doc/en/getting-started.rst
.. _get-started:
.. _getstarted:
.. _installation:
pytest.. code-block:: bash
pip install -U pytest
2. Check that you installed the correct version:
.. code-block:: bash
$ pytest --version
pytest 9.0.2
.. _simpletest:
Create a new file called test_sample.py, containing a function, and a test:
.. code-block:: python
# content of test_sample.py
def func(x):
return x + 1
def test_answer():
assert func(3) == 5
The test
.. code-block:: pytest
$ pytest
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-9.x.y, pluggy-1.x.y
rootdir: /home/sweet/project
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 :ref:`Advanced assertion introspection <python:assert>` will intelligently report intermediate values of the assert expression so you can avoid the many names :ref:`of JUnit legacy methods <testcase-objects>`.
pytest will run all files of the form test_*.py or *_test.py in the current directory and its subdirectories. More generally, it follows :ref:standard test discovery rules <test discovery>.
Use the :ref:raises <assertraises> helper to assert that some code raises an exception:
.. code-block:: python
# 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:
.. code-block:: pytest
$ 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.
See :ref:assertraises for specifying more details about the expected exception.
.. regendoc:wipe
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:
.. code-block:: python
# 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 :ref: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:
.. code-block:: pytest
$ pytest -q test_class.py
.F [100%]
================================= FAILURES =================================
____________________________ TestClass.test_two ____________________________
self = <test_class.TestClass object at 0xdeadbeef0001>
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:
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:
.. regendoc:wipe
.. code-block:: python
# content of test_class_demo.py
class TestClassDemoInstance:
value = 0
def test_one(self):
self.value = 1
assert self.value == 1
def test_two(self):
assert self.value == 1
.. code-block:: pytest
$ pytest -k TestClassDemoInstance -q
.F [100%]
================================= FAILURES =================================
______________________ TestClassDemoInstance.test_two ______________________
self = <test_class_demo.TestClassDemoInstance object at 0xdeadbeef0002>
def test_two(self):
> assert self.value == 1
E assert 0 == 1
E + where 0 = <test_class_demo.TestClassDemoInstance object at 0xdeadbeef0002>.value
test_class_demo.py:9: AssertionError
========================= short test summary info ==========================
FAILED test_class_demo.py::TestClassDemoInstance::test_two - assert 0 == 1
1 failed, 1 passed in 0.12s
Note that attributes added at class level are class attributes, so they will be shared between tests.
pytest also provides a number of utilities to make writing tests easier.
For example, you can use :func:pytest.approx to compare floating-point
values that may have small rounding errors:
.. code-block:: python
# content of test_approx.py
import pytest
def test_sum():
assert (0.1 + 0.2) == pytest.approx(0.3)
This avoids the need for manual tolerance checks or using
math.isclose and works with scalars, lists, and NumPy arrays.
pytest provides :std:doc:Builtin fixtures/function arguments <builtin> to request arbitrary resources, like a unique temporary directory:
.. code-block:: python
# content of test_tmp_path.py
def test_needsfiles(tmp_path):
print(tmp_path)
assert 0
List the name tmp_path 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:
.. code-block:: pytest
$ pytest -q test_tmp_path.py
F [100%]
================================= FAILURES =================================
_____________________________ test_needsfiles ______________________________
tmp_path = PosixPath('PYTEST_TMPDIR/test_needsfiles0')
def test_needsfiles(tmp_path):
print(tmp_path)
> assert 0
E assert 0
test_tmp_path.py:3: AssertionError
--------------------------- Captured stdout call ---------------------------
PYTEST_TMPDIR/test_needsfiles0
========================= short test summary info ==========================
FAILED test_tmp_path.py::test_needsfiles - assert 0
1 failed in 0.12s
More info on temporary directory handling is available at :ref:Temporary directories and files <tmp_path handling>.
Find out what kind of builtin :ref:pytest fixtures <fixtures> exist with the command:
.. code-block:: bash
pytest --fixtures # shows builtin and custom fixtures
Note that this command omits fixtures with leading _ unless the :option:-v option is added.
Check out additional pytest resources to help you customize tests for your unique workflow:
usage" for command line invocation examplesexistingtestsuite" for working with preexisting testsmark" for information on the pytest.mark mechanismfixtures" for providing a functional baseline to your testsplugins" for managing and writing pluginsgoodpractices" for virtualenv and test layouts