pytest fixtures

Once you start writing many tests – more than you wrote in Lent1A, you’ll find that setting up the context in which to run each test can take a while. pytest’s fixtures are a way to ease the effort of writing the set-up code, and to make the running of the tests faster by using caching. Here’s some code adapted from pythontesting.net to demonstrate some features.

import pytest
 
@pytest.fixture(scope='module')
def resource_a_setup(request):
    '''Creating resource a, shared across a module'''
    print('\nSetting up resources a')
    def resource_a_teardown():
        print('\nTearing down resources a')
    request.addfinalizer(resource_a_teardown)
 
@pytest.fixture(scope='function')
def resource_b_setup(request):
    '''Create resource b, not shared'''
    print('\nSetting up resources b')
    def resource_b_teardown():
        print('\nTearing down resources b')
    request.addfinalizer(resource_b_teardown)
 
def test_1(resource_a_setup, resource_b_setup):
    print('\ntest_1 - needs resource a and b')
 
def test_2():
    print('\ntest_2 - does not need resource a or b')
 
def test_3(resource_a_setup, resource_b_setup):
    print('\ntest_3  - needs_resource a and b')

Put this code into (say) test_fixturedemo.py and run

   pytest -v -s

Note how the resources are created and destroyed. Use

 
   pytest --fixtures

to get a list of fixtures (including build-in ones)

Using your own exceptions with pytest

Try this (adapted from stackoverflow)

class CustomError(Exception):
    pass

def foo():
    raise ValueError('everything is broken')

def bar():
    raise CustomError('still broken')

import pytest

def test_fooErrorHandling():
    with pytest.raises(ValueError) as excinfo:
        foo()
    assert str(excinfo.value) == 'everything is broken'

def test_barSimpleErrorHandling():
    # don't care about the specific message
    with pytest.raises(CustomError):
        bar()

def test_barSpecificErrorHandling():
    # check the specific error message
    with pytest.raises(CustomError) as excinfo:
        bar()
    assert str(excinfo.value) == 'still broken'

def test_barWithoutImportingExceptionClass():
    # if for some reason you can't import the specific exception class,
    # catch it as generic and verify it's in the str(excinfo)
    with pytest.raises(Exception) as excinfo:
        bar()
    assert 'CustomError:' in str(excinfo)

parametrize

If you’re doing many tests where only the parameters vary, you can use
parametrize.
Here’s an example from the pytest documention which does 3 tests at once –

import pytest

@pytest.mark.parametrize("test_input,expected", [("3+5", 8), ("2+4", 6), ("6*9", 42)])
def test_eval(test_input, expected):
    assert eval(test_input) == expected