PyTest Tutorial: What is, How to Install, Framework, Assertions

Details

What is PyTest?

PyTest is a testing framework that allows users to write test codes using Python programming language. It helps you to write simple and scalable test cases for databases, APIs, or UI. PyTest is mainly used for writing tests for APIs. It helps to write tests from simple unit tests to complex functional tests.

Why use PyTest?

Some of the advantages of pytest are

In this Python PyTest tutorial, you will learn:

How to install PyTest

Following is a process on how to install PyTest:

Step 1) You can install pytest by 

pip install pytest==2.9.1

Once the installation is complete you can confirm it with by 

py.test -h

This will display the help

How to install PyTest

First Basic PyTest

Now, we will learn how to use Pytest with a basic PyTest example.

Create a folder study_pytest. We are going to create our test files inside this folder.

Please navigate to that folder in your command line.

Create a file named test_sample1.py inside the folder

Add the below code into it and save 

import pytest
def test_file1_method1():
	x=5
	y=6
	assert x+1 == y,"test failed"
	assert x == y,"test failed"
def test_file1_method2():
	x=5
	y=6
	assert x+1 == y,"test failed"

Run the test using the command 

py.test

You'll get output as 

test_sample1.py F.
============================================== FAILURES ========================================
____________________________________________ test_sample1 ______________________________________
    def test_file1_method1():
    	x=5
    	y=6
       	assert x+1 == y,"test failed"
>      	assert x == y,"test failed"
E       AssertionError: test failed
E       assert 5 == 6
test_sample1.py:6: AssertionError

Here in test_sample1.py F.

F says failure

Dot(.) says success.

In the failures section, you can see the failed method(s) and the line of failure. Here x==y means 5==6 which is false.

Next in this PyTest tutorial, we will learn about assertion in PyTest.

Assertions in PyTest

Pytest assertions are checks that return either True or False status. In Python Pytest, if an assertion fails in a test method, then that method execution is stopped there. The remaining code in that test method is not executed, and Pytest assertions will continue with the next test method.

Pytest Assert examples: 

assert "hello" == "Hai" is an assertion failure.
assert 4==4 is a successful assertion
assert True is a successful assertion
assert False is an assertion failure.

Consider 

assert x == y,"test failed because x=" + str(x) + " y=" + str(y)

Place this code in test_file1_method1() instead of the assertion 

assert x == y,"test failed"

Running the test will give the failure as AssertionError: test failed x=5 y=6

How PyTest Identifies the Test Files and Test Methods

By default pytest only identifies the file names starting with test_ or ending with _test as the test files. We can explicitly mention other filenames though (explained later). Pytest requires the test method names to start with "test." All other method names will be ignored even if we explicitly ask to run those methods.

See some examples of valid and invalid pytest file names 

test_login.py - valid
login_test.py - valid
testlogin.py -invalid
logintest.py -invalid

Note: Yes we can explicitly ask pytest to pick testlogin.py and logintest.py

See some examples of valid and invalid pytest test methods 

def test_file1_method1(): - valid
def testfile1_method1(): - valid
def file1_method1(): - invalid

Note: Even if we explicitly mention file1_method1() pytest will not run this method.

Run Multiple Tests From a Specific File and Multiple Files

Currently, inside the folder study_pytest, we have a file test_sample1.py. Suppose we have multiple files , say test_sample2.py , test_sample3.py. To run all the tests from all the files in the folder and subfolders we need to just run the pytest command. 

py.test

This will run all the filenames starting with test_ and the filenames ending with _test in that folder and subfolders under that folder.

