Unittest for a function throws an exception in Python - python-3.x

Question

class Kill_Tasks:
def __init__(self):
self.action = None
self.cmd=[]
def kill_tasks(self):
self.action = False
#exp task = chrome.exe
target = 'chrome.exe'
try:
self.cmd = suprocess.run(['taskkill','/im', target,'/f'],capture_output=True, check=True)
self.action = True
except subprocess.CalledProcessError as e:
print(e)
self.action = False
return self.cmd
Unit Test for a function throws an exception in Python.
The question is: how can I do the Unit Testing for the exception part. I'm doing Code Coverage for this function. But I am not able to check the exception because it's always True. Do I need to do a fake connection to be able to test the exception? Please help me out! Thanks

Answer 1

You should use unittest.mock.Mock.side_effect to mock a raised exception.
E.g.
kill_tasks.py:
import subprocess
class Kill_Tasks:
def __init__(self):
self.action = None
self.cmd = []
def kill_tasks(self):
self.action = False
# exp task = chrome.exe
target = 'chrome.exe'
try:
self.cmd = subprocess.run(['taskkill', '/im', target, '/f'], capture_output=True, check=True)
self.action = True
except subprocess.CalledProcessError as e:
print(e)
self.action = False
return self.cmd
test_kill_tasks.py:
import unittest
from unittest.mock import patch
from subprocess import CompletedProcess, CalledProcessError
from kill_tasks import Kill_Tasks
class TestKillTasks(unittest.TestCase):
#patch('kill_tasks.subprocess.run')
def test_kill_tasks_success(self, mock_run):
mock_run.return_value = CompletedProcess(args=['taskkill', '/im', 'chrome.exe', '/f'], returncode=0)
kill_tasks_instance = Kill_Tasks()
cmd = kill_tasks_instance.kill_tasks()
mock_run.assert_called_once_with(['taskkill', '/im', 'chrome.exe', '/f'], capture_output=True, check=True)
self.assertIsInstance(cmd, CompletedProcess)
self.assertTrue(kill_tasks_instance.action)
#patch('kill_tasks.subprocess.run')
def test_kill_tasks_exception(self, mock_run):
mock_run.side_effect = CalledProcessError(returncode=1, cmd='123')
kill_tasks_instance = Kill_Tasks()
kill_tasks_instance.kill_tasks()
self.assertFalse(kill_tasks_instance.action)
if __name__ == '__main__':
unittest.main()
unit test result:
coverage run /Users/dulin/workspace/github.com/mrdulin/python-codelab/src/stackoverflow/66848842/test_kill_tasks.py && coverage report -m --include='./src/**'
Command '123' returned non-zero exit status 1.
..
----------------------------------------------------------------------
Ran 2 tests in 0.001s
OK
Name Stmts Miss Cover Missing
-----------------------------------------------------------------------------
src/stackoverflow/66848842/kill_tasks.py 15 0 100%
src/stackoverflow/66848842/test_kill_tasks.py 21 0 100%
-----------------------------------------------------------------------------
TOTAL 36 0 100%