py.test is one of several third party testing libraries that are available for Python. It can be installed using [pip](<http://stackoverflow.com/documentation/python/1781/pip-pypi-package-manager#t=201607221328338568674>) with

pip install pytest

The Code to Test

Say we are testing an addition function in projectroot/module/code.py:

# projectroot/module/code.py
def add(a, b):
    return a + b

The Testing Code

We create a test file in projectroot/tests/test_code.py. The file must begin with test_ to be recognized as a testing file.

# projectroot/tests/test_code.py
from module import code

def test_add():
    assert code.add(1, 2) == 3

Running The Test

From projectroot we simply run py.test:

# ensure we have the modules
$ touch tests/__init__.py
$ touch module/__init__.py
$ py.test
================================================== test session starts ===================================================
platform darwin -- Python 2.7.10, pytest-2.9.2, py-1.4.31, pluggy-0.3.1
rootdir: /projectroot, inifile:
collected 1 items

tests/test_code.py .

================================================ 1 passed in 0.01 seconds ================================================