Pytest Testing Idioms

Internal

• Pytest

Overview

Test files start with test_ (?) A function that starts with test is automatically considered a test to be executed. A function that does NOT start with test is ignored as test, so if one tries to specifically execute it, they get:

ERROR: not found: /Users/ovidiu/src/.../test_template.py::null_template_variables

A test function can be injected with a test fixture: Available fixtures: cache, capfd, capfdbinary, caplog, capsys, capsysbinary, doctest_namespace, monkeypatch, pytestconfig, record_property, record_testsuite_property, record_xml_attribute, recwarn, tmp_path, tmp_path_factory, tmpdir, tmpdir_factory.

Tested Code is Supposed to Throw Exception

import pytest

def test_something_that_should_throw_exception():
    with pytest.raises(Exception) as info:
        my_method()

If the tested code raises a more specific exception, you can use that instead:

def test_something_that_should_throw_exception():
    with pytest.raises(ValueError) as info:
        my_method()

This tests the exception arguments. ⚠️ In this case, it is very import to place the assert statements outside the with block, otherwise they will be silently ignored.

def test_something_that_should_throw_exception():
    with pytest.raises(ValueError) as info:
        my_method()
    # important: interaction with 'info' must take place outside the with block
    assert info.value.args[0] == "some message"
    assert "some message" in str(info.value)

Also see:

Python Language | Exceptions

xunit-Style Setup

https://docs.pytest.org/en/7.1.x/how-to/xunit_setup.html

def setup_function(function):
    """setup any state tied to the execution of the given function.
    Invoked for every test function in the module.

    'function' is optional.
    """
    Path(Path(__file__).parent, './data/tmp').mkdir()


def teardown_function(function):
    """teardown any state that was previously setup with a setup_function
    call.

    'function' is optional.
    """
    shutil.rmtree(str(Path(Path(__file__).parent, './data/tmp/')))

Failing a Test

from _pytest.outcomes import fail

def test_something():
  fail("this test failed because I wanted it to")

Mocking

Mocking with unittest