# Basic patterns and examples

## Pass different values to a test function, depending on command line options

Suppose we want to write a test that depends on a command line option. Here is a basic pattern to achieve this:

    # content of test_sample.py
    def test_answer(cmdopt):
        if cmdopt == "type1":
            print ("first")
        elif cmdopt == "type2":
            print ("second")
        assert 0 # to see what was printed

For this to work we need to add a command line option and provide the `cmdopt` through a fixture function:

    # content of conftest.py
    import pytest

    def pytest_addoption(parser):
        parser.addoption("--cmdopt", action="store", default="type1",
            help="my option: type1 or type2")

    @pytest.fixture
    def cmdopt(request):
        return request.config.getoption("--cmdopt")

Let's run this without supplying our new option:

    $ pytest -q test_sample.py
    F                                                                    [100%]
    ================================= FAILURES =================================
    _______________________________ test_answer ________________________________

    cmdopt = 'type1'

        def test_answer(cmdopt):
            if cmdopt == "type1":
                print ("first")
            elif cmdopt == "type2":
                print ("second")
    >       assert 0 # to see what was printed
    E       assert 0

    test_sample.py:6: AssertionError
    --------------------------- Captured stdout call ---------------------------
    first
    1 failed in 0.12 seconds

And now with supplying a command line option:

    $ pytest -q --cmdopt=type2
    F                                                                    [100%]
    ================================= FAILURES =================================
    _______________________________ test_answer ________________________________

    cmdopt = 'type2'

        def test_answer(cmdopt):
            if cmdopt == "type1":
                print ("first")
            elif cmdopt == "type2":
                print ("second")
    >       assert 0 # to see what was printed
    E       assert 0

    test_sample.py:6: AssertionError
    --------------------------- Captured stdout call ---------------------------
    second
    1 failed in 0.12 seconds

You can see that the command line option arrived in our test. This completes the basic pattern. However, one often rather wants to process command line options outside of the test and rather pass in different or more complex objects.

## Dynamically adding command line options

Through `addopts` you can statically add command line options for your project. You can also dynamically modify the command line arguments before they get processed:

    # content of conftest.py
    import sys
    def pytest_cmdline_preparse(args):
        if 'xdist' in sys.modules: # pytest-xdist plugin
            import multiprocessing
            num = max(multiprocessing.cpu_count() / 2, 1)
            args[:] = ["-n", str(num)] + args

If you have the xdist plugin installed you will now always perform test runs using a number of subprocesses close to your CPU. Running in an empty directory with the above conftest.py:

    $ pytest
    =========================== test session starts ============================
    platform linux -- Python 3.x.y, pytest-3.x.y, py-1.x.y, pluggy-0.x.y
    rootdir: $REGENDOC_TMPDIR, inifile:
    collected 0 items

    ======================= no tests ran in 0.12 seconds =======================

## Control skipping of tests according to command line option

Here is a `conftest.py` file adding a `--runslow` command line option to control skipping of `pytest.mark.slow` marked tests:

    # content of conftest.py

    import pytest
    def pytest_addoption(parser):
        parser.addoption("--runslow", action="store_true",
                         default=False, help="run slow tests")

    def pytest_collection_modifyitems(config, items):
        if config.getoption("--runslow"):
            # --runslow given in cli: do not skip slow tests
            return
        skip_slow = pytest.mark.skip(reason="need --runslow option to run")
        for item in items:
            if "slow" in item.keywords:
                item.add_marker(skip_slow)

We can now write a test module like this:

    # content of test_module.py
    import pytest

    def test_func_fast():
        pass

    @pytest.mark.slow
    def test_func_slow():
        pass

and when running it will see a skipped "slow" test:

    $ pytest -rs    # "-rs" means report details on the little 's'
    =========================== test session starts ============================
    platform linux -- Python 3.x.y, pytest-3.x.y, py-1.x.y, pluggy-0.x.y
    rootdir: $REGENDOC_TMPDIR, inifile:
    collected 2 items

    test_module.py .s                                                    [100%]
    ========================= short test summary info ==========================
    SKIP [1] test_module.py:8: need --runslow option to run

    =================== 1 passed, 1 skipped in 0.12 seconds ====================

Or run it including the `slow` marked test:

    $ pytest --runslow
    =========================== test session starts ============================
    platform linux -- Python 3.x.y, pytest-3.x.y, py-1.x.y, pluggy-0.x.y
    rootdir: $REGENDOC_TMPDIR, inifile:
    collected 2 items

    test_module.py ..                                                    [100%]

    ========================= 2 passed in 0.12 seconds =========================

## Writing well integrated assertion helpers

If you have a test helper function called from a test you can use the `pytest.fail` marker to fail a test with a certain message. The test support function will not show up in the traceback if you set the `__tracebackhide__` option somewhere in the helper function. Example:

    # content of test_checkconfig.py
    import pytest
    def checkconfig(x):
        __tracebackhide__ = True
        if not hasattr(x, "config"):
            pytest.fail("not configured: %s" %(x,))

    def test_something():
        checkconfig(42)

The `__tracebackhide__` setting influences `pytest` showing of tracebacks: the `checkconfig` function will not be shown unless the `--full-trace` command line option is specified. Let's run our little function:

    $ pytest -q test_checkconfig.py
    F                                                                    [100%]
    ================================= FAILURES =================================
    ______________________________ test_something ______________________________

        def test_something():
    >       checkconfig(42)
    E       Failed: not configured: 42

    test_checkconfig.py:8: Failed
    1 failed in 0.12 seconds

If you only want to hide certain exceptions, you can set `__tracebackhide__` to a callable which gets the `ExceptionInfo` object. You can for example use this to make sure unexpected exception types aren't hidden:

    import operator
    import pytest

    class ConfigException(Exception):
        pass

    def checkconfig(x):
        __tracebackhide__ = operator.methodcaller('errisinstance', ConfigException)
        if not hasattr(x, "config"):
            raise ConfigException("not configured: %s" %(x,))

    def test_something():
        checkconfig(42)

This will avoid hiding the exception traceback on unrelated exceptions (i.e. bugs in assertion helpers).

## Detect if running from within a pytest run

Usually it is a bad idea to make application code behave differently if called from a test. But if you absolutely must find out if your application code is running from a test you can do something like this:

    # content of conftest.py

    def pytest_configure(config):
        import sys
        sys._called_from_test = True

    def pytest_unconfigure(config):
        import sys
        del sys._called_from_test

and then check for the `sys._called_from_test` flag:

    if hasattr(sys, '_called_from_test'):
        # called from within a test run
    else:
        # called "normally"

accordingly in your application. It's also a good idea to use your own application module rather than `sys` for handling flag.

## Adding info to test report header

It's easy to present extra information in a `pytest` run:

    # content of conftest.py

    def pytest_report_header(config):
        return "project deps: mylib-1.1"

which will add the string to the test header accordingly:

    $ pytest
    =========================== test session starts ============================
    platform linux -- Python 3.x.y, pytest-3.x.y, py-1.x.y, pluggy-0.x.y
    project deps: mylib-1.1
    rootdir: $REGENDOC_TMPDIR, inifile:
    collected 0 items

    ======================= no tests ran in 0.12 seconds =======================

It is also possible to return a list of strings which will be considered as several lines of information. You may consider `config.getoption('verbose')` in order to display more information if applicable:

    # content of conftest.py

    def pytest_report_header(config):
        if config.getoption('verbose') > 0:
            return ["info1: did you know that ...", "did you?"]

which will add info only when run with "--v":

    $ pytest -v
    =========================== test session starts ============================
    platform linux -- Python 3.x.y, pytest-3.x.y, py-1.x.y, pluggy-0.x.y -- $PYTHON_PREFIX/bin/python3.5
    cachedir: .cache
    info1: did you know that ...
    did you?
    rootdir: $REGENDOC_TMPDIR, inifile:
    collecting ... collected 0 items

    ======================= no tests ran in 0.12 seconds =======================

and nothing when run plainly:

    $ pytest
    =========================== test session starts ============================
    platform linux -- Python 3.x.y, pytest-3.x.y, py-1.x.y, pluggy-0.x.y
    rootdir: $REGENDOC_TMPDIR, inifile:
    collected 0 items

    ======================= no tests ran in 0.12 seconds =======================

## profiling test duration

If you have a slow running large test suite you might want to find out which tests are the slowest. Let's make an artificial test suite:

    # content of test_some_are_slow.py
    import time

    def test_funcfast():
        time.sleep(0.1)

    def test_funcslow1():
        time.sleep(0.2)

    def test_funcslow2():
        time.sleep(0.3)

