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Pytest-Basic patterns and examples

How to change command line options defaults

It can be tedious to type the same series of command line options every
time you use pytest. For example, if you always want to see detailed
info on skipped and xfailed tests, as well as have terser “dot”
progress output, you can write it into a configuration file:

# content of pytest.ini
[pytest]
addopts = -ra -q

Alternatively, you can set a PYTEST_ADDOPTS environment variable to
add command line options while the environment is in use:

export PYTEST_ADDOPTS="-v"

Here’s how the command-line is built in the presence of addopts or
the environment variable:

<pytest.ini:addopts> $PYTEST_ADDOPTS <extra command-line arguments>

So if the user executes in the command-line:

pytest -m slow

The actual command line executed is:

pytest -ra -q -v -m slow

Note that as usual for other command-line applications, in case of
conflicting options the last one wins, so the example above will show
verbose output because -v overwrites -q.

Pass different values to a test function, depending on command line options {#request example}

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):
        print("first")
        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 <fixture function>{.interpreted-text role=“ref”}:

# 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%]
_______________________________ test_answer ________________________________

cmdopt = 'type1'

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

test_sample.py:6: AssertionError
--------------------------- Captured stdout call ---------------------------
first
FAILED test_sample.py::test_answer - assert 0
1 failed in 0.12s

And now with supplying a command line option:

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

cmdopt = 'type2'

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

test_sample.py:6: AssertionError
--------------------------- Captured stdout call ---------------------------
second
FAILED test_sample.py::test_answer - assert 0
1 failed in 0.12s

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{.interpreted-text role=“confval”} you can statically
add command line options for your project. You can also dynamically
modify the command line arguments before they get processed:

# setuptools plugin
import sys


def pytest_load_initial_conftests(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
platform linux -- Python 3.x.y, pytest-6.x.y, py-1.x.y, pluggy-0.x.y
cachedir: $PYTHON_PREFIX/.pytest_cache
rootdir: $REGENDOC_TMPDIR
collected 0 items

Control skipping of tests according to command line option {#excontrolskip}

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_configure(config):
    config.addinivalue_line("markers", "slow: mark test as slow to run")


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'
platform linux -- Python 3.x.y, pytest-6.x.y, py-1.x.y, pluggy-0.x.y
cachedir: $PYTHON_PREFIX/.pytest_cache
rootdir: $REGENDOC_TMPDIR
collected 2 items

test_module.py .s                                                    [100%]

SKIPPED [1] test_module.py:8: need --runslow option to run

Or run it including the slow marked test:

$ pytest --runslow
platform linux -- Python 3.x.y, pytest-6.x.y, py-1.x.y, pluggy-0.x.y
cachedir: $PYTHON_PREFIX/.pytest_cache
rootdir: $REGENDOC_TMPDIR
collected 2 items

test_module.py ..                                                    [100%]

Writing well integrated assertion helpers {#tracebackhide}

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: {}".format(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%]
______________________________ test_something ______________________________

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

test_checkconfig.py:11: Failed
FAILED test_checkconfig.py::test_something - Failed: not configured: 42
1 failed in 0.12s

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: {}".format(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 your_module.py


_called_from_test = False
# content of conftest.py


def pytest_configure(config):
    your_module._called_from_test = True

and then check for the your_module._called_from_test flag:

if your_module._called_from_test:
    # called from within a test run
    ...
else:
    # called "normally"
    ...

accordingly in your application.

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
platform linux -- Python 3.x.y, pytest-6.x.y, py-1.x.y, pluggy-0.x.y
cachedir: $PYTHON_PREFIX/.pytest_cache
project deps: mylib-1.1
rootdir: $REGENDOC_TMPDIR
collected 0 items

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
platform linux -- Python 3.x.y, pytest-6.x.y, py-1.x.y, pluggy-0.x.y -- $PYTHON_PREFIX/bin/python
cachedir: $PYTHON_PREFIX/.pytest_cache
info1: did you know that ...
did you?
rootdir: $REGENDOC_TMPDIR
collecting ... collected 0 items

and nothing when run plainly:

$ pytest
platform linux -- Python 3.x.y, pytest-6.x.y, py-1.x.y, pluggy-0.x.y
cachedir: $PYTHON_PREFIX/.pytest_cache
rootdir: $REGENDOC_TMPDIR
collected 0 items

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
platform linux -- Python 3.x.y, pytest-6.x.y, py-1.x.y, pluggy-0.x.y
cachedir: $PYTHON_PREFIX/.pytest_cache
rootdir: $REGENDOC_TMPDIR
collected 3 items

test_some_are_slow.py ...                                            [100%]

0.30s call     test_some_are_slow.py::test_funcslow2
0.20s call     test_some_are_slow.py::test_funcslow1
0.10s call     test_some_are_slow.py::test_funcfast