To run tests only from a specific file, we can use py.test <filename> 

py.test test_sample1.py

Run a subset of Entire Test with PyTest

Sometimes we don't want to run the entire test suite. Pytest allows us to run specific tests. We can do it in 2 ways

We already have test_sample1.py. Create a file test_sample2.py and add the below code into it 

def test_file2_method1():
	x=5
	y=6
	assert x+1 == y,"test failed"
	assert x == y,"test failed because x=" + str(x) + " y=" + str(y)
def test_file2_method2():
	x=5
	y=6
	assert x+1 == y,"test failed"

So we have currently 

  • test_sample1.py
    • test_file1_method1()
    • test_file1_method2()
    

  • test_sample2.py
    • test_file2_method1()
    • test_file2_method2()

Option 1) Run tests by substring matching

Here to run all the tests having method1 in its name we have to run 

py.test -k method1 -v
-k <expression> is used to represent the substring to match
-v increases the verbosity

So running py.test -k method1 -v will give you the following result 

test_sample2.py::test_file2_method1 FAILED
test_sample1.py::test_file1_method1 FAILED

============================================== FAILURES ==============================================
_________________________________________ test_file2_method1 _________________________________________
    def test_file2_method1():
    	x=5
    	y=6
       	assert x+1 == y,"test failed"
>      	assert x == y,"test failed because x=" + str(x) + " y=" + str(y)
E       AssertionError: test failed because x=5 y=6
E       assert 5 == 6
test_sample2.py:5: AssertionError

_________________________________________ test_file1_method1 _________________________________________
    @pytest.mark.only
    def test_file1_method1():
    	x=5
    	y=6
       	assert x+1 == y,"test failed"
>      	assert x == y,"test failed because x=" + str(x) + " y=" + str(y)
E       AssertionError: test failed because x=5 y=6
E       assert 5 == 6
test_sample1.py:8: AssertionError

================================= 2 tests deselected by '-kmethod1' ==================================
=============================== 2 failed, 2 deselected in 0.02 seconds ===============================

Here you can see towards the end 2 tests deselected by '-kmethod1' which are test_file1_method2 and test_file2_method2

Try running with various combinations like:- 

py.test -k method -v - will run all the four methods
py.test -k methods -v – will not run any test as there is no test name matches the substring 'methods'

Option 2) Run tests by markers

Pytest allows us to set various attributes for the test methods using pytest markers, @pytest.mark . To use markers in the test file, we need to import pytest on the test files.

Here we will apply different marker names to test methods and run specific tests based on marker names. We can define the markers on each test names by using 

@pytest.mark.<name>.

We are defining markers set1 and set2 on the test methods, and we will run the test using the marker names. Update the test files with the following code

test_sample1.py 

import pytest
@pytest.mark.set1
def test_file1_method1():
	x=5
	y=6
	assert x+1 == y,"test failed"
	assert x == y,"test failed because x=" + str(x) + " y=" + str(y)

@pytest.mark.set2
def test_file1_method2():
	x=5
	y=6
	assert x+1 == y,"test failed"

test_sample2.py 

import pytest
@pytest.mark.set1
def test_file2_method1():
	x=5
	y=6
	assert x+1 == y,"test failed"
	assert x == y,"test failed because x=" + str(x) + " y=" + str(y)

@pytest.mark.set1
def test_file2_method2():
	x=5
	y=6
	assert x+1 == y,"test failed"

We can run the marked test by 

py.test -m <name>
-m <name> mentions the marker name

Run py.test -m set1.This will run the methods test_file1_method1, test_file2_method1, test_file2_method2.

Running py.test -m set2 will run test_file1_method2.

Run Tests in Parallel with Pytest

Usually, a test suite will have multiple test files and hundreds of test methods which will take a considerable amount of time to execute. Pytest allows us to run tests in parallel.

For that we need to first install pytest-xdist by running 

pip install pytest-xdist

You can run tests now by 

py.test -n 4

-n <num> runs the tests by using multiple workers. In the above command, there will be 4 workers to run the test.

Pytest Fixtures

Fixtures are used when we want to run some code before every test method. So instead of repeating the same code in every test we define fixtures. Usually, fixtures are used to initialize database connections, pass the base , etc

A method is marked as a Pytest fixture by marking with 

@pytest.fixture

A test method can use a Pytest fixture by mentioning the fixture as an input parameter.

