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Define pytest fixtures with lambda functions.

Project description


Define pytest fixtures with lambda functions.


pip install pytest-lambda

from pytest_lambda import lambda_fixture, static_fixture

first = static_fixture('John')
middle = static_fixture('Jacob')
last = static_fixture('Jingleheimer-Schmidt')

full_name = lambda_fixture(lambda first, middle, last: f'{first} {middle} {last}')

def test_the_namerator(full_name):
    assert full_name == 'John Jacob Jingleheimer-Schmidt'


import asyncio
import pytest
from pytest_lambda import (

# Basic usage
fixture_name = lambda_fixture(lambda other_fixture: 'expression', scope='session', autouse=True)

# Async fixtures (awaitables automatically awaited) — requires an async plugin, like pytest-asyncio
fixture_name = lambda_fixture(lambda: asyncio.sleep(0, 'expression'), async_=True)

# Request fixtures by name
fixture_name = lambda_fixture('other_fixture')
fixture_name = lambda_fixture('other_fixture', 'another_fixture', 'cant_believe_its_not_fixture')
ren, ame, it = lambda_fixture('other_fixture', 'another_fixture', 'cant_believe_its_not_fixture')

# Reference `self` inside a class
class TestContext:
   fixture_name = lambda_fixture(lambda self: self.__class__.__name__, bind=True)

# Parametrize
fixture_name = lambda_fixture(params=['a', 'b'])
fixture_name = lambda_fixture(params=['a', 'b'], ids=['A!', 'B!'])
fixture_name = lambda_fixture(params=[pytest.param('a', id='A!'),
                                     pytest.param('b', id='B!')])
alpha, omega = lambda_fixture(params=[pytest.param('start', 'end', id='uno'),
                                     pytest.param('born', 'dead', id='dos')])

# Use literal value (not lazily evaluated)
fixture_name = static_fixture(42)
fixture_name = static_fixture('just six sevens', autouse=True, scope='module')

# Raise an exception if fixture is requested
fixture_name = error_fixture(lambda: ValueError('my life has no intrinsic value'))

# Or maybe don't raise the exception
fixture_name = error_fixture(lambda other_fixture: TypeError('nope') if other_fixture else None)

# Create an abstract fixture (to be overridden by the user)
fixture_name = not_implemented_fixture()
fixture_name = not_implemented_fixture(autouse=True, scope='session')

# Disable usage of a fixture (fail early to save future head scratching)
fixture_name = disabled_fixture()

What else is possible?

Of course, you can use lambda fixtures inside test classes:


from pytest_lambda import lambda_fixture

class TestClassiness:
    classiness = lambda_fixture(lambda: 9000 + 1)

    def test_how_classy_we_is(self, classiness):
        assert classiness == 9001

Aliasing other fixtures

You can also pass the name of another fixture, instead of a lambda:


from pytest_lambda import lambda_fixture, static_fixture

agent = static_fixture('Bourne')
who_i_am = lambda_fixture('agent')

def test_my_identity(who_i_am):
    assert who_i_am == 'Bourne'

Even multiple fixture names can be used:


from pytest_lambda import lambda_fixture, static_fixture

agent_first = static_fixture('Jason')
agent_last = static_fixture('Bourne')
who_i_am = lambda_fixture('agent_first', 'agent_last')

def test_my_identity(who_i_am):
    assert who_i_am == ('Jason', 'Bourne')

Destructuring assignment is also supported, allowing multiple fixtures to be renamed in one statement:


from pytest_lambda import lambda_fixture, static_fixture

agent_first = static_fixture('Jason')
agent_last = static_fixture('Bourne')
first, last = lambda_fixture('agent_first', 'agent_last')

def test_my_identity(first, last):
    assert first == 'Jason'
    assert last == 'Bourne'

Annotating aliased fixtures

You can force the loading of fixtures without trying to remember the name of pytest.mark.usefixtures


from pytest_lambda import lambda_fixture, static_fixture

car = static_fixture({
    'type': 'Sweet-ass Cadillac',
    'is_started': False,
turn_the_key = lambda_fixture(lambda car: car.update(is_started=True))

preconditions = lambda_fixture('turn_the_key', autouse=True)

def test_my_caddy(car):
    assert car['is_started']


