pytest-cache: working with cross-testrun state

Usage

install via:

pip install pytest-cache

after which other plugins can access a new config.cache object which helps sharing values between py.test invocations.

The plugin also introduces a new --lf option to rerun the last failing tests and a --clearcache option to remove cache contents ahead of a test run.

The new –lf (rerun last failing) option

The cache plugin introduces the --lf option to py.test which alows to rerun all test failures of a previous test run. If not tests failed, all tests will be run as normal. It is thus perfectly fine to always pass --lf.

As an example, let’s create 50 test invocation of which only 2 fail:

# content of test_50.py
import pytest

@pytest.mark.parametrize("i", range(50))
def test_num(i):
    if i in (17,25):
       pytest.fail("bad luck")

If you run this for the first time you will see two failures:

$ py.test -q
collecting ... collected 50 items
.................F.......F........................
================================= FAILURES =================================
_______________________________ test_num[17] _______________________________

i = 17

    @pytest.mark.parametrize("i", range(50))
    def test_num(i):
        if i in (17,25):
>          pytest.fail("bad luck")
E          Failed: bad luck

test_50.py:6: Failed
_______________________________ test_num[25] _______________________________

i = 25

    @pytest.mark.parametrize("i", range(50))
    def test_num(i):
        if i in (17,25):
>          pytest.fail("bad luck")
E          Failed: bad luck

test_50.py:6: Failed
2 failed, 48 passed in 0.06 seconds

If you then run it with --lf you will re-run the last two failures:

$ py.test -q --lf
collecting ... collected 50 items
FF
================================= FAILURES =================================
_______________________________ test_num[17] _______________________________

i = 17

    @pytest.mark.parametrize("i", range(50))
    def test_num(i):
        if i in (17,25):
>          pytest.fail("bad luck")
E          Failed: bad luck

test_50.py:6: Failed
_______________________________ test_num[25] _______________________________

i = 25

    @pytest.mark.parametrize("i", range(50))
    def test_num(i):
        if i in (17,25):
>          pytest.fail("bad luck")
E          Failed: bad luck

test_50.py:6: Failed
2 failed, 48 deselected in 0.01 seconds

The last line indicates that 48 tests have not been run.

The new config.cache object

Plugins or conftest.py support code can get a cached value using the pytest config object. Here is a basic example plugin which implements a funcarg which re-uses previously created state across py.test invocations:

# content of test_caching.py
import time

def pytest_funcarg__mydata(request):
    val = request.config.cache.get("example/value", None)
    if val is None:
        time.sleep(9*0.6) # expensive computation :)
        val = 42
        request.config.cache.set("example/value", val)
    return val

def test_function(mydata):
    assert mydata == 23

If you run this command once, it will take a while because of the sleep:

$ py.test -q
collecting ... collected 1 items
F
================================= FAILURES =================================
______________________________ test_function _______________________________

mydata = 42

    def test_function(mydata):
>       assert mydata == 23
E       assert 42 == 23

test_caching.py:12: AssertionError
1 failed in 5.43 seconds

If you run it a second time the value will be retrieved from the cache and this will be quick:

$ py.test -q
collecting ... collected 1 items
F
================================= FAILURES =================================
______________________________ test_function _______________________________

mydata = 42

    def test_function(mydata):
>       assert mydata == 23
E       assert 42 == 23

test_caching.py:12: AssertionError
1 failed in 0.02 seconds

Consult the pytest-cache API for more details.

Inspecting Cache content

You can always peek at the content of the cache using the --cache command line option:

$ py.test --cache
=========================== test session starts ============================
platform linux2 -- Python 2.7.3 -- pytest-2.2.5.dev3
cache base dir: /home/hpk/tmp/doc-exec-246/.cache

=============================  in 0.01 seconds =============================
cache/lastfailed contains:
  set(['test_caching.py::test_function'])
example/value contains:
  42

Removing Cache content

You can instruct pytest to clear all cache files and values by adding the --clearcache option like this:

py.test --clearcache

This is recommended for invocations from Continous Integration servers where isolation and correctness is more important than speed.

Notes

repository: http://bitbucket.org/hpk42/pytest-cache

Issues: repository: http://bitbucket.org/hpk42/pytest-cache/issues

more info on py.test: http://pytest.org