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Basic patterns and examples
==========================================================
.. _request example:
Pass different values to a test function, depending on command line options
----------------------------------------------------------------------------
.. regendoc:wipe
Suppose we want to write a test that depends on a command line option.
Here is a basic pattern to achieve this:
.. code-block:: python
# 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 :ref:`fixture function <fixture function>`:
.. code-block:: python
# 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:
.. code-block:: pytest
$ 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:
.. code-block:: pytest
$ 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
--------------------------------------------------------------
.. regendoc:wipe
Through :confval:`addopts` you can statically add command line
options for your project. You can also dynamically modify
the command line arguments before they get processed:
.. code-block:: python
# 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 <https://pypi.org/project/pytest-xdist/>`_ 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:
.. code-block:: pytest
$ pytest
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-4.x.y, py-1.x.y, pluggy-0.x.y
cachedir: $PYTHON_PREFIX/.pytest_cache
rootdir: $REGENDOC_TMPDIR
collected 0 items
======================= no tests ran in 0.12 seconds =======================
.. _`excontrolskip`:
Control skipping of tests according to command line option
--------------------------------------------------------------
.. regendoc:wipe
Here is a ``conftest.py`` file adding a ``--runslow`` command
line option to control skipping of ``pytest.mark.slow`` marked tests:
.. code-block:: python
# 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:
.. code-block:: python
# 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:
.. code-block:: pytest
$ pytest -rs # "-rs" means report details on the little 's'
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-4.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%]
========================= short test summary info ==========================
SKIPPED [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:
.. code-block:: pytest
$ pytest --runslow
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-4.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%]
========================= 2 passed in 0.12 seconds =========================
Writing well integrated assertion helpers
--------------------------------------------------
.. regendoc:wipe
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:
.. code-block:: python
# 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:
.. code-block:: pytest
$ pytest -q test_checkconfig.py
F [100%]
================================= FAILURES =================================
______________________________ test_something ______________________________
def test_something():
> checkconfig(42)
E Failed: not configured: 42
test_checkconfig.py:11: 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:
.. code-block:: python
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
--------------------------------------------------------------
.. regendoc:wipe
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:
.. code-block:: python
# 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:
.. code-block:: python
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
--------------------------------------------------------------
.. regendoc:wipe
It's easy to present extra information in a ``pytest`` run:
.. code-block:: python
# 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:
.. code-block:: pytest
$ pytest
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-4.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
======================= no tests ran in 0.12 seconds =======================
.. regendoc:wipe
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:
.. code-block:: python
# 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":
.. code-block:: pytest
$ pytest -v
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-4.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
======================= no tests ran in 0.12 seconds =======================
and nothing when run plainly:
.. code-block:: pytest
$ pytest
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-4.x.y, py-1.x.y, pluggy-0.x.y
cachedir: $PYTHON_PREFIX/.pytest_cache
rootdir: $REGENDOC_TMPDIR
collected 0 items
======================= no tests ran in 0.12 seconds =======================
profiling test duration
--------------------------
.. regendoc:wipe
.. versionadded: 2.2
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:
.. code-block:: python
# 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:
.. code-block:: pytest
$ pytest --durations=3
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-4.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%]
========================= slowest 3 test durations =========================
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
========================= 3 passed in 0.12 seconds =========================
incremental testing - test steps
---------------------------------------------------
.. regendoc:wipe
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:
.. code-block:: python
# 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:
.. code-block:: python
# 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:
.. code-block:: pytest
$ pytest -rx
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-4.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%]
================================= FAILURES =================================
____________________ TestUserHandling.test_modification ____________________
self = <test_step.TestUserHandling object at 0xdeadbeef>
def test_modification(self):
> assert 0
E assert 0
test_step.py:11: AssertionError
========================= short test summary info ==========================
XFAIL test_step.py::TestUserHandling::test_deletion
reason: previous test failed (test_modification)
============== 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 :ref:`autouse fixtures
<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:
.. code-block:: python
# 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:
.. code-block:: python
# content of a/test_db.py
def test_a1(db):
assert 0, db # to show value
another test module:
.. code-block:: python
# 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:
.. code-block:: python
# content of b/test_error.py
def test_root(db): # no db here, will error out
pass
We can run this:
.. code-block:: pytest
$ pytest
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-4.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%]
================================== 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_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
================================= FAILURES =================================
____________________ 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
========== 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:
.. code-block:: python
# 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:
.. code-block:: python
# content of test_module.py
def test_fail1(tmpdir):
assert 0
def test_fail2():
assert 0
and run them:
.. code-block:: pytest
$ pytest test_module.py
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-4.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%]
================================= 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:6: AssertionError
========================= 2 failed in 0.12 seconds =========================
you will have a "failures" file which contains the failing test ids:
.. code-block:: bash
$ cat failures
test_module.py::test_fail1 (PYTEST_TMPDIR/test_fail10)
test_module.py::test_fail2
Making test result information available in fixtures
-----------------------------------------------------------
.. regendoc:wipe
If you want to make test result reports available in fixture finalizers
here is a little example implemented via a local plugin:
.. code-block:: python
# 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:
.. code-block:: python
# 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:
.. code-block:: pytest
$ pytest -s test_module.py
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-4.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
================================== ERRORS ==================================
____________________ ERROR at setup of test_setup_fails ____________________
@pytest.fixture
def other():
> assert 0
E assert 0
test_module.py:7: AssertionError
================================= FAILURES =================================
_____________________________ 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
==================== 2 failed, 1 error in 0.12 seconds =====================
You'll see that the fixture finalizers could use the precise reporting
information.
.. _pytest current test env:
``PYTEST_CURRENT_TEST`` environment variable
--------------------------------------------
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 <https://pypi.org/project/psutil/>`_ to discover which
test got stuck if necessary:
.. code-block:: python
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
:ref:`nodeid <nodeids>` 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:
#. ``foo_module.py::test_foo (setup)``
#. ``foo_module.py::test_foo (call)``
#. ``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:
Freezing pytest
---------------
If you freeze your application using a tool like
`PyInstaller <https://pyinstaller.readthedocs.io>`_
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.
.. code-block:: python
# contents of app_main.py
import sys
import pytest_timeout # Third party plugin
if len(sys.argv) > 1 and sys.argv[1] == "--pytest":
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:
.. code-block:: bash
./app_main --pytest --verbose --tb=long --junitxml=results.xml test-suite/
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