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 how 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:

$ py.test -q test_sample.py
F
================================= 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 ------------------------------
first
1 failed in 0.01 seconds

And now with supplying a command line option:

$ py.test -q --cmdopt=type2
F
================================= 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 ------------------------------
second
1 failed in 0.01 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:

$ py.test
=========================== test session starts ============================
platform linux2 -- Python 2.7.3 -- py-1.4.20 -- pytest-2.5.2
collected 0 items

=============================  in 0.00 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 slow marked tests:

# content of conftest.py

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

def pytest_runtest_setup(item):
    if 'slow' in item.keywords and not item.config.getoption("--runslow"):
        pytest.skip("need --runslow option to run")

We can now write a test module like this:

# content of test_module.py

import pytest
slow = pytest.mark.slow

def test_func_fast():
    pass

@slow
def test_func_slow():
    pass

and when running it will see a skipped “slow” test:

$ py.test -rs    # "-rs" means report details on the little 's'
=========================== test session starts ============================
platform linux2 -- Python 2.7.3 -- py-1.4.20 -- pytest-2.5.2
collected 2 items

test_module.py .s
========================= short test summary info ==========================
SKIP [1] /tmp/doc-exec-70/conftest.py:9: need --runslow option to run

=================== 1 passed, 1 skipped in 0.01 seconds ====================

Or run it including the slow marked test:

$ py.test --runslow
=========================== test session starts ============================
platform linux2 -- Python 2.7.3 -- py-1.4.20 -- pytest-2.5.2
collected 2 items

test_module.py ..

========================= 2 passed in 0.01 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 --fulltrace command line option is specified. Let’s run our little function:

$ py.test -q test_checkconfig.py
F
================================= FAILURES =================================
______________________________ test_something ______________________________

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

test_checkconfig.py:8: Failed
1 failed in 0.01 seconds

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):
    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:

$ py.test
=========================== test session starts ============================
platform linux2 -- Python 2.7.3 -- py-1.4.20 -- pytest-2.5.2
project deps: mylib-1.1
collected 0 items

=============================  in 0.00 seconds =============================

You can also return a list of strings which will be considered as several lines of information. You can of course also make the amount of reporting information on e.g. the value of config.option.verbose so that you present more information appropriately:

# content of conftest.py

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

which will add info only when run with “–v”:

$ py.test -v
=========================== test session starts ============================
platform linux2 -- Python 2.7.3 -- py-1.4.20 -- pytest-2.5.2 -- /home/hpk/p/pytest/.tox/regen/bin/python
info1: did you know that ...
did you?
collecting ... collected 0 items

=============================  in 0.00 seconds =============================

and nothing when run plainly:

$ py.test
=========================== test session starts ============================
platform linux2 -- Python 2.7.3 -- py-1.4.20 -- pytest-2.5.2
collected 0 items

=============================  in 0.00 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 artifical test suite:

# content of test_some_are_slow.py

import time

def test_funcfast():
    pass

def test_funcslow1():
    time.sleep(0.1)

def test_funcslow2():
    time.sleep(0.2)

Now we can profile which test functions execute the slowest:

$ py.test --durations=3
=========================== test session starts ============================
platform linux2 -- Python 2.7.3 -- py-1.4.20 -- pytest-2.5.2
collected 3 items

test_some_are_slow.py ...

========================= slowest 3 test durations =========================
0.20s call     test_some_are_slow.py::test_funcslow2
0.10s call     test_some_are_slow.py::test_funcslow1
0.00s setup    test_some_are_slow.py::test_funcfast
========================= 3 passed in 0.31 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:
    def test_login(self):
        pass
    def test_modification(self):
        assert 0
    def test_deletion(self):
        pass

def test_normal():
    pass

If we run this:

$ py.test -rx
=========================== test session starts ============================
platform linux2 -- Python 2.7.3 -- py-1.4.20 -- pytest-2.5.2
collected 4 items

test_step.py .Fx.

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

self = <test_step.TestUserHandling instance at 0x2768dd0>

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

test_step.py:9: 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 0.01 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 implicitely executing setup/teardown functions, especially if they are far away from the actual tests.

Here is a 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:

$ py.test
=========================== test session starts ============================
platform linux2 -- Python 2.7.3 -- py-1.4.20 -- pytest-2.5.2
collected 7 items

test_step.py .Fx.
a/test_db.py F
a/test_db2.py F
b/test_error.py E

================================== ERRORS ==================================
_______________________ ERROR at setup of test_root ________________________
file /tmp/doc-exec-70/b/test_error.py, line 1
  def test_root(db):  # no db here, will error out
        fixture 'db' not found
        available fixtures: pytestconfig, capfd, monkeypatch, capsys, recwarn, tmpdir
        use 'py.test --fixtures [testpath]' for help on them.

/tmp/doc-exec-70/b/test_error.py:1
================================= FAILURES =================================
____________________ TestUserHandling.test_modification ____________________

self = <test_step.TestUserHandling instance at 0x238fdd0>

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

test_step.py:9: AssertionError
_________________________________ test_a1 __________________________________

db = <conftest.DB instance at 0x23f9998>

    def test_a1(db):
>       assert 0, db  # to show value
E       AssertionError: <conftest.DB instance at 0x23f9998>

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

db = <conftest.DB instance at 0x23f9998>

    def test_a2(db):
>       assert 0, db  # to show value
E       AssertionError: <conftest.DB instance at 0x23f9998>

a/test_db2.py:2: AssertionError
========== 3 failed, 2 passed, 1 xfailed, 1 error in 0.03 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 informations out to a failures file:

# content of conftest.py

import pytest
import os.path

@pytest.mark.tryfirst
def pytest_runtest_makereport(item, call, __multicall__):
    # execute all other hooks to obtain the report object
    rep = __multicall__.execute()

    # 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.funcargs:
                extra = " (%s)" % item.funcargs["tmpdir"]
            else:
                extra = ""

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

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:

$ py.test test_module.py
=========================== test session starts ============================
platform linux2 -- Python 2.7.3 -- py-1.4.20 -- pytest-2.5.2
collected 2 items

test_module.py FF

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

tmpdir = local('/tmp/pytest-1012/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 0.01 seconds =========================

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

$ cat failures
test_module.py::test_fail1 (/tmp/pytest-1012/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.mark.tryfirst
def pytest_runtest_makereport(item, call, __multicall__):
    # execute all other hooks to obtain the report object
    rep = __multicall__.execute()

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

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


@pytest.fixture
def something(request):
    def fin():
        # 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
    request.addfinalizer(fin)

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:

$ py.test -s test_module.py
=========================== test session starts ============================
platform linux2 -- Python 2.7.3 -- py-1.4.20 -- pytest-2.5.2
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: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 0.01 seconds =====================

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