Introduction to py.test

Presentation by Jean Cruypenynck/@nonatomiclabs

nonatomiclabs.com

Why testing is important?

• a bug is cheap if discovered early
• more flexible workflow (CI, deployment)

             Coding is good…

… with testing, it is better…

… with automated testing, it is the best

Testing in Python?

• easy to write
• easy to read
• easy to run
• memory & CPU are not problems
• deployment is not a problem

What is py.test?

pip install pytest
easy_install pytest

• open-source Python testing tool
• "helps you write better programs"
• not included in stdlib but very well-known
• active since 2007 and actively developed

Write and run your first test

2.  run tests

def test_division():
    assert division(1, 1) == 1

1.  create a function with the test_ prefix

py.test division.py

# Function to test
def division(a, b):
    return a / float(b)

Passing test

Failing test

How are tests discovered?

Any way you can imagine!!!

py.test

py.test test_module.py

py.test test_module.py::test_function

py.test test_module.py -k my_keyword

Skip, xfail, Xpass (1/3)

import pytest

@pytest.mark.skipif()
def test_skipped():
    assert 1

Simple

Skip, xfail, Xpass (2/3)

@pytest.mark.xfail
def test_division_fail():
    assert division(1, 0) == 1

• Mark test expected to fail
• Also allows conditional use

Skip, xfail, Xpass (3/3)

We add exception handling to our function

def division(a, b):
    try:
	return a / float(b)
    except ZeroDivisionError:
	return 1

Output:

Parametrisation

Output:

@pytest.mark.parametrize("a", [1, 2, 3, 4])
def test_division_param(a):
    assert division(a, 1) == a

Fixtures (1/3)

• Create the test context
  • Test setup
  • Test cleanup
• Often used to provide instances
• "Dependency injection"

Fixtures (2/3)

Example: tmpdir

def test_tmpdir(tmpdir):
    assert 0

Output:

Fixtures (3/3)

import pytest

@pytest.fixture
def structure():
    class MyStruct(object):
        text = ""
        num = 10
    return MyStruct()

def test_fix(structure):
    assert isinstance(structure.text, str)
    assert isinstance(structure.num, int)

Plugins

• web frameworks (django, flask, …)
• code coverage
• xdist: multicore test distribution
• timeout: long tests management
• pep8
• … (150+ plugins)

No plugin, write it!

• good documentation
• hooks for all steps

Other modules?

So, why py.test?

• runs unittest and nose tests
• easy
• advanced features
• plugins
• runs on everything
• good documentation
• active development
• used by Fedora, Mozilla, …

Thank you!

Questions?

nonatomiclabs.com

git clone https://github.com/filaton/pytest-intro.git

http://nonatomiclabs.com/blog/?p=89

Ressources

• pytest.org
• pythontesting.net
• Testing Python, David Sale