python class method mocking failure

Trying to understand mocking/patching and I have a restful API project with three files (FYI, I'm using flask)

1. class1.py
2. domain.py
3. test_domain.py

class1.py file content:

class one:
    def addition(self):
       return 4+5

domain.py file content:

from class1 import one

class DomainClass(Resource):

    def post(self):

        test1 = one()
        val = test1.addition()

        return {'test' : val } 

test_domain.py file content:

import my_app
from flask_api import status
from mock import patch

app = my_app.app.test_client()

def test_post():
    with patch('domain.one') as mock:
        instance = mock.return_value
        instance.addition.return_value = 'yello'

    url = '/domain'
    response = app.post(url)
    print response.data

    assert status.HTTP_200_OK == response.status_code
    assert mock.called

For my test_domain.py file, I've also tried this...

@patch('domain.one')
def test_post(mock_domain):
    mock_domain.addition.return_value = 1

    url = '/domain'
    response = app.post(url)
    print response.data

    assert status.HTTP_200_OK == response.status_code

My assert for the status of 200 passes, however, the problem is that I'm not able to mock or patch the addition method to give me value of 1 in place of 9 (4+5). I also tried doing 'assert mock.called' and it failes as well. I know I should be mocking/patching where the 'one()' method is used, i.e. in domain.py not in class1.py. But I tried even mocking class1.one in place of domain.one and I still kept getting 9 and not 1. What am I doing wrong ?

******** Update I've another dilemma on the same issue, I tried doing this in the test_domain file instead of patching....

from common.class1 import one
def test_post():
    one.addition = MagicMock(return_value=40)

    url = '/domain'
    response = app.post(url)
    print response.data

    assert status.HTTP_200_OK == response.status_code

Question

1. In update above, I did not do a mock at the place where it is used (i.e.: domain.one.addition = MagicMock(...) and it still worked !!!! It seems it may be doing a global change. Why did this work ?

2. In the above example, 'one' is a class in the module class1.py. If I change this class 'one' to a function in class1.py, mocking does not work. It seems this function 'one' residing in module class1.py can not be mocked like this...one.return_value = 'xyz', why? Can it be mocked globally ?

Copyright Notice：Content Author:「LuckyStarr」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/30092414/python-class-method-mocking-failure