World!

To test, you could make this:

import pytest
import hello  # ← imports the hello.py module


def test_main_prints_hello_world(capsys):
    """Check that main() prints exactly 'Hello, World!'"""
    
    # Run the student's main function
    hello.main()
    
    # Capture everything that was printed
    captured = capsys.readouterr()
    
    # Clean up the output (remove extra spaces/newlines)
    output = captured.out.strip()
    expected = "Hello, World!"
    
    # Use pytest.fail() instead of assert → no ugly diff at the bottom!
    if output != expected:
        pytest.fail(f"Expected exactly this: {expected} But your main() printed: \
{output}")

The output of this failed program looks like this:

➜  hello git:(main) ✗ pytest              
================================================================================= test session starts =================================================================================
platform darwin -- Python 3.14.3, pytest-9.0.2, pluggy-1.6.0
rootdir: /Users/user/Documents/IntroToPythonTemplate/IntroToPythonTemplate/hello
collected 1 item                                                                                                                                                                      

test_hello.py F                                                                                                                                                                 [100%]

====================================================================================== FAILURES =======================================================================================
____________________________________________________________________________ test_main_prints_hello_world _____________________________________________________________________________

capsys = <_pytest.capture.CaptureFixture object at 0x10be817f0>

    def test_main_prints_hello_world(capsys):
        """Check that main() prints exactly 'Hello, World!'"""
    
        # Run the student's main function
        hello.main()
    
        # Capture everything that was printed
        captured = capsys.readouterr()
    
        # Clean up the output (remove extra spaces/newlines)
        output = captured.out.strip()
        expected = "Hello, World!"
    
        # Use pytest.fail() instead of assert → no ugly diff at the bottom!
        if output != expected:
>           pytest.fail(f"Expected exactly this: {expected} But your main() printed: \
    {output}")
E           Failed: Expected exactly this: Hello, World! But your main() printed: Hello World!

test_hello.py:20: Failed
=============================================================================== short test summary info ===============================================================================
FAILED test_hello.py::test_main_prints_hello_world - Failed: Expected exactly this: Hello, World! But your main() printed: Hello World!
================================================================================== 1 failed in 0.05s ==================================================================================

Your turn!

Implement

1. pip install pytest (or use your template’s env).
2. Write greet(name) that returns a string and a say_hi() that prints a fixed greeting; verify both locally with pytest and capsys.
3. Delete or keep your practice tests only on disk—do not push them.
4. Commit and push only your solution source (e.g. hello.py, main.py) per the rubric.