Pythons: Python 3.6, 3.7, 3.8, 3.9, PyPy3
Platforms: Linux and Windows
PyPI package name: pytest
Documentation as PDF: download latest
pytest
is a framework that makes building simple and scalable tests easy. Tests are expressive and readable—no boilerplate code required. Get started in minutes with a small unit test or complex functional test for your application or library.
- Run the following command in your command line:
pip install -U pytest
- Check that you installed the correct version:
$ pytest --version
pytest 6.2.2
Create a simple test function with just four lines of code:
# content of test_sample.py
def func(x):
return x + 1
def test_answer():
assert func(3) == 5
That’s it. You can now execute the test function:
$ pytest
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-6.x.y, py-1.x.y, pluggy-0.x.y
cachedir: $PYTHON_PREFIX/.pytest_cache
rootdir: $REGENDOC_TMPDIR
collected 1 item
test_sample.py F [100%]
================================= FAILURES =================================
_______________________________ test_answer ________________________________
def test_answer():
> assert func(3) == 5
E assert 4 == 5
E + where 4 = func(3)
test_sample.py:6: AssertionError
========================= short test summary info ==========================
FAILED test_sample.py::test_answer - assert 4 == 5
============================ 1 failed in 0.12s =============================
The [100%]
refers to the overall progress of running all test cases. After it finishes, pytest then shows a failure report because func(3)
does not return 5
.
Note
You can use the assert
statement to verify test expectations. pytest’s Advanced assertion introspection will intelligently report intermediate values of the assert expression so you can avoid the many names of JUnit legacy methods.
pytest
will run all files of the form test*.py or *_test.py in the current directory and its subdirectories. More generally, it follows standard test discovery rules <test discovery>
.
Use the raises <assertraises>
helper to assert that some code raises an exception:
# content of test_sysexit.py
import pytest
def f():
raise SystemExit(1)
def test_mytest():
with pytest.raises(SystemExit):
f()
Execute the test function with “quiet” reporting mode:
$ pytest -q test_sysexit.py
. [100%]
1 passed in 0.12s
Note
The -q/--quiet
flag keeps the output brief in this and following examples.
Once you develop multiple tests, you may want to group them into a class. pytest makes it easy to create a class containing more than one test:
# content of test_class.py
class TestClass:
def test_one(self):
x = "this"
assert "h" in x
def test_two(self):
x = "hello"
assert hasattr(x, "check")
pytest
discovers all tests following its Conventions for Python test discovery <test discovery>
, so it finds both test_
prefixed functions. There is no need to subclass anything, but make sure to prefix your class with Test
otherwise the class will be skipped. We can simply run the module by passing its filename:
$ pytest -q test_class.py
.F [100%]
================================= FAILURES =================================
____________________________ TestClass.test_two ____________________________
self = <test_class.TestClass object at 0xdeadbeef>
def test_two(self):
x = "hello"
> assert hasattr(x, "check")
E AssertionError: assert False
E + where False = hasattr('hello', 'check')
test_class.py:8: AssertionError
========================= short test summary info ==========================
FAILED test_class.py::TestClass::test_two - AssertionError: assert False
1 failed, 1 passed in 0.12s
The first test passed and the second failed. You can easily see the intermediate values in the assertion to help you understand the reason for the failure.
Grouping tests in classes can be beneficial for the following reasons:
- Test organization
- Sharing fixtures for tests only in that particular class
- Applying marks at the class level and having them implicitly apply to all tests
Something to be aware of when grouping tests inside classes is that each test has a unique instance of the class. Having each test share the same class instance would be very detrimental to test isolation and would promote poor test practices. This is outlined below:
# content of test_class_demo.py
class TestClassDemoInstance:
def test_one(self):
assert 0
def test_two(self):
assert 0
$ pytest -k TestClassDemoInstance -q
FF [100%]
================================= FAILURES =================================
______________________ TestClassDemoInstance.test_one ______________________
self = <test_class_demo.TestClassDemoInstance object at 0xdeadbeef>
def test_one(self):
> assert 0
E assert 0
test_class_demo.py:3: AssertionError
______________________ TestClassDemoInstance.test_two ______________________
self = <test_class_demo.TestClassDemoInstance object at 0xdeadbeef>
def test_two(self):
> assert 0
E assert 0
test_class_demo.py:6: AssertionError
========================= short test summary info ==========================
FAILED test_class_demo.py::TestClassDemoInstance::test_one - assert 0
FAILED test_class_demo.py::TestClassDemoInstance::test_two - assert 0
2 failed in 0.12s
pytest
provides Builtin fixtures/function arguments to request arbitrary resources, like a unique temporary directory:
# content of test_tmpdir.py
def test_needsfiles(tmpdir):
print(tmpdir)
assert 0
List the name tmpdir
in the test function signature and pytest
will lookup and call a fixture factory to create the resource before performing the test function call. Before the test runs, pytest
creates a unique-per-test-invocation temporary directory:
$ pytest -q test_tmpdir.py
F [100%]
================================= FAILURES =================================
_____________________________ test_needsfiles ______________________________
tmpdir = local('PYTEST_TMPDIR/test_needsfiles0')
def test_needsfiles(tmpdir):
print(tmpdir)
> assert 0
E assert 0
test_tmpdir.py:3: AssertionError
--------------------------- Captured stdout call ---------------------------
PYTEST_TMPDIR/test_needsfiles0
========================= short test summary info ==========================
FAILED test_tmpdir.py::test_needsfiles - assert 0
1 failed in 0.12s
More info on tmpdir handling is available at Temporary directories and files <tmpdir handling>
.
Find out what kind of builtin pytest fixtures <fixtures>
exist with the command:
pytest --fixtures # shows builtin and custom fixtures
Note that this command omits fixtures with leading _
unless the -v
option is added.
Check out additional pytest resources to help you customize tests for your unique workflow:
- "
cmdline
" for command line invocation examples - "
existingtestsuite
" for working with pre-existing tests - "
mark
" for information on thepytest.mark
mechanism - "
fixtures
" for providing a functional baseline to your tests - "
plugins
" for managing and writing plugins - "
goodpractices
" for virtualenv and test layouts