Testing: pytest

The pytest framework makes it easy to write small tests, yet scales to support complex functional testing for applications and libraries.

• Detailed info on failing assert statements (no need to remember self.assert* names)

• Auto-discovery of test modules and functions

• Modular fixtures for managing small or parametrized long-lived test resources

• Can run unittest (including trial) and nose test suites out of the box

• Python 3.5+ and PyPy 3

• Rich plugin architecture, with over 315+ external plugins and thriving community

For more information visit https://docs.pytest.org/en/stable/

Installation

$ poetry add pytest --dev

Configuration

Add the following in pyproject.toml:

[tool.pytest.ini_options]
testpaths = "tests"

Usage

Let’s write a test for the simple calculator in test_simple.py

from python_lifecycle.calculator import simple


def test_add():
    assert simple.add(1, 2) == 3

Note

You can replace the version test and remove the variable __version__ from your package init file

To execute pytest run:

$ pytest

pytest will run all files of the form test_*.py or *_test.py in the current directory and its subdirectories. Thus we can write any sort of tests we want.

from python_lifecycle_training.calculator import simple


def test_add_positives():
    assert simple.add(1, 2) == 3


def test_add_negatives():
    assert simple.add(-1, -2) == -3


def test_add_mixed():
    assert simple.add(1, -2) == -1


def test_add_float():
    assert simple.add(0.1, 0.2) == 0.3


def test_add_str():
    assert simple.add("hello ", "world") == "hello world"

Run test:

$ pytest

Whoops! Something doesn’t seem right.

We need to handle float values a little differently when writing tests. We just need to check whether the value obtained is close enough.

import pytest

def test_add_float():
    assert simple.add(0.1, 0.2) == pytest.approx(0.3)

Now run the tests again.

$ pytest

All tests have passed. Congratulations!

There is, however, one more thing to notice here. The tests passed for “hello world” as well even though we’ve explicitly said that the method add takes nothing but real numbers. This is because Python computes the types dynamically and type check is just a convention we use for the ease of code maintenance and future development.

Advanced feature

We can represent all these tests by a single test method with different test params.

import pytest

from python_lifecycle_training.calculator import simple


def isfloat(num):
    return isinstance(num, float)


@pytest.mark.parametrize(
    "a, b",
    [
        (1, 2),
        (-1, -2),
        (1, -2),
        (0.1, 0.2),
        ("hello ", "world"),
    ],
)
def test_add(a, b):
    expected = a + b

    if any(map(isfloat, (a, b))):
        expected = pytest.approx(expected)

    assert simple.add(a, b) == expected

Pre-commit

We can run tests before every commit by adding the following in .pre-commit-config.yaml file:

- repo: local
  hooks:
    - id: install-dependencies
      name: Install Dependencies
      entry: poetry install
      language: python
      always_run: true
      pass_filenames: false

    - id: test
      name: Run tests
      entry: poetry run pytest
      language: python
      always_run: true
      pass_filenames: false

Next Step

To move on to the next step commit or stash your changes then checkout to the branch setup/test/coverage

$ git stash
$ git checkout setup/test/coverage

Uninstall

$ poetry remove pytest --dev