Tests can be parametrized with lambda_fixture's params kwarg


from pytest_lambda import lambda_fixture

lady = lambda_fixture(params=[
    'Monica', 'Erica', 'Rita', 'Tina', 'Sandra', 'Mary', 'Jessica'

def test_your_man(lady):
    assert lady[:0] in 'my life'

Destructuring assignment of a parametrized lambda fixture is also supported


import pytest
from pytest_lambda import lambda_fixture

lady, where = lambda_fixture(params=[
    pytest.param('Monica', 'in my life'),
    pytest.param('Erica', 'by my side'),
    pytest.param('Rita', 'is all I need'),
    pytest.param('Tina', 'is what I see'),
    pytest.param('Sandra', 'in the sun'),
    pytest.param('Mary', 'all night long'),
    pytest.param('Jessica', 'here I am'),

def test_your_man(lady, where):
    assert lady[:0] in where

Declaring abstract things

not_implemented_fixture is perfect for labeling abstract parameter fixtures of test mixins


import pytest
from pytest_lambda import static_fixture, not_implemented_fixture

class Dials1900MixinALot:
    butt_shape = not_implemented_fixture()
    desires = not_implemented_fixture()

    def it_kicks_them_nasty_thoughts(self, butt_shape, desires):
        assert butt_shape == 'round' and 'triple X throw down' in desires

class DescribeMissThing(Dials1900MixinALot):
    butt_shape = static_fixture('flat')
    desires = static_fixture(['playin workout tapes by Fonda'])

class DescribeSistaICantResista(Dials1900MixinALot):
    butt_shape = static_fixture('round')
    desires = static_fixture(['gettin in yo Benz', 'triple X throw down'])

Use disabled_fixture to mark a fixture as disabled. Go figure.


import pytest
from pytest_lambda import disabled_fixture

wheelchair = disabled_fixture()

def test_stairs(wheelchair):
    assert wheelchair + 'floats'

Raising exceptions

You can also raise an arbitrary exception when a fixture is requested, using error_fixture


import pytest
from pytest_lambda import error_fixture, not_implemented_fixture, static_fixture

bicycle = static_fixture('a sledgehammer')

def it_does_sweet_jumps(bicycle):
    assert bicycle + 'jump' >= '3 feet'

class ContextOcean:
    depth = not_implemented_fixture()
    bicycle = error_fixture(lambda bicycle, depth: (
        RuntimeError(f'Now is not the time to use that! ({bicycle})') if depth > '1 league' else None))

    class ContextDeep:
        depth = static_fixture('20,000 leagues')

        @pytest.mark.xfail(strict=True, raises=RuntimeError)
        def it_doesnt_flip_and_shit(self, bicycle):
            assert bicycle + 'floats'

    class ContextBeach:
        depth = static_fixture('1 inch')

        def it_gets_you_all_wet_but_otherwise_rides_like_a_champ(self, bicycle):
            assert 'im wet'

Async fixtures

By passing async_=True to lambda_fixture, the fixture will be defined as an async function, and if the returned value is awaitable, it will be automatically awaited before exposing it to pytest. This allows the usage of async things while only being slightly salty that Python, TO THIS DAY, still does not support await expressions within lambdas! Yes, only slightly salty!

NOTE: an asyncio pytest plugin is required to use async fixtures, such as pytest-asyncio


import asyncio
import pytest
from pytest_lambda import lambda_fixture

async def hows_the_sink():
    await asyncio.sleep(1)
    return 'leaky'

a_sink = lambda_fixture(lambda: hows_the_sink(), async_=True)

class DescribeASink:
    async def it_is_leaky(self, a_sink):
        assert a_sink is 'leaky'


How can I build and test the thing locally?

  1. Create a virtualenv, however you prefer. Or don't, if you prefer.
  2. pip install poetry
  3. poetry install to install setuptools entrypoint, so pytest automatically loads the plugin (otherwise, you'll have to run py.test -p pytest_lambda.plugin)
  4. Run py.test. The tests will be collected from the (thanks to pytest-markdown).

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