Now we can profile which test functions execute the slowest:

    $ pytest --durations=3
    =========================== test session starts ============================
    platform linux -- Python 3.x.y, pytest-3.x.y, py-1.x.y, pluggy-0.x.y
    rootdir: $REGENDOC_TMPDIR, inifile:
    collected 3 items

    test_some_are_slow.py ...                                            [100%]

    ========================= slowest 3 test durations =========================
    0.31s call     test_some_are_slow.py::test_funcslow2
    0.20s call     test_some_are_slow.py::test_funcslow1
    0.17s call     test_some_are_slow.py::test_funcfast
    ========================= 3 passed in 0.12 seconds =========================

## incremental testing - test steps

Sometimes you may have a testing situation which consists of a series of test steps. If one step fails it makes no sense to execute further steps as they are all expected to fail anyway and their tracebacks add no insight. Here is a simple `conftest.py` file which introduces an `incremental` marker which is to be used on classes:

    # content of conftest.py

    import pytest

    def pytest_runtest_makereport(item, call):
        if "incremental" in item.keywords:
            if call.excinfo is not None:
                parent = item.parent
                parent._previousfailed = item

    def pytest_runtest_setup(item):
        if "incremental" in item.keywords:
            previousfailed = getattr(item.parent, "_previousfailed", None)
            if previousfailed is not None:
                pytest.xfail("previous test failed (%s)" %previousfailed.name)

These two hook implementations work together to abort incremental-marked tests in a class. Here is a test module example:

    # content of test_step.py

    import pytest

    @pytest.mark.incremental
    class TestUserHandling(object):
        def test_login(self):
            pass
        def test_modification(self):
            assert 0
        def test_deletion(self):
            pass

    def test_normal():
        pass

If we run this:

    $ pytest -rx
    =========================== test session starts ============================
    platform linux -- Python 3.x.y, pytest-3.x.y, py-1.x.y, pluggy-0.x.y
    rootdir: $REGENDOC_TMPDIR, inifile:
    collected 4 items

    test_step.py .Fx.                                                    [100%]
    ========================= short test summary info ==========================
    XFAIL test_step.py::TestUserHandling::()::test_deletion
      reason: previous test failed (test_modification)

    ================================= FAILURES =================================
    ____________________ TestUserHandling.test_modification ____________________

    self = <test_step.TestUserHandling object at 0xdeadbeef>

        def test_modification(self):
    >       assert 0
    E       assert 0

    test_step.py:9: AssertionError
    ============== 1 failed, 2 passed, 1 xfailed in 0.12 seconds ===============

We'll see that `test_deletion` was not executed because `test_modification` failed. It is reported as an "expected failure".

## Package/Directory-level fixtures (setups)

If you have nested test directories, you can have per-directory fixture scopes by placing fixture functions in a `conftest.py` file in that directory You can use all types of fixtures including autouse fixtures which are the equivalent of xUnit's setup/teardown concept. It's however recommended to have explicit fixture references in your tests or test classes rather than relying on implicitly executing setup/teardown functions, especially if they are far away from the actual tests.

Here is an example for making a `db` fixture available in a directory:

    # content of a/conftest.py
    import pytest

    class DB(object):
        pass

    @pytest.fixture(scope="session")
    def db():
        return DB()

and then a test module in that directory:

    # content of a/test_db.py
    def test_a1(db):
        assert 0, db  # to show value

another test module:

    # content of a/test_db2.py
    def test_a2(db):
        assert 0, db  # to show value

and then a module in a sister directory which will not see the `db` fixture:

    # content of b/test_error.py
    def test_root(db):  # no db here, will error out
        pass

We can run this:

    $ pytest
    =========================== test session starts ============================
    platform linux -- Python 3.x.y, pytest-3.x.y, py-1.x.y, pluggy-0.x.y
    rootdir: $REGENDOC_TMPDIR, inifile:
    collected 7 items

    test_step.py .Fx.                                                    [ 57%]
    a/test_db.py F                                                       [ 71%]
    a/test_db2.py F                                                      [ 85%]
    b/test_error.py E                                                    [100%]

    ================================== ERRORS ==================================
    _______________________ ERROR at setup of test_root ________________________
    file $REGENDOC_TMPDIR/b/test_error.py, line 1
      def test_root(db):  # no db here, will error out
    E       fixture 'db' not found
    >       available fixtures: cache, capfd, capfdbinary, caplog, capsys, capsysbinary, doctest_namespace, monkeypatch, pytestconfig, record_xml_property, recwarn, tmpdir, tmpdir_factory
    >       use 'pytest --fixtures [testpath]' for help on them.