Create a new file test_basic_fixture.py with following code 

import pytest
@pytest.fixture
def supply_AA_BB_CC():
	aa=25
	bb =35
	cc=45
	return [aa,bb,cc]

def test_comparewithAA(supply_AA_BB_CC):
	zz=35
	assert supply_AA_BB_CC[0]==zz,"aa and zz comparison failed"

def test_comparewithBB(supply_AA_BB_CC):
	zz=35
	assert supply_AA_BB_CC[1]==zz,"bb and zz comparison failed"

def test_comparewithCC(supply_AA_BB_CC):
	zz=35
	assert supply_AA_BB_CC[2]==zz,"cc and zz comparison failed"

Here

Each of the test function has an input argument whose name is matching with an available fixture. Pytest then invokes the corresponding fixture method and the returned values will be stored in the input argument , here the list [25,35,45]. Now the list items are being used in test methods for the comparison.

Now run the test and see the result 

 py.test test_basic_fixture
test_basic_fixture.py::test_comparewithAA FAILED                                                                                                                                                                                       
test_basic_fixture.py::test_comparewithBB PASSED                                                                                                                                                                                       
test_basic_fixture.py::test_comparewithCC FAILED
                                                                                                                                                                                       
============================================== FAILURES ==============================================
_________________________________________ test_comparewithAA _________________________________________
supply_AA_BB_CC = [25, 35, 45]
    def test_comparewithAA(supply_AA_BB_CC):
    	zz=35
>   	assert supply_AA_BB_CC[0]==zz,"aa and zz comparison failed"
E    AssertionError: aa and zz comparison failed
E    assert 25 == 35
test_basic_fixture.py:10: AssertionError

_________________________________________ test_comparewithCC _________________________________________
supply_AA_BB_CC = [25, 35, 45]
    def test_comparewithCC(supply_AA_BB_CC):
    	zz=35
>   	assert supply_AA_BB_CC[2]==zz,"cc and zz comparison failed"
E    AssertionError: cc and zz comparison failed
E    assert 45 == 35
test_basic_fixture.py:16: AssertionError
================================= 2 failed, 1 passed in 0.05 seconds =================================

The test test_comparewithBB is passed since zz=BB=35, and the remaining 2 tests are failed.

The fixture method has a scope only within that test file it is defined. If we try to access the fixture in some other test file , we will get an error saying fixture 'supply_AA_BB_CC' not found for the test methods in other files.

To use the same fixture against multiple test files, we will create fixture methods in a file called conftest.py.

Let's see this by the below PyTest example. Create 3 files conftest.py, test_basic_fixture.py, test_basic_fixture2.py with the following code

conftest.py 

import pytest
@pytest.fixture
def supply_AA_BB_CC():
	aa=25
	bb =35
	cc=45
	return [aa,bb,cc]

test_basic_fixture.py 

import pytest
def test_comparewithAA(supply_AA_BB_CC):
	zz=35
	assert supply_AA_BB_CC[0]==zz,"aa and zz comparison failed"

def test_comparewithBB(supply_AA_BB_CC):
	zz=35
	assert supply_AA_BB_CC[1]==zz,"bb and zz comparison failed"

def test_comparewithCC(supply_AA_BB_CC):
	zz=35
	assert supply_AA_BB_CC[2]==zz,"cc and zz comparison failed"

test_basic_fixture2.py 

import pytest
def test_comparewithAA_file2(supply_AA_BB_CC):
	zz=25
	assert supply_AA_BB_CC[0]==zz,"aa and zz comparison failed"

def test_comparewithBB_file2(supply_AA_BB_CC):
	zz=25
	assert supply_AA_BB_CC[1]==zz,"bb and zz comparison failed"

def test_comparewithCC_file2(supply_AA_BB_CC):
	zz=25
	assert supply_AA_BB_CC[2]==zz,"cc and zz comparison failed"

pytest will look for the fixture in the test file first and if not found it will look in the conftest.py

Run the test by py.test -k test_comparewith -v to get the result as below 

test_basic_fixture.py::test_comparewithAA FAILED  
test_basic_fixture.py::test_comparewithBB PASSED 
test_basic_fixture.py::test_comparewithCC FAILED 
test_basic_fixture2.py::test_comparewithAA_file2 PASSED 
test_basic_fixture2.py::test_comparewithBB_file2 FAILED 
test_basic_fixture2.py::test_comparewithCC_file2 FAILED

Pytest Parameterized Test

The purpose of parameterizing a test is to run a test against multiple sets of arguments. We can do this by @pytest.mark.parametrize.

We will see this with the below PyTest example. Here we will pass 3 arguments to a test method. This test method will add the first 2 arguments and compare it with the 3rd argument.

Create the test file test_addition.py with the below code 

import pytest
@pytest.mark.parametrize("input1, input2, output",[(5,5,10),(3,5,12)])
def test_add(input1, input2, output):
	assert input1+input2 == output,"failed"

Here the test method accepts 3 arguments- input1, input2, output. It adds input1 and input2 and compares against the output.

Let's run the test by py.test -k test_add -v and see the result 

test_addition.py::test_add[5-5-10] PASSED                                                                                                                                                                                              
test_addition.py::test_add[3-5-12] FAILED                                                                                                                                                                                              
============================================== FAILURES ==============================================
__________________________________________ test_add[3-5-12] __________________________________________
input1 = 3, input2 = 5, output = 12
    @pytest.mark.parametrize("input1, input2, output",[(5,5,10),(3,5,12)])
    def test_add(input1, input2, output):
>   	assert input1+input2 == output,"failed"
E    AssertionError: failed
E    assert (3 + 5) == 12
test_addition.py:5: AssertionError

You can see the tests ran 2 times – one checking 5+5 ==10 and other checking 3+5 ==12

test_addition.py::test_add[5-5-10] PASSED

test_addition.py::test_add[3-5-12] FAILED

Pytest Xfail / Skip Tests

There will be some situations where we don't want to execute a test, or a test case is not relevant for a particular time. In those situations, we have the option to Xfail the test or skip the tests

The xfailed test will be executed, but it will not be counted as part failed or passed tests. There will be no traceback displayed if that test fails. We can xfail tests using

@pytest.mark.xfail.

Skipping a test means that the test will not be executed. We can skip tests using

@pytest.mark.skip.

Edit the test_addition.py with the below code 

import pytest
@pytest.mark.skip
def test_add_1():
	assert 100+200 == 400,"failed"

@pytest.mark.skip
def test_add_2():
	assert 100+200 == 300,"failed"

@pytest.mark.xfail
def test_add_3():
	assert 15+13 == 28,"failed"

@pytest.mark.xfail
def test_add_4():
	assert 15+13 == 100,"failed"

def test_add_5():
	assert 3+2 == 5,"failed"

def test_add_6():
	assert 3+2 == 6,"failed"

Here

Execute the test by py.test test_addition.py -v and see the result 

test_addition.py::test_add_1 SKIPPED
test_addition.py::test_add_2 SKIPPED
test_addition.py::test_add_3 XPASS
test_addition.py::test_add_4 xfail
test_addition.py::test_add_5 PASSED
test_addition.py::test_add_6 FAILED

============================================== FAILURES ==============================================
_____________________________________________ test_add_6 _____________________________________________
    def test_add_6():
>   	assert 3+2 == 6,"failed"
E    AssertionError: failed
E    assert (3 + 2) == 6
test_addition.py:24: AssertionError

================ 1 failed, 1 passed, 2 skipped, 1 xfailed, 1 xpassed in 0.07 seconds =================

Results XML

We can create test results in XML format which we can feed to Continuous Integration servers for further processing and so. This can be done by

py.test test_sample1.py -v --junitxml="result.xml"

The result.xml will record the test execution result. Find a sample result.xml below 

<?xml version="1.0" encoding="UTF-8"?>
<testsuite errors="0" failures="1" name="pytest" skips="0" tests="2" time="0.046">
   <testcase classname="test_sample1" file="test_sample1.py" line="3" name="test_file1_method1" time="0.001384973526">
     <failure message="AssertionError:test failed because x=5 y=6 assert 5 ==6">
    @pytest.mark.set1
    def test_file1_method1():
    	x=5
    	y=6
       	assert x+1 == y,"test failed"
>      	assert x == y,"test failed because x=" + str(x) + " y=" + str(y)
E       AssertionError: test failed because x=5 y=6
E       assert 5 == 6
         test_sample1.py:9: AssertionError
    </failure>
   </testcase>
   <testcase classname="test_sample1" file="test_sample1.py" line="10" name="test_file1_method2" time="0.000830173492432" />
</testsuite>

From <testsuite errors="0" failures="1" name="pytest" skips="0" tests="2" time="0.046"> we can see a total of two tests of which one is failed. Below that you can see the details regarding each executed test under <testcase> tag.

Pytest Framework Testing an API

Now we will create a small pytest framework to test an API. The API here used is a free one from https://reqres.in/. This website is just to provide testable API. This website doesn't store our data.

Here we will write some tests for

Create the below files with the code given

conftest.py - have a fixture which will supply base url for all the test methods 

import pytest
@pytest.fixture
def supply_url():
	return "https://reqres.in/api"

test_list_user.py – contains the test methods for listing valid and invalid users 

import pytest
import requests
import json
@pytest.mark.parametrize("userid, firstname",[(1,"George"),(2,"Janet")])
def test_list_valid_user(supply_url,userid,firstname):
	url = supply_url + "/users/" + str(userid)
	resp = requests.get(url)
	j = json.loads(resp.text)
	assert resp.status_code == 200, resp.text
	assert j['data']['id'] == userid, resp.text
	assert j['data']['first_name'] == firstname, resp.text

def test_list_invaliduser(supply_url):
	url = supply_url + "/users/50"
	resp = requests.get(url)
	assert resp.status_code == 404, resp.text

test_login_user.py – contains test methods for testing login functionality. 

import pytest
import requests
import json
def test_login_valid(supply_url):
	url = supply_url + "/login/" 
	data = {'email':This email address is being protected from spambots. You need JavaScript enabled to view it.','password':'something'}
	resp = requests.post(url, data=data)
	j = json.loads(resp.text)
	assert resp.status_code == 200, resp.text
	assert j['token'] == "QpwL5tke4Pnpja7X", resp.text

def test_login_no_password(supply_url):
	url = supply_url + "/login/" 
	data = {'email':This email address is being protected from spambots. You need JavaScript enabled to view it.'}
	resp = requests.post(url, data=data)
	j = json.loads(resp.text)
	assert resp.status_code == 400, resp.text
	assert j['error'] == "Missing password", resp.text

def test_login_no_email(supply_url):
	url = supply_url + "/login/" 
	data = {}
	resp = requests.post(url, data=data)
	j = json.loads(resp.text)
	assert resp.status_code == 400, resp.text
	assert j['error'] == "Missing email or username", resp.text

Run the test using py.test -v

See the result as 

test_list_user.py::test_list_valid_user[1-George] PASSED                                                                                                                                                                               
test_list_user.py::test_list_valid_user[2-Janet] PASSED                                                                                                                                                                                
test_list_user.py::test_list_invaliduser PASSED                                                                                                                                                                                        
test_login_user.py::test_login_valid PASSED                                                                                                                                                                                            
test_login_user.py::test_login_no_password PASSED                                                                                                                                                                                      
test_login_user.py::test_login_no_email PASSED

Update the tests and try various outputs

Summary

In this PyTest tutorial, we covered

 

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