quicklinks

home TOC/contents
install changelog
examples customize
issues[bb] contact

Table Of Contents

Previous topic

Demo of Python failure reports with py.test

Next topic

Parametrizing tests

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.option.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

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

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 -- pytest-2.3.2
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.getvalue("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 -- pytest-2.3.2
collected 2 items

test_module.py .s
========================= short test summary info ==========================
SKIP [1] /tmp/doc-exec-104/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 -- pytest-2.3.2
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 py.test 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

Detect if running from within a py.test 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 py.test 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 -- pytest-2.3.2
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 -- pytest-2.3.2 -- /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 -- pytest-2.3.2
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 -- pytest-2.3.2
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 call     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 -- pytest-2.3.2
collected 4 items

test_step.py .Fx.

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

self = <test_step.TestUserHandling instance at 0x269efc8>

    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”.