    $REGENDOC_TMPDIR/b/test_error.py:1
    ================================= FAILURES =================================
    ____________________ TestUserHandling.test_modification ____________________

    self = <test_step.TestUserHandling object at 0xdeadbeef>

        def test_modification(self):
    >       assert 0
    E       assert 0

    test_step.py:9: AssertionError
    _________________________________ test_a1 __________________________________

    db = <conftest.DB object at 0xdeadbeef>

        def test_a1(db):
    >       assert 0, db  # to show value
    E       AssertionError: <conftest.DB object at 0xdeadbeef>
    E       assert 0

    a/test_db.py:2: AssertionError
    _________________________________ test_a2 __________________________________

    db = <conftest.DB object at 0xdeadbeef>

        def test_a2(db):
    >       assert 0, db  # to show value
    E       AssertionError: <conftest.DB object at 0xdeadbeef>
    E       assert 0

    a/test_db2.py:2: AssertionError
    ========== 3 failed, 2 passed, 1 xfailed, 1 error in 0.12 seconds ==========

The two test modules in the `a` directory see the same `db` fixture instance while the one test in the sister-directory `b` doesn't see it. We could of course also define a `db` fixture in that sister directory's `conftest.py` file. Note that each fixture is only instantiated if there is a test actually needing it (unless you use "autouse" fixture which are always executed ahead of the first test executing).

## post-process test reports / failures

If you want to postprocess test reports and need access to the executing environment you can implement a hook that gets called when the test "report" object is about to be created. Here we write out all failing test calls and also access a fixture (if it was used by the test) in case you want to query/look at it during your post processing. In our case we just write some information out to a `failures` file:

    # content of conftest.py

    import pytest
    import os.path

    @pytest.hookimpl(tryfirst=True, hookwrapper=True)
    def pytest_runtest_makereport(item, call):
        # execute all other hooks to obtain the report object
        outcome = yield
        rep = outcome.get_result()

        # we only look at actual failing test calls, not setup/teardown
        if rep.when == "call" and rep.failed:
            mode = "a" if os.path.exists("failures") else "w"
            with open("failures", mode) as f:
                # let's also access a fixture for the fun of it
                if "tmpdir" in item.fixturenames:
                    extra = " (%s)" % item.funcargs["tmpdir"]
                else:
                    extra = ""

                f.write(rep.nodeid + extra + "\n")

if you then have failing tests:

    # content of test_module.py
    def test_fail1(tmpdir):
        assert 0
    def test_fail2():
        assert 0

and run them:

    $ pytest test_module.py
    =========================== test session starts ============================
    platform linux -- Python 3.x.y, pytest-3.x.y, py-1.x.y, pluggy-0.x.y
    rootdir: $REGENDOC_TMPDIR, inifile:
    collected 2 items

    test_module.py FF                                                    [100%]

    ================================= FAILURES =================================
    ________________________________ test_fail1 ________________________________

    tmpdir = local('PYTEST_TMPDIR/test_fail10')

        def test_fail1(tmpdir):
    >       assert 0
    E       assert 0

    test_module.py:2: AssertionError
    ________________________________ test_fail2 ________________________________

        def test_fail2():
    >       assert 0
    E       assert 0

    test_module.py:4: AssertionError
    ========================= 2 failed in 0.12 seconds =========================

you will have a "failures" file which contains the failing test ids:

    $ cat failures
    test_module.py::test_fail1 (PYTEST_TMPDIR/test_fail10)
    test_module.py::test_fail2

## Making test result information available in fixtures

If you want to make test result reports available in fixture finalizers here is a little example implemented via a local plugin:

    # content of conftest.py

    import pytest

    @pytest.hookimpl(tryfirst=True, hookwrapper=True)
    def pytest_runtest_makereport(item, call):
        # execute all other hooks to obtain the report object
        outcome = yield
        rep = outcome.get_result()

        # set a report attribute for each phase of a call, which can
        # be "setup", "call", "teardown"

        setattr(item, "rep_" + rep.when, rep)

