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Changing standard (Python) test discovery

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simple hook using patterns

adding custom options

py.test supports adding of standard optparse Options. A plugin may implement the addoption hook for registering custom options:

def pytest_addoption(parser):
    parser.addoption("-M", "--myopt", action="store",
        help="specify string to set myopt")

def pytest_configure(config):
    if config.option.myopt:
        # do action based on option value

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 function argument factory:

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

def pytest_funcarg__cmdopt(request):
    return request.config.option.cmdopt

Let’s run this without supplying our new command line option:

$ py.test -q
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 0.02 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 0.02 seconds

Ok, 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. See the next example or refer to mysetup pattern: application specific test fixtures for more information on real-life examples.

generating parameters combinations, depending on command line

Let’s say we want to execute a test with different parameters and the parameter range shall be determined by a command line argument. Let’s first write a simple computation test:

# content of test_compute.py

def test_compute(param1):
    assert param1 < 4

Now we add a test configuration like this:

# content of conftest.py

def pytest_addoption(parser):
    parser.addoption("--all", action="store_true",
        help="run all combinations")

def pytest_generate_tests(metafunc):
    if 'param1' in metafunc.funcargnames:
        if metafunc.config.option.all:
            end = 5
        else:
            end = 2
        for i in range(end):
            metafunc.addcall(funcargs={'param1': i})

This means that we only run 2 tests if we do not pass --all:

$ py.test -q test_compute.py
..
2 passed in 0.01 seconds

We run only two computations, so we see two dots. let’s run the full monty:

$ py.test -q --all test_compute.py
....F
================================= FAILURES =================================
_____________________________ test_compute[4] ______________________________

param1 = 4

    def test_compute(param1):
>       assert param1 < 4
E       assert 4 < 4

test_compute.py:3: AssertionError
1 failed, 4 passed in 0.03 seconds

As expected when running the full range of param1 values we’ll get an error on the last one.

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 test_module.py -rs    # "-rs" means report details on the little 's'
=========================== test session starts ============================
platform linux2 -- Python 2.6.5 -- pytest-2.0.0.dev30
test path 1: test_module.py

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 0.02 seconds ====================

Or run it including the slow marked test:

$ py.test test_module.py --runslow
=========================== test session starts ============================
platform linux2 -- Python 2.6.5 -- pytest-2.0.0.dev30
test path 1: test_module.py

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
F
================================= FAILURES =================================
______________________________ test_something ______________________________

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

test_checkconfig.py:8: Failed
1 failed in 0.02 seconds

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 in your testing directory

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 rather use your own application module rather than sys for handling flag.