pytest-2.3: reasoning for fixture/funcarg evolution¶
Target audience: Reading this document requires basic knowledge of python testing, xUnit setup methods and the (previous) basic pytest funcarg mechanism, see https://docs.pytest.org/en/stable/historical-notes.html#funcargs-and-pytest-funcarg. If you are new to pytest, then you can simply ignore this section and read the other sections.
Shortcomings of the previous
The pre pytest-2.3 funcarg mechanism calls a factory each time a
funcarg for a test function is required. If a factory wants to
re-use a resource across different scopes, it often used
request.cached_setup() helper to manage caching of
resources. Here is a basic example how we could implement
a per-session Database object:
# content of conftest.py class Database: def __init__(self): print("database instance created") def destroy(self): print("database instance destroyed") def pytest_funcarg__db(request): return request.cached_setup( setup=DataBase, teardown=lambda db: db.destroy, scope="session" )
There are several limitations and difficulties with this approach:
Scoping funcarg resource creation is not straight forward, instead one must understand the intricate cached_setup() method mechanics.
parametrizing the “db” resource is not straight forward: you need to apply a “parametrize” decorator or implement a
parametrize()which performs parametrization at the places where the resource is used. Moreover, you need to modify the factory to use an
Multiple parametrized session-scoped resources will be active at the same time, making it hard for them to affect global state of the application under test.
there is no way how you can make use of funcarg factories in xUnit setup methods.
A non-parametrized fixture function cannot use a parametrized funcarg resource if it isn’t stated in the test function signature.
All of these limitations are addressed with pytest-2.3 and its improved fixture mechanism.
Direct scoping of fixture/funcarg factories¶
Instead of calling cached_setup() with a cache scope, you can use the @pytest.fixture decorator and directly state the scope:
@pytest.fixture(scope="session") def db(request): # factory will only be invoked once per session - db = DataBase() request.addfinalizer(db.destroy) # destroy when session is finished return db
This factory implementation does not need to call
because it will only be invoked once per session. Moreover, the
request.addfinalizer() registers a finalizer according to the specified
resource scope on which the factory function is operating.
Direct parametrization of funcarg resource factories¶
Previously, funcarg factories could not directly cause parametrization.
You needed to specify a
@parametrize decorator on your test function
or implement a
pytest_generate_tests hook to perform
parametrization, i.e. calling a test multiple times with different value
sets. pytest-2.3 introduces a decorator for use on the factory itself:
@pytest.fixture(params=["mysql", "pg"]) def db(request): ... # use request.param
Here the factory will be invoked twice (with the respective “mysql”
and “pg” values set as
request.param attributes) and all of
the tests requiring “db” will run twice as well. The “mysql” and
“pg” values will also be used for reporting the test-invocation variants.
This new way of parametrizing funcarg factories should in many cases
allow to re-use already written factories because effectively
request.param was already used when test functions/classes were
Of course it’s perfectly fine to combine parametrization and scoping:
@pytest.fixture(scope="session", params=["mysql", "pg"]) def db(request): if request.param == "mysql": db = MySQL() elif request.param == "pg": db = PG() request.addfinalizer(db.destroy) # destroy when session is finished return db
This would execute all tests requiring the per-session “db” resource twice, receiving the values created by the two respective invocations to the factory function.
pytest_funcarg__ prefix when using @fixture decorator¶
When using the
@fixture decorator the name of the function
denotes the name under which the resource can be accessed as a function
@pytest.fixture() def db(request): ...
The name under which the funcarg resource can be requested is
You can still use the “old” non-decorator way of specifying funcarg factories aka:
def pytest_funcarg__db(request): ...
But it is then not possible to define scoping and parametrization. It is thus recommended to use the factory decorator.
solving per-session setup / autouse fixtures¶
pytest for a long time offered a pytest_configure and a pytest_sessionstart hook which are often used to setup global resources. This suffers from several problems:
in distributed testing the master process would setup test resources that are never needed because it only co-ordinates the test run activities of the worker processes.
if you only perform a collection (with “–collect-only”) resource-setup will still be executed.
If a pytest_sessionstart is contained in some subdirectories conftest.py file, it will not be called. This stems from the fact that this hook is actually used for reporting, in particular the test-header with platform/custom information.
Moreover, it was not easy to define a scoped setup from plugins or
conftest files other than to implement a
and caring for scoping/caching yourself. And it’s virtually impossible
to do this with parametrization as
pytest_runtest_setup() is called
during test execution and parametrization happens at collection time.
It follows that pytest_configure/session/runtest_setup are often not appropriate for implementing common fixture needs. Therefore, pytest-2.3 introduces Autouse fixtures (xUnit setup on steroids) which fully integrate with the generic fixture mechanism and obsolete many prior uses of pytest hooks.
funcargs/fixture discovery now happens at collection time¶
Since pytest-2.3, discovery of fixture/funcarg factories are taken care of at collection time. This is more efficient especially for large test suites. Moreover, a call to “pytest –collect-only” should be able to in the future show a lot of setup-information and thus presents a nice method to get an overview of fixture management in your project.
Conclusion and compatibility notes¶
funcargs were originally introduced to pytest-2.0. In pytest-2.3 the mechanism was extended and refined and is now described as fixtures:
previously funcarg factories were specified with a special
pytest_funcarg__NAMEprefix instead of using the
Factories received a
requestobject which managed caching through
request.cached_setup()calls and allowed using other funcargs via
request.getfuncargvalue()calls. These intricate APIs made it hard to do proper parametrization and implement resource caching. The new
pytest.fixture()decorator allows to declare the scope and let pytest figure things out for you.
if you used parametrization and funcarg factories which made use of
request.cached_setup()it is recommended to invest a few minutes and simplify your fixture function code to use the Fixtures as Function arguments decorator instead. This will also allow to take advantage of the automatic per-resource grouping of tests.