Getting Started-Pytest

  5 mins read  

Installation and Getting Started

Pythons: Python 2.7, 3.4, 3.5, 3.6, 3.7, Jython, PyPy-2.3

Platforms: Unix/Posix 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.

1.1 Install pytest

  1. Run the following command in your command line:
    pip install -U pytest
  2. Check that you installed the correct version:
    $ pytest --version
This is pytest version 4.x.y, imported from $PYTHON_PREFIX/lib/python3.6/site-
˓→packages/pytest.py


1.2 Create your first test

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-4.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:5: AssertionError
========================= 1 failed in 0.12 seconds =========================

This test returns 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

1.3 Run multiple tests

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.

1.4 Assert that a certain exception is raised

Use the raises 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 0.12 seconds


1.5 Group multiple tests in a class

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(object):
	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, so it finds both test_ prefixed

$ 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
1 failed, 1 passed in 0.12 seconds

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.

1.6 Request a unique temporary directory for functional tests

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
1 failed in 0.12 seconds

More info on tmpdir handling is available at Temporary directories and files. Find out what kind of builtin [pytest 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

1.7 Continue reading

Check out additional pytest resources to help you customize tests for your unique workflow: