Python Unit Testing(Pytest)
Published in
5 min readMay 5, 2017
A good programmer always tests his code before merging moving his code to production. The way to do it is by writing automated test.
Pytest is a full-featured Python testing tool which means it does everything from test collection to run the test, to give you the output on whether which test failed or passed.
Let’s take an example of simple function(mathexample.py), which takes two argument and return the sum/difference of two number
def add_two(num1,num2):
return num1 + num2
def sub_two(num1,num2):
return num1 - num2First step is to install pytest
pip3 install pytestUnittest look like this
import mathexample
def test_add_two():
add = mathexample.add_two(1,2)
assert add == 3
def test_sub_two():
substract = mathexample.sub_two(1,2)
assert substract == -1Let take a look what’s going on here
- First we import the module(or the script we wrote above)
- We appended test in front of the function we define above and the reason for that pytest using some discovery rules to find out these test cases and one of the discovery rule is to look for a prefix which has test underscore
- To verify your output we are going to use assert
Now there are two ways to execute these tests
- python -m pytest if we just run to run this command, it will do it will recursively goes into the directory and look for any file with test_ and execute any method with test_ prefix
Output
python -m pytest
=========================================== test session starts ============================================
platform darwin -- Python 3.6.0, pytest-3.0.5, py-1.4.32, pluggy-0.4.0
rootdir: /Users/plakhera/Downloads/advanced_python, inifile:
collected 2 items
test_mathexample.py ..
========================================= 2 passed in 0.01 seconds =========================================- Second way to running the same test is via py.test(with verbose output)
py.test -v
=========================================================================================== test session starts ============================================================================================
platform darwin — Python 3.6.0, pytest-3.0.5, py-1.4.32, pluggy-0.4.0 — /Users/plakhera/anaconda/bin/python
cachedir: .cache
rootdir: /Users/plakhera/Downloads/advanced_python, inifile:
collected 2 items test_mathexample.py::test_add_two PASSED
test_mathexample.py::test_sub_two PASSED========================================================================================= 2 passed in 0.01 seconds =========================================================================================
Failure output will look like this
py.test -v
=========================================================================================== test session starts ============================================================================================
platform darwin — Python 3.6.0, pytest-3.0.5, py-1.4.32, pluggy-0.4.0 — /Users/plakhera/anaconda/bin/python
cachedir: .cache
rootdir: /Users/plakhera/Downloads/advanced_python, inifile:
collected 2 items test_mathexample.py::test_add_two FAILED
test_mathexample.py::test_sub_two PASSED================================================================================================= FAILURES =================================================================================================
_______________________________________________________________________________________________ test_add_two _______________________________________________________________________________________________ def test_add_two():
add = mathexample.add_two(1,2)
> assert add == 4
E assert 3 == 4test_mathexample.py:4: AssertionError
==================================================================================== 1 failed, 1 passed in 0.03 seconds ====================================================================================
Generally we don’t write single test case for testing purpose but we wrote multiple test cases
import mathexample
def test_add_two_1():
add = mathexample.add_two(1,2)
assert add == 3
def test_add_two_2():
add = mathexample.add_two(1,6)
assert add == 7
def test_sub_two():
substract = mathexample.sub_two(1,2)
assert substract == -1Output
py.test -v
=========================================================================================== test session starts ============================================================================================
platform darwin — Python 3.6.0, pytest-3.0.5, py-1.4.32, pluggy-0.4.0 — /Users/plakhera/anaconda/bin/python
cachedir: .cache
rootdir: /Users/plakhera/Downloads/advanced_python, inifile:
collected 3 items test_mathexample.py::test_add_two_1 PASSED
test_mathexample.py::test_add_two_2 PASSED
test_mathexample.py::test_sub_two PASSED========================================================================================= 3 passed in 0.01 seconds =========================================================================================
But as you can see we have a problem in the above code, we are defining two function to perform the same task test_add_two_1 and test_add_two_2, so there is a duplication of code here.
