New in version 2.0/2.3/2.4.
The purpose of test fixtures is to provide a fixed baseline upon which tests can reliably and repeatedly execute. pytest fixtures offer dramatic improvements over the classic xUnit style of setup/teardown functions:
In addition, pytest continues to support classic xunit-style setup. You can mix both styles, moving incrementally from classic to new style, as you prefer. You can also start out from existing unittest.TestCase style or nose based projects.
pytest-2.4 introduced an additional experimental yield fixture mechanism for easier context manager integration and more linear writing of teardown code.
Test functions can receive fixture objects by naming them as an input argument. For each argument name, a fixture function with that name provides the fixture object. Fixture functions are registered by marking them with @pytest.fixture. Let’s look at a simple self-contained test module containing a fixture and a test function using it:
# content of ./test_smtpsimple.py import pytest @pytest.fixture def smtp(): import smtplib return smtplib.SMTP("merlinux.eu") def test_ehlo(smtp): response, msg = smtp.ehlo() assert response == 250 assert "merlinux" in msg assert 0 # for demo purposes
Here, the test_ehlo needs the smtp fixture value. pytest will discover and call the @pytest.fixture marked smtp fixture function. Running the test looks like this:
$ py.test test_smtpsimple.py =========================== test session starts ============================ platform linux2 -- Python 2.7.3 -- pytest-2.4.2 collected 1 items test_smtpsimple.py F ================================= FAILURES ================================= ________________________________ test_ehlo _________________________________ smtp = <smtplib.SMTP instance at 0x2bb9d88> def test_ehlo(smtp): response, msg = smtp.ehlo() assert response == 250 assert "merlinux" in msg > assert 0 # for demo purposes E assert 0 test_smtpsimple.py:12: AssertionError ========================= 1 failed in 0.18 seconds =========================
In the failure traceback we see that the test function was called with a smtp argument, the smtplib.SMTP() instance created by the fixture function. The test function fails on our deliberate assert 0. Here is an exact protocol of how py.test comes to call the test function this way:
Note that if you misspell a function argument or want to use one that isn’t available, you’ll see an error with a list of available function arguments.
You can always issue:
py.test --fixtures test_simplefactory.py
to see available fixtures.
In versions prior to 2.3 there was no @pytest.fixture marker and you had to use a magic pytest_funcarg__NAME prefix for the fixture factory. This remains and will remain supported but is not anymore advertised as the primary means of declaring fixture functions.
When injecting fixtures to test functions, pytest-2.0 introduced the term “funcargs” or “funcarg mechanism” which continues to be present also in docs today. It now refers to the specific case of injecting fixture values as arguments to test functions. With pytest-2.3 there are more possibilities to use fixtures but “funcargs” remain as the main way as they allow to directly state the dependencies of a test function.
As the following examples show in more detail, funcargs allow test functions to easily receive and work against specific pre-initialized application objects without having to care about import/setup/cleanup details. It’s a prime example of dependency injection where fixture functions take the role of the injector and test functions are the consumers of fixture objects.
pytest supports execution of fixture specific finalization code when the fixture goes out of scope. By accepting a request object into your fixture function you can call its request.addfinalizer one or multiple times:
# content of conftest.py import smtplib import pytest @pytest.fixture(scope="module") def smtp(request): smtp = smtplib.SMTP("merlinux.eu") def fin(): print ("teardown smtp") smtp.close() request.addfinalizer(fin) return smtp # provide the fixture value
The fin function will execute when the last test using the fixture in the module has finished execution.
Let’s execute it:
$ py.test -s -q --tb=no FFteardown smtp 2 failed in 0.15 seconds
We see that the smtp instance is finalized after the two tests finished execution. Note that if we decorated our fixture function with scope='function' then fixture setup and cleanup would occur around each single test. In either case the test module itself does not need to change or know about these details of fixture setup.
Fixture function can accept the request object to introspect the “requesting” test function, class or module context. Further extending the previous smtp fixture example, let’s read an optional server URL from the test module which uses our fixture:
# content of conftest.py import pytest import smtplib @pytest.fixture(scope="module") def smtp(request): server = getattr(request.module, "smtpserver", "merlinux.eu") smtp = smtplib.SMTP(server) def fin(): print ("finalizing %s (%s)" % (smtp, server)) smtp.close() return smtp
We use the request.module attribute to optionally obtain an smtpserver attribute from the test module. If we just execute again, nothing much has changed:
$ py.test -s -q --tb=no FF 2 failed in 0.16 seconds
Let’s quickly create another test module that actually sets the server URL in its module namespace:
# content of test_anothersmtp.py smtpserver = "mail.python.org" # will be read by smtp fixture def test_showhelo(smtp): assert 0, smtp.helo()
$ py.test -qq --tb=short test_anothersmtp.py F ================================= FAILURES ================================= ______________________________ test_showhelo _______________________________ test_anothersmtp.py:5: in test_showhelo > assert 0, smtp.helo() E AssertionError: (250, 'mail.python.org')
voila! The smtp fixture function picked up our mail server name from the module namespace.
