pytest / doc / en / funcarg_compare.txt

V5: changes to new resource/setup facilities
=============================================================

**Target audience**: Reading this document requires basic knowledge of 
python testing, xUnit setup methods and the basic pytest funcarg mechanism,
see http://pytest.org/latest/funcargs.html 


**Changes**: This V5 draft is based on incorporating and thinking about
feedback on previous versions provided by Floris Bruynooghe, Carl Meyer,
Ronny Pfannschmidt and Samuele Pedroni.  I have also now implemented it
which triggered a number of refinements as well.  The main changes are:

* Collapse funcarg factory decorators into a single "@resource" one.
  You can specify scopes and params with it.  When using the decorator
  the "pytest_funcarg__" prefix is not allowed and the old-style 
  ``request`` object cannot be received.

* funcarg resource factories can now use funcargs themselves

* Drop setup/directory scope from this draft

* introduce a new @setup decorator similar to the @funcarg one
  except that setup-markers cannot define parametriation themselves.
  Instead they can easily depend on a parametrized funcarg (which
  must not be visible at test function signatures).

* drop consideration of setup_X support for funcargs because
  it is less flexible and probably causes more implementation
  troubles than the current @setup approach which can share
  a lot of logic with the @funcarg one.

* tests are grouped by parametrized funcargs and according to scope
  (sounds like a small thing but is a big deal)

* make the new-style funcargs/setup use a "testcontext" object
  which offers test context info and addfinalizer() methods but no
  getfuncargvalue()/cached_setup()/applymarker anymore.  Reason
  being that getfuncargvalue()/cached_setup breaks other features
  such as sorting by resource-scope and parametrization


.. currentmodule:: _pytest

Shortcomings of the previous pytest_funcarg__ mechanism
---------------------------------------------------------

The previous funcarg mechanism calls a factory each time a
funcarg for a test function is testcontexted.  If a factory wants
t re-use a resource across different scopes, it often used 
the ``testcontext.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:

1. Scoping funcarg resource creation is not straight forward, instead one must
   understand the intricate cached_setup() method mechanics.

2. parametrizing the "db" resource is not straight forward: 
   you need to apply a "parametrize" decorator or implement a
   :py:func:`~hookspec.pytest_generate_tests` hook 
   calling :py:func:`~python.Metafunc.parametrize` which
   performs parametrization at the places where the resource 
   is used.  Moreover, you need to modify the factory to use an 
   ``extrakey`` parameter containing ``request.param`` to the 
   :py:func:`~python.Request.cached_setup` call.

3. 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.

4. there is no way how you can make use of funcarg factories
   in xUnit setup methods.

5. A non-parametrized funcarg factory 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
new facilities.

Direct scoping of funcarg factories
--------------------------------------------------------

Instead of calling cached_setup(), you can decorate your factory
to state its scope::

    @pytest.mark.resource(scope="session")
    def db(testcontext):
        # factory will only be invoked once per session - 
        db = DataBase()
        testcontext.addfinalizer(db.destroy)  # destroy when session is finished
        return db

This factory implementation does not need to call ``cached_setup()`` anymore
because it will only be invoked once per session.  Moreover, the 
``testcontext.addfinalizer()`` registers a finalizer according to the specified
resource scope on which the factory function is operating.  With this new
scoping, the still existing ``cached_setup()`` should be much less used
but will remain for compatibility reasons and for the case where you
still want to have your factory get called on a per-item basis.


Direct parametrization of funcarg resource factories 
----------------------------------------------------------

.. note:: Implemented

Previously, funcarg factories could not directly cause parametrization.
You needed to specify a ``@parametrize`` or implement a ``pytest_generate_tests`` hook to perform parametrization, i.e. calling a test multiple times
with different value sets.  pytest-2.X introduces a decorator for use
on the factory itself::

    @pytest.mark.resource(params=["mysql", "pg"])
    def pytest_funcarg__db(testcontext):
        ...

Here the factory will be invoked twice (with the respective "mysql" 
and "pg" values set as ``testcontext.param`` attributes) and 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
``testcontext.param`` are already the parametrization attribute for test 
functions/classes were parametrized via
:py:func:`~_pytest.python.Metafunc.parametrize(indirect=True)` calls.

Of course it's perfectly fine to combine parametrization and scoping::

    @pytest.mark.resource(scope="session", params=["mysql", "pg"])
    def pytest_funcarg__db(testcontext):
        if testcontext.param == "mysql":
            db = MySQL()
        elif testcontext.param == "pg":
            db = PG()
        testcontext.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.


No ``pytest_funcarg__`` prefix when using @resource decorator
-------------------------------------------------------------------


.. note:: Implemented

When using the ``@funcarg`` decorator the name of the function
does not need to (and in fact cannot) use the ``pytest_funcarg__``
naming::

    @pytest.mark.resource
    def db(testcontext):
        ...

The name under which the funcarg resource can be requested is ``db``.

You can also use the "old" non-decorator way of specifying funcarg factories 
aka::

    def pytest_funcarg__db(testcontext):
        ...

It is recommended to use the resource decorator, however.


solving per-session setup / the new @setup marker
--------------------------------------------------------------

.. note:: Implemented, at least working for basic situations.

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:

1. 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 slave processes.  

2. if you only perform a collection (with "--collectonly") 
   resource-setup will still be executed.  

3. 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 is today not easy to define a scoped setup from plugins or
conftest files other than to implement a ``pytest_runtest_setup()`` hook
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.X introduces a new "@pytest.mark.setup" marker which takes
an optional "scope" parameter.  

See :ref:`new_setup` for examples.

funcarg and setup discovery now happens at collection time
---------------------------------------------------------------------

.. note:: 
    Partially implemented - collectonly shows no extra information however.

pytest-2.X takes care to discover funcarg factories and @setup methods
at collection time.  This is more efficient especially for large test suites. 
Moreover, a call to "py.test --collectonly" should be able to show
a lot of setup-information and thus presents a nice method to get an
overview of resource management in your project.
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