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

from typing import Dict, Tuple
import pytest

# store history of failures per test class name and per index in parametrize (if parametrize used)
_test_failed_incremental: Dict[str, Dict[Tuple[int, ...], str]] = {}


def pytest_runtest_makereport(item, call):
    if "incremental" in item.keywords:
        # incremental marker is used
        if call.excinfo is not None:
            # the test has failed
            # retrieve the class name of the test
            cls_name = str(item.cls)
            # retrieve the index of the test (if parametrize is used in combination with incremental)
            parametrize_index = (
                tuple(item.callspec.indices.values())
                if hasattr(item, "callspec")
                else ()
            )
            # retrieve the name of the test function
            test_name = item.originalname or item.name
            # store in _test_failed_incremental the original name of the failed test
            _test_failed_incremental.setdefault(cls_name, {}).setdefault(
                parametrize_index, test_name
            )


def pytest_runtest_setup(item):
    if "incremental" in item.keywords:
        # retrieve the class name of the test
        cls_name = str(item.cls)
        # check if a previous test has failed for this class
        if cls_name in _test_failed_incremental:
            # retrieve the index of the test (if parametrize is used in combination with incremental)
            parametrize_index = (
                tuple(item.callspec.indices.values())
                if hasattr(item, "callspec")
                else ()
            )
            # retrieve the name of the first test function to fail for this class name and index
            test_name = _test_failed_incremental[cls_name].get(parametrize_index, None)
            # if name found, test has failed for the combination of class name & test name
            if test_name is not None:
                pytest.xfail("previous test failed ({})".format(test_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:
    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
platform linux -- Python 3.x.y, pytest-6.x.y, py-1.x.y, pluggy-0.x.y
cachedir: $PYTHON_PREFIX/.pytest_cache
rootdir: $REGENDOC_TMPDIR
collected 4 items

test_step.py .Fx.                                                    [100%]

____________________ TestUserHandling.test_modification ____________________

self = <test_step.TestUserHandling object at 0xdeadbeef>

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

test_step.py:11: AssertionError
XFAIL test_step.py::TestUserHandling::test_deletion
  reason: previous test failed (test_modification)

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 <autouse fixtures>{.interpreted-text role=“ref”} 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:
    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
platform linux -- Python 3.x.y, pytest-6.x.y, py-1.x.y, pluggy-0.x.y
cachedir: $PYTHON_PREFIX/.pytest_cache
rootdir: $REGENDOC_TMPDIR
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%]

_______________________ 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_property, record_testsuite_property, record_xml_attribute, recwarn, tmp_path, tmp_path_factory, tmpdir, tmpdir_factory
>       use 'pytest --fixtures [testpath]' for help on them.

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

self = <test_step.TestUserHandling object at 0xdeadbeef>

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

test_step.py:11: 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
FAILED test_step.py::TestUserHandling::test_modification - assert 0
FAILED a/test_db.py::test_a1 - AssertionError: <conftest.DB object at 0x7...
FAILED a/test_db2.py::test_a2 - AssertionError: <conftest.DB object at 0x...
ERROR b/test_error.py::test_root

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
        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 = " ({})".format(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
platform linux -- Python 3.x.y, pytest-6.x.y, py-1.x.y, pluggy-0.x.y
cachedir: $PYTHON_PREFIX/.pytest_cache
rootdir: $REGENDOC_TMPDIR
collected 2 items

test_module.py FF                                                    [100%]

________________________________ 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:6: AssertionError
FAILED test_module.py::test_fail1 - assert 0
FAILED test_module.py::test_fail2 - assert 0

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
platform linux -- Python 3.x.y, pytest-6.x.y, py-1.x.y, pluggy-0.x.y
cachedir: $PYTHON_PREFIX/.pytest_cache
rootdir: $REGENDOC_TMPDIR
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

____________________ ERROR at setup of test_setup_fails ____________________

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

test_module.py:7: AssertionError
_____________________________ test_call_fails ______________________________

something = None

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

test_module.py:15: AssertionError
________________________________ test_fail2 ________________________________

    def test_fail2():
>       assert 0
E       assert 0

test_module.py:19: AssertionError
FAILED test_module.py::test_call_fails - assert 0
FAILED test_module.py::test_fail2 - assert 0
ERROR test_module.py::test_setup_fails - assert 0

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

PYTEST_CURRENT_TEST environment variable {#pytest current test env}

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 happens only sporadically, the
famous “flaky” kind of tests.

pytest sets the PYTEST_CURRENT_TEST{.interpreted-text role=“envvar”}
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 <nodeids>{.interpreted-text role=“ref”} and the
current stage, which can be setup, call, or 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}
::: {.title}
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. Please note that
the mechanism for plugin discovery used by pytest (setupttools entry
points) doesn’t work with frozen executables so pytest can’t find any
third party plugins automatically. To include third party plugins like
pytest-timeout they must be imported explicitly and passed on to
pytest.main.

# contents of app_main.py
import sys
import pytest_timeout  # Third party plugin

    import pytest

    sys.exit(pytest.main(sys.argv[2:], plugins=[pytest_timeout]))
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/