Fixture functions can be parametrized in which case they will be called multiple times, each time executing the set of dependent tests, i. e. the tests that depend on this fixture. Test functions do usually not need to be aware of their re-running. Fixture parametrization helps to write exhaustive functional tests for components which themselves can be configured in multiple ways.
Extending the previous example, we can flag the fixture to create two smtp fixture instances which will cause all tests using the fixture to run twice. The fixture function gets access to each parameter through the special request object:
# content of conftest.py import pytest import smtplib @pytest.fixture(scope="module", params=["merlinux.eu", "mail.python.org"]) def smtp(request): smtp = smtplib.SMTP(request.param) def fin(): print ("finalizing %s" % smtp) smtp.close() request.addfinalizer(fin) return smtp
The main change is the declaration of params with @pytest.fixture, a list of values for each of which the fixture function will execute and can access a value via request.param. No test function code needs to change. So let’s just do another run:
$ py.test -q test_module.py FFFF ================================= FAILURES ================================= __________________________ test_ehlo[merlinux.eu] __________________________ smtp = <smtplib.SMTP instance at 0x2662290> def test_ehlo(smtp): response = smtp.ehlo() assert response == 250 assert "merlinux" in response > assert 0 # for demo purposes E assert 0 test_module.py:6: AssertionError __________________________ test_noop[merlinux.eu] __________________________ smtp = <smtplib.SMTP instance at 0x2662290> def test_noop(smtp): response = smtp.noop() assert response == 250 > assert 0 # for demo purposes E assert 0 test_module.py:11: AssertionError ________________________ test_ehlo[mail.python.org] ________________________ smtp = <smtplib.SMTP instance at 0x26c2dd0> def test_ehlo(smtp): response = smtp.ehlo() assert response == 250 > assert "merlinux" in response E assert 'merlinux' in 'mail.python.org\nSIZE 25600000\nETRN\nSTARTTLS\nENHANCEDSTATUSCODES\n8BITMIME\nDSN' test_module.py:5: AssertionError ________________________ test_noop[mail.python.org] ________________________ smtp = <smtplib.SMTP instance at 0x26c2dd0> def test_noop(smtp): response = smtp.noop() assert response == 250 > assert 0 # for demo purposes E assert 0 test_module.py:11: AssertionError 4 failed in 6.32 seconds
We see that our two test functions each ran twice, against the different smtp instances. Note also, that with the mail.python.org connection the second test fails in test_ehlo because a different server string is expected than what arrived.
You can not only use fixtures in test functions but fixture functions can use other fixtures themselves. This contributes to a modular design of your fixtures and allows re-use of framework-specific fixtures across many projects. As a simple example, we can extend the previous example and instantiate an object app where we stick the already defined smtp resource into it:
# content of test_appsetup.py import pytest class App: def __init__(self, smtp): self.smtp = smtp @pytest.fixture(scope="module") def app(smtp): return App(smtp) def test_smtp_exists(app): assert app.smtp
Here we declare an app fixture which receives the previously defined smtp fixture and instantiates an App object with it. Let’s run it:
$ py.test -v test_appsetup.py =========================== test session starts ============================ platform linux2 -- Python 2.7.3 -- pytest-2.4.2 -- /home/hpk/p/pytest/.tox/regen/bin/python collecting ... collected 2 items test_appsetup.py:12: test_smtp_exists[mail.python.org] PASSED test_appsetup.py:12: test_smtp_exists[merlinux.eu] PASSED ========================= 2 passed in 5.75 seconds =========================
Due to the parametrization of smtp the test will run twice with two different App instances and respective smtp servers. There is no need for the app fixture to be aware of the smtp parametrization as pytest will fully analyse the fixture dependency graph.
Note, that the app fixture has a scope of module and uses a module-scoped smtp fixture. The example would still work if smtp was cached on a session scope: it is fine for fixtures to use “broader” scoped fixtures but not the other way round: A session-scoped fixture could not use a module-scoped one in a meaningful way.
pytest minimizes the number of active fixtures during test runs. If you have a parametrized fixture, then all the tests using it will first execute with one instance and then finalizers are called before the next fixture instance is created. Among other things, this eases testing of applications which create and use global state.
