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2016, Apr 16 - Dimitri Merejkowsky License: CC By 4.0
Today I thought I'd share my experience with various test tools for the Python programming language.
I was maintaining a Python tool that did a lot of `subprocess` calling, reading and writing files. This kind of code is not easy to test. My solution to this problem was to create a lot of temporary directories, so that I could exercise the code under test safely, without interfering with the rest of my filesystem. Also, temporary directories are created empty, which is a good way to isolate tests from each other. (No files read or created by `test_one` in `/tmp/test_one` can interfere with `test_two` running in `/tmp/test_two`)
I naturally started with `unittest`, which is in the standard library. (Note: this was a long time ago, `unittest` did many progress since then)
This is what the code looked like:
# in test_one.py
import unnitest
class TestOne(unittest.TestCase):
def setUp(self):
self.tmpdir = tmpfile.mkdtemp("test-one-")
def test_one(self):
# do stuff in self.tmpdir
rc = ...
self.assertEquals(rc, 0)
def tearDown(self):
shutil.rmtree(self.tmpdir)
# in run_tests.py
import unittest
TESTCASES = [
TestOne,
# ....
]
suite = unittest.TestSuite()
for test_case in TEST_CASES:
suite.addTests(unittest.makeSuite(test_case))
runner = unittest.TextTestRunner()
result = runner.run(suite)
if not result.wasSuccessful():
sys.exit(1)
Also, I had tests that shared a common set up and tear down. I found two solutions, but neither was really satisfying:
So in the end there was a lot of code duplication among tests...
The last two points illustrate a fundamental problem with `unittest`: since it's part of the standard library, you are stuck with the version coming with your Python installation, and you cannot get new features without upgrading Python too.
Yes, I know `unittest2` exists, but if you're going to use an external package to run the tests, why stick with `unittest`?
Before diving into `pytest` specific features, let me point out that **pytest is fully compatible with unittest**, so if you want to switch, you don't have to rewrite all your tests right away :)
Here's what the same code looks like when using `pytest`
def test_one(tmpdir):
# do stuff in tmpdir
rc = ....
assert rc == 0
Well, that's nicer isn't it?
file test_foo.py, line 1
def test_foo():
actual = "foo" + "bar"
expected = "fooBar"
> assert actual == expected
E assert 'foobar' == 'fooBar'
E - foobar
E ? ^
E + fooBar
E ? ^
test_foo.py:4: AssertionError
The nice thing about `pytest`is that the code of the "fixtures" (the setup / tear down) is completely separated from the code that exercise the production code.
1: https://pytest.org/latest/builtin.html#builtin-fixtures-function-arguments
`pytest` encourages you to write them in a special file called `conftest.py`. Sharing fixtures is then as easy as writing a function, decorate it with `@pytest.fixture` and then pass it as parameter to whatever function needs it.
Here's an example:
# in conftest.py
import pytest
@pytest.fixture
def db():
connection = DataBaseConnection("...")
yield connection
connection.close()
# in test_one
def test_one(db):
# ...
# in test_two
def test_two(db):
# ...
Note how the code that deals with closing the connection to the database is right next to the code that opens it, and how `pytest` uses the `yield` keyword to stop executing the fixture code while the test is running.
Also note how the tests do not care where the database come from: they just use it as a parameter. (This is Dependency Injection at its finest)
Finally, by default fixtures have a scope of "function" (meaning the database will be opened and then closed for each test function), but you can chose to have a "module scope" or even a "session scope".
(This is quite hard to do with `unittest`)
You can even have fixtures that are always implicitly called, by using `autouse=True` in the fixture definition.
You can also extend the command line API: this is especially useful if you need some kind of token to run your tests.
Here's an example:
# in conftest.py
import pytest
def pytest_addoption(parser):
parser.addoption("--token", action="store", help="secret token")
# in test_foo.py
def test_foo(request):
token = request.config.getoption("--token")
Again, this is quite hard to do with `unittest`
Last but not least, there are a lot of plugins available to use with `pytest`.
Here are a few:
You can even use `pytest` with tests written in C++ using `gtest` or `boost::test` thanks to the pytest-cpp[6] plugin
2: https://pypi.python.org/pypi/pytest-sugar
3: https://pypi.python.org/pypi/pytest-cache
4: https://pypi.python.org/pypi/pytest-xdist
5: https://pypi.python.org/pypi/pytest-cov
6: https://github.com/pytest-dev/pytest-cpp
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