What is PyTest?

Pytest is a testing framework which allows us to write test codes using python. You can write code to test anything like database , API, even UI if you want. But pytest is mainly being used in industry to write tests for APIs.

Why use PyTest?

Some of the advantages of pytest are

  • Very easy to start with because of its simple and easy syntax.
  • Can run tests in parallel.
  • Can run a specific test or a subset of tests
  • Automatically detect tests
  • Skip tests
  • Open source

In this tutorial, you will learn:

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

First Basic PyTest

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.

Assertions in PyTest

Assertions are checks that return either True or False status. In 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 will continue with the next test method.

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

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

  • Grouping of test names by substring matching
  • Grouping of tests by markers

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.

Running tests in parallel

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 fixture by marking with

@pytest.fixture

A test method can use a 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

  • We have a fixture named supply_AA_BB_CC. This method will return a list of 3 values.
  • We have 3 test methods comparing against each of the values.

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 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

Parameterized tests

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 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

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

  • test_add_1 and test_add_2 are skipped and will not be executed.
  • test_add_3 and test_add_4 are xfailed. These tests will be executed and will be part of xfailed(on test failure) or xpassed(on test pass) tests. There won't be any traceback for failures.
  • test_add_5 and test_add_6 will be executed and test_add_6 will report failure with traceback while the test_add_5 passes

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.

A 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

  • listing some users
  • login with users

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

  • test_list_valid_user tests for valid user fetch and verifies the response
  • test_list_invaliduser tests for invalid user fetch and verifies the response
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.

  • test_login_valid tests the valid login attempt with email and password
  • test_login_no_password tests the invalid login attempt without passing password
  • test_login_no_email tests the invalid login attempt without passing email.
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

  • Install pytest using pip install pytest=2.9.1
  • Simple pytest program and run it with py.test command.
  • Assertion statements, assert x==y, will return either True or False.
  • How pytest identifies test files and methods.
    • Test files starting with test_ or ending with _test
    • Test methods starting with test
  • py.test command will run all the test files in that folder and subfolders. To run a specific file, we can use the command py.test <filename>
  • Run a subset of test methods
    • Grouping of test names by substring matching.
      py.test -k <name> -v will run all the tests having <name> in its name.
    • Run test by markers.Mark the tests using @pytest.mark.<name> and run the tests using pytest -m <name> to run tests marked as <name>.
  • Run tests in parallel
    • Install pytest-xdist using pip install pytest-xdist
    • Run tests using py.test -n NUM where NUM is the number of workers
  • Creating fixture methods to run code before every test by marking the method with @pytest.fixture
    • The scope of a fixture method is within the file it is defined.
    • A fixture method can be accessed across multiple test files by defining it in conftest.py file.
    • A test method can access a fixture by using it as an input argument.
  • Parametrizing tests to run it against multiple set of inputs.
    @pytest.mark.parametrize("input1, input2, output",[(5,5,10),(3,5,12)]) def test_add(input1, input2, output):
    assert input1+input2 == output,"failed"
    will run the test with inputs (5,5,10) and (3,5,12)
  • Skip/xfail tests using @pytets.mark.skip and @pytest.mark.xfail
  • Create test results in XML format which covers executed test details using py.test test_sample1.py -v --junitxml="result.xml"
  • A sample pytest framework to test an API

 

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