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

Unit, integration, system?

Unit &

Component

Integration

System

3 main testing layers

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

import pytest
import sys

@pytest.mark.skipif(sys.version_info[0] == 3,
                    reason="Only Python 2")
def test_skipped():
    assert 1

Simple

Conditional

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

unittest

included in stdlib
easy to use
lot of code to write
limited

nose

advanced features compared to unittest
more configuration needed
no fixtures

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?