
pytest fixtures: explicit, modular, scalable
********************************************

New in version 2.0/2.3.

The general purpose of test fixtures is to provide a fixed baseline
upon which tests can reliably and repeatedly execute.   pytest-2.3
fixtures offer dramatic improvements over the classic xUnit style of
setup/teardown functions:

* fixtures have explicit names and are activated by declaring their
  use from test functions, modules, classes or whole projects.

* fixtures are implemented in a modular manner, as each fixture name
  triggers a *fixture function* which can itself easily use other
  fixtures.

* fixture management scales from simple unit to complex functional
  testing, allowing to parametrize fixtures and tests according to
  configuration and component options, or to re-use fixtures across
  class, module or whole test session scopes.

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.


Fixtures as Function arguments (funcargs)
=========================================

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-2.3.5
   collected 1 items

   test_smtpsimple.py F

   ================================= FAILURES =================================
   ________________________________ test_ehlo _________________________________

   smtp = <smtplib.SMTP instance at 0x226cc20>

       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 0.20 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:

1. pytest *finds* the "test_ehlo" because of the "test_" prefix.  The
   test function needs a function argument named "smtp".  A matching
   fixture function is discovered by looking for a fixture-marked
   function named "smtp".

2. "smtp()" is called to create an instance.

3. "test_ehlo(<SMTP instance>)" is called and fails in the last line
   of the test function.

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.

Note: 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.


Funcargs a prime example of dependency injection
================================================

When injecting fixtures to test functions, pytest-2.0 introduced the
term "funcargs" or "funcarg mechanism" which continues to be present
also in pytest-2.3 docs.  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"
probably will remain as the main way of dealing with fixtures.

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.


Working with a module-shared fixture
====================================

Fixtures requiring network access depend on connectivity and are
usually time-expensive to create.  Extending the previous example, we
can add a "scope='module'" parameter to the "@pytest.fixture"
invocation to cause the decorated "smtp" fixture function to only be
invoked once per test module.  Multiple test functions in a test
module will thus each receive the same "smtp" fixture instance.  The
next example also extracts the fixture function into a separate
"conftest.py" file so that all tests in test modules in the directory
can access the fixture function:

   # content of conftest.py
   import pytest
   import smtplib

   @pytest.fixture(scope="module")
   def smtp():
       return smtplib.SMTP("merlinux.eu")

The name of the fixture again is "smtp" and you can access its result
by listing the name "smtp" as an input parameter in any test or
fixture function (in or below the directory where "conftest.py" is
located):

   # content of test_module.py

   def test_ehlo(smtp):
       response = smtp.ehlo()
       assert response[0] == 250
       assert "merlinux" in response[1]
       assert 0  # for demo purposes

   def test_noop(smtp):
       response = smtp.noop()
       assert response[0] == 250
       assert 0  # for demo purposes

We deliberately insert failing "assert 0" statements in order to
inspect what is going on and can now run the tests:

   $ py.test test_module.py
   =========================== test session starts ============================
   platform linux2 -- Python 2.7.3 -- pytest-2.3.5
   collected 2 items

   test_module.py FF

   ================================= FAILURES =================================
   ________________________________ test_ehlo _________________________________

   smtp = <smtplib.SMTP instance at 0x18a6368>

       def test_ehlo(smtp):
           response = smtp.ehlo()
           assert response[0] == 250
           assert "merlinux" in response[1]
   >       assert 0  # for demo purposes
   E       assert 0

   test_module.py:6: AssertionError
   ________________________________ test_noop _________________________________

   smtp = <smtplib.SMTP instance at 0x18a6368>

       def test_noop(smtp):
           response = smtp.noop()
           assert response[0] == 250
   >       assert 0  # for demo purposes
   E       assert 0

   test_module.py:11: AssertionError
   ========================= 2 failed in 0.26 seconds =========================

You see the two "assert 0" failing and more importantly you can also
see that the same (module-scoped) "smtp" object was passed into the
two test functions because pytest shows the incoming argument values
in the traceback.  As a result, the two test functions using "smtp"
run as quick as a single one because they reuse the same instance.

If you decide that you rather want to have a session-scoped "smtp"
instance, you can simply declare it:

   @pytest.fixture(scope=``session``)
   def smtp(...):
       # the returned fixture value will be shared for
       # all tests needing it


Fixtures can interact with the requesting test context
======================================================

Fixture functions can themselves use other fixtures by naming them as
an input argument just like test functions do, see *Modularity: using
fixtures from a fixture function*.  Moreover, pytest provides a
builtin "request" object, which fixture functions can use to
introspect the function, class or module for which they are invoked or
to register finalizing (cleanup) functions which are called when the
last test finished execution.

Further extending the previous "smtp" fixture example, let's read an
optional server URL from the module namespace and register a finalizer
that closes the smtp connection after the last test in a module
finished execution:

   # 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" % smtp)
           smtp.close()
       request.addfinalizer(fin)
       return smtp

The registered "fin" function will be called when the last test using
it has executed:

   $ py.test -s -q --tb=no
   FF
   finalizing <smtplib.SMTP instance at 0x1e10248>

We see that the "smtp" instance is finalized after the two tests using
it tests executed.  If we had specified "scope='function'" then
fixture setup and cleanup would occur around each single test. Note
that either case the test module itself does not need to change!

Let's quickly create another test module that actually sets the server
URL and has a test to verify the fixture picks it up:

   # content of test_anothersmtp.py

   smtpserver = "mail.python.org"  # will be read by smtp fixture

   def test_showhelo(smtp):
       assert 0, smtp.helo()

Running it:

   $ 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')


Parametrizing a fixture
=======================

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 0x1b38a28>

       def test_ehlo(smtp):
           response = smtp.ehlo()
           assert response[0] == 250
           assert "merlinux" in response[1]
   >       assert 0  # for demo purposes
   E       assert 0

   test_module.py:6: AssertionError
   __________________________ test_noop[merlinux.eu] __________________________

   smtp = <smtplib.SMTP instance at 0x1b38a28>

       def test_noop(smtp):
           response = smtp.noop()
           assert response[0] == 250
   >       assert 0  # for demo purposes
   E       assert 0

   test_module.py:11: AssertionError
   ________________________ test_ehlo[mail.python.org] ________________________

   smtp = <smtplib.SMTP instance at 0x1b496c8>

       def test_ehlo(smtp):
           response = smtp.ehlo()
           assert response[0] == 250
   >       assert "merlinux" in response[1]
   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 0x1b496c8>

       def test_noop(smtp):
           response = smtp.noop()
           assert response[0] == 250
   >       assert 0  # for demo purposes
   E       assert 0

   test_module.py:11: AssertionError

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.


Modularity: using fixtures from a fixture function
==================================================

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-2.3.5 -- /home/hpk/p/pytest/.tox/regen/bin/python
   collecting ... collected 2 items

   test_appsetup.py:12: test_smtp_exists[merlinux.eu] PASSED
   test_appsetup.py:12: test_smtp_exists[mail.python.org] PASSED

   ========================= 2 passed in 5.38 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.


Automatic grouping of tests by fixture instances
================================================

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", param
       request.addfinalizer(fin)
       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-2.3.5 -- /home/hpk/p/pytest/.tox/regen/bin/python
   collecting ... collected 8 items

   test_module.py:16: test_0[1] PASSED
   test_module.py:16: test_0[2] PASSED
   test_module.py:18: test_1[mod1] PASSED
   test_module.py:20: test_2[1-mod1] PASSED
   test_module.py:20: test_2[2-mod1] PASSED
   test_module.py:18: test_1[mod2] PASSED
   test_module.py:20: test_2[1-mod2] PASSED
   test_module.py:20: test_2[2-mod2] PASSED

   ========================= 8 passed in 0.01 seconds =========================
     test0 1
     test0 2
   create mod1
     test1 mod1
     test2 1 mod1
     test2 2 mod1
   fin mod1
   create mod2
     test1 mod2
     test2 1 mod2
     test2 2 mod2
   fin mod2

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.


using fixtures from classes, modules or projects
================================================

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
   ..

You can specify multiple fixtures like this:

   @pytest.mark.usefixtures("cleandir", "anotherfixture")

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


autouse fixtures (xUnit setup on steroids)
==========================================

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
   ..

Here is how autouse fixtures work in other scopes:

* if an autouse fixture is defined in a test module, all its test
  functions automatically use it.

* if an autouse fixture is defined in a conftest.py file then all
  tests in all test modules belows its directory will invoke the
  fixture.

* lastly, and **please use that with care**: if you define an autouse
  fixture in a plugin, it will be invoked for all tests in all
  projects where the plugin is installed.  This can be useful if a
  fixture only anyway works in the presence of certain settings e. g.
  in the ini-file.  Such a global fixture should always quickly
  determine if it should do any work and avoid expensive imports or
  computation otherwise.

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.


Shifting (visibility of) fixture functions
==========================================

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.