The much better way to write the same code is to parametrize the same code
import mathexample
import pytest# We need to use this special decorator when we need to parametrize our code(We are passing input/output as tuple)
@pytest.mark.parametrize("input1, input2, output",
[
(1,2,3),
(1,6,7)
]
)
def test_add_two(input1, input2,output):
add = mathexample.add_two(input1, input2)
assert add == output
@pytest.mark.parametrize("input1, input2, output",
[
(1,2,-1)
]
)
def test_sub_two(input1, input2,output):
substract = mathexample.sub_two(input1, input2)
assert substract == output
Output will look like this
py.test -v
=========================================================================================== test session starts ============================================================================================
platform darwin — Python 3.6.0, pytest-3.0.5, py-1.4.32, pluggy-0.4.0 — /Users/plakhera/anaconda/bin/python
cachedir: .cache
rootdir: /Users/plakhera/Downloads/advanced_python, inifile:
collected 3 items test_mathexample.py::test_add_two[1–2–3] PASSED
test_mathexample.py::test_add_two[1–6–7] PASSED
test_mathexample.py::test_sub_two[1–2–1] PASSED========================================================================================= 3 passed in 0.01 seconds =========================================================================================
There might be times where we selectively need to run test cases, to do that we need to use special decorator pytest.mark.skip
import mathexample
import pytest
@pytest.mark.parametrize("input1, input2, output",
[
(1,2,3),
(1,6,7)
]
)
@pytest.mark.skip(reason="Not required for demo purpose")
def test_add_two(input1, input2,output):
add = mathexample.add_two(input1, input2)
assert add == output
@pytest.mark.parametrize("input1, input2, output",
[
(1,2,-1)
]
)
def test_sub_two(input1, input2,output):
substract = mathexample.sub_two(input1, input2)
assert substract == outputOutput
py.test -v
=========================================================================================== test session starts ============================================================================================
platform darwin — Python 3.6.0, pytest-3.0.5, py-1.4.32, pluggy-0.4.0 — /Users/plakhera/anaconda/bin/python
cachedir: .cache
rootdir: /Users/plakhera/Downloads/advanced_python, inifile:
collected 3 items test_mathexample.py::test_add_two[1–2–3] SKIPPED
test_mathexample.py::test_add_two[1–6–7] SKIPPED
test_mathexample.py::test_sub_two[1–2–1] PASSED=================================================================================== 1 passed, 2 skipped in 0.01 seconds ====================================================================================
We can also skip the test based on certain conditions and for that we need to use pytest.mark.skipif
import mathexample
import pytest
import sys
@pytest.mark.parametrize("input1, input2, output",
[
(1,2,3),
(1,6,7)
]
)
@pytest.mark.skipif(sys.version_info < (3,5),reason="Not required for demo purpose")
def test_add_two(input1, input2,output):
add = mathexample.add_two(input1, input2)
assert add == output
@pytest.mark.parametrize("input1, input2, output",
[
(1,2,-1)
]
)
def test_sub_two(input1, input2,output):
substract = mathexample.sub_two(input1, input2)
assert substract == output- Here we are saying if Python version is < 3.5 please skip these tests
Now there is one much easy way to run specific test
import mathexample
@pytest.mark.parametrize("input1, input2, output",
[
(1,2,3),
(1,6,7)
]
)
def test_add_two(input1, input2,output):
add = mathexample.add_two(input1, input2)
assert add == output
@pytest.mark.parametrize("input1, input2, output",
[
(1,2,-1)
]
)
def test_sub_two(input1, input2,output):
substract = mathexample.sub_two(input1, input2)
assert substract == outputNow here if I want to run only subtract test case, I need to pass -k option
pytest -k sub
=========================================== test session starts ============================================
platform darwin — Python 3.6.0, pytest-3.0.5, py-1.4.32, pluggy-0.4.0
rootdir: /Users/plakhera/Downloads/advanced_python, inifile:
collected 3 items test_mathexample.py .============================================ 2 tests deselected ============================================
================================== 1 passed, 2 deselected in 0.01 seconds ====================
where -k
-k EXPRESSION only run tests which match the given substring
expression. An expression is a python evaluatable
expression where all names are substring-matched
against test names and their parent classes. Example:
-k ‘test_method or test_other’ matches all test
functions and classes whose name contains
‘test_method’ or ‘test_other’. Additionally keywords
are matched to classes and functions containing extra
names in their ‘extra_keyword_matches’ set, as well as
functions which have names assigned directly to them.We can also set our own custom marker
import mathexample
import pytest
@pytest.mark.add
def test_add_two():
add = mathexample.add_two(1,2)
assert add == 3
@pytest.mark.sub
def test_sub_two():
substract = mathexample.sub_two(1,2)
assert substract == -1and the way to run it
pytest -m sub -v
=========================================================================================== test session starts ============================================================================================
platform darwin — Python 3.6.0, pytest-3.0.5, py-1.4.32, pluggy-0.4.0 — /Users/plakhera/anaconda/bin/python
cachedir: .cache
rootdir: /Users/plakhera/Downloads/advanced_python, inifile:
collected 2 items test_mathexample.py::test_sub_two PASSED============================================================================================ 1 tests deselected ============================================================================================
================================================================================== 1 passed, 1 deselected in 0.01 seconds ==================================================================================
Same way we can check for add
pytest -m add -v
=========================================================================================== test session starts ============================================================================================
platform darwin — Python 3.6.0, pytest-3.0.5, py-1.4.32, pluggy-0.4.0 — /Users/plakhera/anaconda/bin/python
cachedir: .cache
rootdir: /Users/plakhera/Downloads/advanced_python, inifile:
collected 2 items test_mathexample.py::test_add_two PASSED============================================================================================ 1 tests deselected ============================================================================================
================================================================================== 1 passed, 1 deselected in 0.01 seconds ==================================================================================
