Why python mock patch doesn't work?

Question

I have two files

spike.py

class T1(object):
    def foo(self, afd):
        return "foo"

    def get_foo(self):
        return self.foo(1)


def bar():
    return "bar"

test_spike.py:

from unittest import TestCase
import unittest
from mock import patch, MagicMock
from spike import T1, bar


class TestStuff(TestCase):
    @patch('spike.T1.foo', MagicMock(return_value='patched'))
    def test_foo(self):
        foo = T1().get_foo()
        self.assertEqual('patched', foo)

    @patch('spike.bar')
    def test_bar(self, mock_obj):
        mock_obj.return_value = 'patched'
        bar = bar()
        self.assertEqual('patched', bar)


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

When I run python test_spike.py, the first test case would pass, but the second would fail. and I switch to use nosetests test_spike.py, then both two are failed.

I don't understand how this happened? These cases supposed to pass all.

Possible duplicate of Python mock patch doesn't work as expected for public method — funky-future
– funky-future, Commented May 24, 2017 at 21:15

Martijn Pieters · Accepted Answer · 2015-08-10 13:19:20Z

71

Access bar using spike.bar. Imported bar is not affected by mock.patch.

from unittest import TestCase
import unittest
from mock import patch, MagicMock
from spike import T1
import spike # <----


class TestShit(TestCase):
    @patch('spike.T1.foo', MagicMock(return_value='patched'))
    def test_foo(self):
        foo = T1().get_foo()
        self.assertEqual('patched', foo)

    @patch('spike.bar')
    def test_bar(self, mock_obj):
        mock_obj.return_value = 'patched'
        bar = spike.bar() # <-----
        self.assertEqual('patched', bar)


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

edited Aug 10, 2015 at 13:19

Martijn Pieters

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answered Nov 27, 2013 at 12:47

falsetru

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2 Comments

LeoShi Over a year ago

Thanks, it works for python test_spike.py, but why it still not works for nosetests test_spike.py?

falsetru Over a year ago

@LeoShi, Sorry, I don't know about that.

matthew-mcallister · Accepted Answer · 2022-03-18 23:54:09Z

65

To elaborate on the very helpful top answer, let me paraphrase the official documentation for unittest.mock.

a.py
    class SomeClass:
        ...

b.py
    import a
    from a import SomeClass
    def some_function():
        a.SomeClass()
        SomeClass()

If you write mock.patch('a.SomeClass'), this will affect the first line of some_function. If you write mock.patch('b.SomeClass'), this will affect the second line.

answered Mar 18, 2022 at 23:54

matthew-mcallister

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3 Comments

Kalle Nijs Over a year ago

Thank you, this made the difference for me! instead of patching core_module.core_function I had to patch module_that_im_testing.core_function, since the module that I'm testing also imported the function.

matthew-mcallister Over a year ago

It's easiest to patch functions globally if you always call them with qualified names. For example, inside of module_that_im_testing, don't write core_function(...), but instead write core_module.core_function(...). That way, you don't have to patch core_function once for every module where it's used.

WakeskaterX Over a year ago

ffs thank you. This wasn't very intuitive for me, but you made it very clear.

vmonteco · Accepted Answer · 2023-12-13 02:40:26Z

For test_foo you are not using patch correctly. You should be using it like this:

class TestFoo(TestCase):
    @patch.object(T1, 'foo', MagicMock(return_value='patched'))
    def test_foo(self):
        foo = T1().get_foo()
        self.assertEqual('patched', foo)

that gives me:

nosetests test_spike.py 
.
----------------------------------------------------------------------
Ran 1 test in 0.000s

OK

Now the second example does not work because you import bar function (get a reference to it) and then try to mock it. When you mock something you can't change what your variables hold (reference to original function). To fix this you should use @falsetru suggested method like:

from unittest import TestCase
import unittest
from mock import patch
import spike


class TestFoo(TestCase):
    @patch('spike.bar')
    def test_bar(self, mock_obj):
        mock_obj.return_value = 'patched'
        value = spike.bar()
        self.assertEqual('patched', value)


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

this gives me:

python test_spike.py
.
----------------------------------------------------------------------
Ran 1 test in 0.000s

OK

But when I try to run it with nose I get:

 nosetests test_spike.py
F
======================================================================
FAIL: test_bar (src.test_spike.TestFoo)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/zilva/envs/test/local/lib/python2.7/site-packages/mock/mock.py", line 1305, in patched
    return func(*args, **keywargs)
  File "/home/zilva/git/test/src/test_spike.py", line 11, in test_bar
    self.assertEqual('patched', value)
AssertionError: 'patched' != 'bar'

----------------------------------------------------------------------
Ran 1 test in 0.001s

FAILED (failures=1)

This happends because I am patching not in the right place. My directory structure is:

test/
└── src/
    ├── spike.py
    ├── test_spike.py
    └── __init__.py

and I run tests from src directory so I should be patching using path from project root directory like:

@patch('src.spike.bar')

and this would give me:

nosetests test_spike.py
.
----------------------------------------------------------------------
Ran 1 test in 0.000s

OK

or if I am at test directory:

nosetests src/test_spike.py
.
----------------------------------------------------------------------
Ran 1 test in 0.001s

OK

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