How to properly use mock in python with unittest setUp

I'll start by answering your questions, and then I'll give a detailed example of how patch() and setUp() interact.

1. I don't think it looks right, see my answer to question #3 in this list for details.
2. Yes, the actual call to patch looks like it should mock the object you want.
3. No, you almost never want to use the @patch() decorator on setUp(). You got lucky, because the object is created in setUp() and never gets created during the test method.
4. I don't know of any way to make a mock object raise an exception without importing that exception into your test case file.
5. I don't see any need for patch.object() here. It just lets you patch attributes of an object instead of specifying the target as a string.

To expand on my answer to question #3, the problem is that the patch() decorator only applies while the decorated function is running. As soon as setUp() returns, the patch is removed. In your case, that works, but I bet it would confuse someone looking at this test. If you really only want the patch to happen during setUp(), I would suggest using the with statement to make it obvious that the patch is going to be removed.

The following example has two test cases. TestPatchAsDecorator shows that decorating the class will apply the patch during the test method, but not during setUp(). TestPatchInSetUp shows how you can apply the patch so that it's in place during both setUp() and the test method. Calling self.addCleanUp() makes sure that the patch will be removed during tearDown().

import unittest
from mock import patch


@patch('__builtin__.sum', return_value=99)
class TestPatchAsDecorator(unittest.TestCase):
    def setUp(self):
        s = sum([1, 2, 3])

        self.assertEqual(6, s)

    def test_sum(self, mock_sum):
        s1 = sum([1, 2, 3])
        mock_sum.return_value = 42
        s2 = sum([1, 2, 3])

        self.assertEqual(99, s1)
        self.assertEqual(42, s2)


class TestPatchInSetUp(unittest.TestCase):
    def setUp(self):
        patcher = patch('__builtin__.sum', return_value=99)
        self.mock_sum = patcher.start()
        self.addCleanup(patcher.stop)

        s = sum([1, 2, 3])

        self.assertEqual(99, s)

    def test_sum(self):
        s1 = sum([1, 2, 3])
        self.mock_sum.return_value = 42
        s2 = sum([1, 2, 3])

        self.assertEqual(99, s1)
        self.assertEqual(42, s2)

You can use patch() as a class decorator, not just as a function decorator. You can then pass in the mocked function as before:

@patch('mymodule.SomeClass')
class MyTest(TestCase):

    def test_one(self, MockSomeClass):
        self.assertIs(mymodule.SomeClass, MockSomeClass)

See: Applying the same patch to every test method (which also lists alternatives)

It makes more sense to set up the patcher this way on setUp if you want the patching to be done for all the test methods.

I'd like to point out a variation of the accepted answer in which a new argument is passed to the patch() decorator:

from unittest.mock import patch, Mock

MockSomeClass = Mock()

@patch('mymodule.SomeClass', new=MockSomeClass)
class MyTest(TestCase):
    def test_one(self):
        # Do your test here

Note that in this case, it is no longer necessary to add the second argument, MockSomeClass, to every test method, which can save a lot of code repetition.

An explanation of this can be found at https://docs.python.org/3/library/unittest.mock.html#patch:

If patch() is used as a decorator and new is omitted, the created mock is passed in as an extra argument to the decorated function.

The answers above all omit new, but it can be convenient to include it.