The following example uses two parametrized funcargs, one of which is scoped on a per-module basis, and all the functions perform print calls to show the setup/teardown flow:
# content of test_module.py import pytest @pytest.fixture(scope="module", params=["mod1", "mod2"]) def modarg(request): param = request.param print "create", param def fin(): print ("fin %s" % param) return param @pytest.fixture(scope="function", params=[1,2]) def otherarg(request): return request.param def test_0(otherarg): print " test0", otherarg def test_1(modarg): print " test1", modarg def test_2(otherarg, modarg): print " test2", otherarg, modarg
Let’s run the tests in verbose mode and with looking at the print-output:
$ py.test -v -s test_module.py =========================== test session starts ============================ platform linux2 -- Python 2.7.3 -- pytest-2.4.2 -- /home/hpk/p/pytest/.tox/regen/bin/python collecting ... collected 8 items test_module.py:15: test_0 test0 1 PASSED test_module.py:15: test_0 test0 2 PASSED test_module.py:17: test_1[mod1] create mod1 test1 mod1 PASSED test_module.py:19: test_2[1-mod1] test2 1 mod1 PASSED test_module.py:19: test_2[2-mod1] test2 2 mod1 PASSED test_module.py:17: test_1[mod2] create mod2 test1 mod2 PASSED test_module.py:19: test_2[1-mod2] test2 1 mod2 PASSED test_module.py:19: test_2[2-mod2] test2 2 mod2 PASSED ========================= 8 passed in 0.01 seconds =========================
You can see that the parametrized module-scoped modarg resource caused an ordering of test execution that lead to the fewest possible “active” resources. The finalizer for the mod1 parametrized resource was executed before the mod2 resource was setup.
Sometimes test functions do not directly need access to a fixture object. For example, tests may require to operate with an empty directory as the current working directory but otherwise do not care for the concrete directory. Here is how you can can use the standard tempfile and pytest fixtures to achieve it. We separate the creation of the fixture into a conftest.py file:
# content of conftest.py import pytest import tempfile import os @pytest.fixture() def cleandir(): newpath = tempfile.mkdtemp() os.chdir(newpath)
and declare its use in a test module via a usefixtures marker:
# content of test_setenv.py import os import pytest @pytest.mark.usefixtures("cleandir") class TestDirectoryInit: def test_cwd_starts_empty(self): assert os.listdir(os.getcwd()) ==  with open("myfile", "w") as f: f.write("hello") def test_cwd_again_starts_empty(self): assert os.listdir(os.getcwd()) == 
Due to the usefixtures marker, the cleandir fixture will be required for the execution of each test method, just as if you specified a “cleandir” function argument to each of them. Let’s run it to verify our fixture is activated and the tests pass:
$ py.test -q .. 2 passed in 0.01 seconds
You can specify multiple fixtures like this:
and you may specify fixture usage at the test module level, using a generic feature of the mark mechanism:
pytestmark = pytest.mark.usefixtures("cleandir")
Lastly you can put fixtures required by all tests in your project into an ini-file:
# content of pytest.ini [pytest] usefixtures = cleandir
Occasionally, you may want to have fixtures get invoked automatically without a usefixtures or funcargs reference. As a practical example, suppose we have a database fixture which has a begin/rollback/commit architecture and we want to automatically surround each test method by a transaction and a rollback. Here is a dummy self-contained implementation of this idea:
# content of test_db_transact.py import pytest class DB: def __init__(self): self.intransaction =  def begin(self, name): self.intransaction.append(name) def rollback(self): self.intransaction.pop() @pytest.fixture(scope="module") def db(): return DB() class TestClass: @pytest.fixture(autouse=True) def transact(self, request, db): db.begin(request.function.__name__) request.addfinalizer(db.rollback) def test_method1(self, db): assert db.intransaction == ["test_method1"] def test_method2(self, db): assert db.intransaction == ["test_method2"]
The class-level transact fixture is marked with autouse=true which implies that all test methods in the class will use this fixture without a need to state it in the test function signature or with a class-level usefixtures decorator.
If we run it, we get two passing tests:
$ py.test -q .. 2 passed in 0.01 seconds
Here is how autouse fixtures work in other scopes:
Note that the above transact fixture may very well be a fixture that you want to make available in your project without having it generally active. The canonical way to do that is to put the transact definition into a conftest.py file without using autouse:
# content of conftest.py @pytest.fixture() def transact(self, request, db): db.begin() request.addfinalizer(db.rollback)
and then e.g. have a TestClass using it by declaring the need:
@pytest.mark.usefixtures("transact") class TestClass: def test_method1(self): ...
All test methods in this TestClass will use the transaction fixture while other test classes or functions in the module will not use it unless they also add a transact reference.
If during implementing your tests you realize that you want to use a fixture function from multiple test files you can move it to a conftest.py file or even separately installable plugins without changing test code. The discovery of fixtures functions starts at test classes, then test modules, then conftest.py files and finally builtin and third party plugins.