Test with mock for a function in python

Question

For the test coverage I want to test also the exception block of this function that is in the file 'signalC':

 class SignalC:
    def readSignal(self, a):
    try:
        with open(os.path.join(self.newSubFolder, "my file" + '.csv'), 'a') as csvfile:
            writer = csv.writer(csvfile, delimiter=',', quotechar='|',
                                quoting=csv.QUOTE_MINIMAL, lineterminator='\n')
            print 'Reading'
            z = random.uniform(-0.1, 0.1)
            readValue = z + setP[element]

            writer.writerow([self.element + '-' + str(element+1),)
    except IOError as message:
        logging.error('Error in writing the csv file ' + str(message))
        print(message.strerror)
        raise IOError

So far I tried with this approach, but still cant go inside the exception block:

def testReadSignal(self):
    sc = signalC.SignalC()
    a = [1, 1, 1]
    with mock.patch("signalC.SignalC.readSignal", side_effect=IOError("IOError")):
        self.assertRaises(IOError, sc.readSignal, a)

Or I should instead raise the exception with a wrong input? Can anyone give any example? Thanks in advance

Simeon Visser · Accepted Answer · 2017-08-05 13:30:20Z

1

It appears you want to create an instance of SignalC and then call that readSignal method. So you shouldn't patch that method itself ("signalC.SignalC.readSignal") because that means it'll be mocked and you won't be calling the real implementation of that method.

You could raise the IOError when calling open for example:

def testReadSignal(self):
    sc = signalC.SignalC()
    a = [1, 1, 1]
    with mock.patch("signalC.open", side_effect=IOError("IOError")):
        self.assertRaises(IOError, sc.readSignal, a)

answered Aug 5, 2017 at 13:30

Simeon Visser

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1 Comment

Joe Over a year ago

Thanks lot! I dont know why I didnt think about open to raise the error

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