Compatibility: Python 2.4-3.2, Jython, PyPy on Unix/Posix and Windows
Installation options:
easy_install -U pytest # or
pip install -U pytest
To check your installation has installed the correct version:
$ py.test --version
This is py.test version 2.0.0.dev30, imported from /home/hpk/p/pytest/pytest.py
If you get an error checkout Known Installation issues.
Let’s create a first test file with a simple test function:
# content of test_sample.py
def func(x):
return x + 1
def test_answer():
assert func(3) == 5
That’s it. You can execute the test function now:
$ py.test
=========================== test session starts ============================
platform linux2 -- Python 2.6.5 -- pytest-2.0.0.dev30
test path 1: /tmp/doc-exec-70
test_sample.py F
================================= 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 0.02 seconds =========================
py.test found the test_answer function by following standard test discovery rules, basically detecting the test_ prefixes. We got a failure report because our little func(3) call did not return 5. The report is formatted using the standard traceback reporting.
Note
You can simply use the assert statement for coding expectations because intermediate values will be presented to you. This is much easier than learning all the the JUnit legacy methods which are even inconsistent with Python’s own coding guidelines (but consistent with Java-style naming).
There is only one seldomly hit caveat to using asserts: if your assertion expression fails and has side effects then re-evaluating it for presenting intermediate values can go wrong. It’s easy to fix: compute the value ahead of the assert and then do the assertion or use the assert “message” syntax:
assert expr, "message" # show "message" if expr is not True
If you want to assert some code raises an exception you can use the raises helper:
# content of test_sysexit.py
import pytest
def f():
raise SystemExit(1)
def test_mytest():
with pytest.raises(SystemExit):
f()
Running it with, this time in “quiet” reporting mode:
$ py.test -q test_sysexit.py
.
1 passed in 0.00 seconds
If you start to have more than a few tests it often makes sense to group tests logically, in classes and modules. Let’s put two tests in a class like this:
# 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')
The two tests are found because of the standard Conventions for Python test discovery. There is no need to subclass anything. We can simply run the module by passing its filename:
$ py.test -q test_class.py
.F
================================= FAILURES =================================
____________________________ TestClass.test_two ____________________________
self = <test_class.TestClass instance at 0x288fc20>
def test_two(self):
x = "hello"
> assert hasattr(x, 'check')
E assert hasattr('hello', 'check')
test_class.py:8: AssertionError
1 failed, 1 passed in 0.02 seconds
The first test passed, the second failed. Again we can easily see the intermediate values used in the assertion, helping us to understand the reason for the failure.
For functional tests one often needs to create some files and pass them to application objects. py.test provides the versatile Test function arguments and factories which allows to request arbitrary resources, for example a unique temporary directory:
# content of test_tmpdir.py
def test_needsfiles(tmpdir):
print tmpdir
assert 0
We list the name tmpdir in the test function signature and py.test will lookup and call a factory to create the resource before performing the test function call. Let’s just run it:
$ py.test -q test_tmpdir.py
F
================================= FAILURES =================================
_____________________________ test_needsfiles ______________________________
tmpdir = local('/tmp/pytest-122/test_needsfiles0')
def test_needsfiles(tmpdir):
print tmpdir
> assert 0
E assert 0
test_tmpdir.py:3: AssertionError
----------------------------- Captured stdout ------------------------------
/tmp/pytest-122/test_needsfiles0
1 failed in 0.05 seconds
Before the test runs, a unique-per-test-invocation temporary directory was created. More info at temporary directories and files.
You can find out what kind of builtin Test function arguments and factories exist by typing:
py.test --funcargs # shows builtin and custom function arguments
Here are a few suggestions where to go next:
Consult distribute docs to install the easy_install tool on your machine. You may also use the older setuptools project but it lacks bug fixes and does not work on Python3. If you use Python2 you may also install pip.
Usages and Examples for more complex examples