    @pytest.fixture
    def something(request):
        yield
        # request.node is an "item" because we use the default
        # "function" scope
        if request.node.rep_setup.failed:
            print ("setting up a test failed!", request.node.nodeid)
        elif request.node.rep_setup.passed:
            if request.node.rep_call.failed:
                print ("executing test failed", request.node.nodeid)

if you then have failing tests:

    # content of test_module.py

    import pytest

    @pytest.fixture
    def other():
        assert 0

    def test_setup_fails(something, other):
        pass

    def test_call_fails(something):
        assert 0

    def test_fail2():
        assert 0

and run it:

    $ pytest -s test_module.py
    =========================== test session starts ============================
    platform linux -- Python 3.x.y, pytest-3.x.y, py-1.x.y, pluggy-0.x.y
    rootdir: $REGENDOC_TMPDIR, inifile:
    collected 3 items

    test_module.py Esetting up a test failed! test_module.py::test_setup_fails
    Fexecuting test failed test_module.py::test_call_fails
    F                                                   [100%]

    ================================== ERRORS ==================================
    ____________________ ERROR at setup of test_setup_fails ____________________

        @pytest.fixture
        def other():
    >       assert 0
    E       assert 0

    test_module.py:6: AssertionError
    ================================= FAILURES =================================
    _____________________________ test_call_fails ______________________________

    something = None

        def test_call_fails(something):
    >       assert 0
    E       assert 0

    test_module.py:12: AssertionError
    ________________________________ test_fail2 ________________________________

        def test_fail2():
    >       assert 0
    E       assert 0

    test_module.py:15: AssertionError
    ==================== 2 failed, 1 error in 0.12 seconds =====================

You'll see that the fixture finalizers could use the precise reporting information.

## `PYTEST_CURRENT_TEST` environment variable

New in version 3.2.

Sometimes a test session might get stuck and there might be no easy way to figure out which test got stuck, for example if pytest was run in quiet mode (`-q`) or you don't have access to the console output. This is particularly a problem if the problem helps only sporadically, the famous "flaky" kind of tests.

`pytest` sets a `PYTEST_CURRENT_TEST` environment variable when running tests, which can be inspected by process monitoring utilities or libraries like psutil to discover which test got stuck if necessary:

    import psutil

    for pid in psutil.pids():
        environ = psutil.Process(pid).environ()
        if 'PYTEST_CURRENT_TEST' in environ:
            print(f'pytest process {pid} running: {environ["PYTEST_CURRENT_TEST"]}')

During the test session pytest will set `PYTEST_CURRENT_TEST` to the current test nodeid and the current stage, which can be `setup`, `call` and `teardown`.

For example, when running a single test function named `test_foo` from `foo_module.py`, `PYTEST_CURRENT_TEST` will be set to:

1. `foo_module.py::test_foo (setup)`
2. `foo_module.py::test_foo (call)`
3. `foo_module.py::test_foo (teardown)`

In that order.

Note

The contents of `PYTEST_CURRENT_TEST` is meant to be human readable and the actual format can be changed between releases (even bug fixes) so it shouldn't be relied on for scripting or automation.

## Freezing pytest

If you freeze your application using a tool like PyInstaller in order to distribute it to your end-users, it is a good idea to also package your test runner and run your tests using the frozen application. This way packaging errors such as dependencies not being included into the executable can be detected early while also allowing you to send test files to users so they can run them in their machines, which can be useful to obtain more information about a hard to reproduce bug.

Fortunately recent `PyInstaller` releases already have a custom hook for pytest, but if you are using another tool to freeze executables such as `cx_freeze` or `py2exe`, you can use `pytest.freeze_includes()` to obtain the full list of internal pytest modules. How to configure the tools to find the internal modules varies from tool to tool, however.

Instead of freezing the pytest runner as a separate executable, you can make your frozen program work as the pytest runner by some clever argument handling during program startup. This allows you to have a single executable, which is usually more convenient.

    # contents of app_main.py
    import sys

    if len(sys.argv) > 1 and sys.argv[1] == '--pytest':
        import pytest
        sys.exit(pytest.main(sys.argv[2:]))
    else:
        # normal application execution: at this point argv can be parsed
        # by your argument-parsing library of choice as usual
        ...

This allows you to execute tests using the frozen application with standard `pytest` command-line options:

    ./app_main --pytest --verbose --tb=long --junitxml=results.xml test-suite/