how to mock an exception call in python?

Question

I have a code like this:

def extract(data):
    if len(data) == 3:
       a = 3
    else:
        component = data.split("-")
        if len(component) == 3:
            a,b,c = component
        else:
            raise globals.myException("data1", "Incorrect format", data)

    return a,b,c

This is a simplified one. I want to mock the exception class globals.myException. I'm doing that:

def test_extract_data_throws_exception(self):
        with patch('globals.myException') as mock: 
            mock.__init__("data1", "Incorrect format", "")
            with self.assertRaises(myException):
                self.assertEqual(extract(""), (""))

And I always get the error: "TypeError: exceptions must be old-style classes or derived from BaseException, not MagicMock"

EDIT: As @Aaron Digulla suggest, monkey patching is the correct solution. I post the solution to help others.

def test_extract_data_throws_exception(self):
        #monkey patching
        class ReplaceClass(myException):
            def __init__(self, module, message, detail = u''):
                pass

        globals.myException = ReplaceClass
        with self.assertRaises(myException:
            self.assertEqual(extract(""), (""))

Why do you want to mock the exception? Why not simply throw it and catch it in the test? — Aaron Digulla, Feb 08 '13 at 11:05
MyException extends from Exception but adding some funcionality as Log services and other things. I know that I could include my exception as part of my test but I'd like to keep clean my unit test. Also, it is a matter of curiosity :) — Killrazor, Feb 08 '13 at 11:09

score 4 · Accepted Answer · edited May 23 '17 at 11:44

4

The reason is raise checks the type of the argument. It must be a string (a.k.a "old style exceptions") or derived from BaseException

Since a mock isn't either, raise refuses to use it.

In this specific case, you either have to raise the exception or use monkey patching (= overwrite the symbol globals.myException in your test and restore it afterwards).

edited May 23 '17 at 11:44

Community

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answered Feb 08 '13 at 11:18

Aaron Digulla

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Thanks for your help. I've posted the solution using monkey patching. – Killrazor Feb 08 '13 at 12:13
You need to restore the original value after the test! – Aaron Digulla Feb 08 '13 at 12:47

how to mock an exception call in python?

1 Answers1