python mock exception handling

Question

I have couple of tasks in my workflow..Now, I want to mock the output of Task1 returning exception and just test task2.run method is not called.

class Task1():
    def run(self):
        try:
            return 2,2
        except Exception as e:
            print e
            raise Exception('Task1 Exception')

class Task2():
    def run(self,x,y):
        try:
            return 2 * (x*y)
        except Exception as e:
            print e
            raise Exception('Task2 Exception')            

class Workflow():
    def run(self,task1, task2):
        x,y = task1.run()
        print task2.run(x,y)


task1 = Task1()
task2 = Task2()

w = Workflow()
w.run(task1,task2)

Here is my unittest -

import unittest
import mock
from test2 import Task1, Task2, Workflow
class TestWorkflow(unittest.TestCase):
    def test_workflow(self):
        self.task1 = mock.MagicMock(Task1()) # mocking task1
        self.task2 = mock.MagicMock(Task2())
        self.task1.side_effect = Exception() # setting task1 exception to an exception object

        self.workflow = Workflow()
        self.workflow.run(self.task1, self.task2)
        self.task2.run.assert_not_called() # checking task2 is not called.


if __name__ == '__main__':
    unittest.main()

Exception:-

    x,y = task1.run()
ValueError: need more than 0 values to unpack

coelhudo · Accepted Answer · 2019-01-31 11:53:38Z

1

Given the syntax, you are using Python 2, but I will refer to the documentation in the Python 3 official documentation anyway. I believe the solution applies.

The first example is:

from unittest.mock import MagicMock

thing = ProductionClass()
thing.method = MagicMock(return_value=3)
thing.method(3, 4, 5, key='value')
#prints 3

thing.method.assert_called_with(3, 4, 5, key='value')

Bringing this your problem you would have the code below instead of your current code. See that I'm assigning the Task1.run and Task2.run methods to a MagicMock instance.

import unittest
import mock
from t import Task1, Task2, Workflow
class TestWorkflow(unittest.TestCase):
    def test_workflow(self):
        self.task1 = Task1()
        self.task1.run = mock.MagicMock() # mocking run from task1
        self.task2 = Task2()
        self.task2.run = mock.MagicMock()
        self.task1.run.side_effect = Exception() # setting task1 exception to an exception object

        self.workflow = Workflow()
        self.workflow.run(self.task1, self.task2)
        self.task2.run.assert_not_called() # checking task2 is not called.


if __name__ == '__main__':
    unittest.main()

edited Jan 31, 2019 at 11:53

answered Dec 7, 2017 at 0:19

coelhudo

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2 Comments

user1050619 Over a year ago

It throws this error- File "/usr/local/lib/python2.7/site-packages/mock/mock.py", line 1118, in _mock_call raise effect Exception - Were you able to run without issues?

coelhudo Over a year ago

Nope. In your example, in the test, you wrote the method run to throw an Exception. This means that you need a try/expect around the call of method task1.run in Workflow to make it work.

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