Mocking a python class which changes an instance variable after a method call?

Question

I've some code where a call to the method update() changes the values of some instance variables. I use the changed value to leave a loop. Here is simplified example of my code:

def do_stuff(self):
    # get a new instance of A
    a = get_a()
    while True:
        a.update()
        if a.state == 'state':
            break

This a simple version of the class (I can't change the class, because it's 3rd party library):

class A(object):
    def __init__(self):
        self.state = ''

    def update(self):
        # call to external system
        self.state = extern_func()

Now I want to test my function do_stuff() by mocking class A. To test every aspect of the function I want to have all the different values of state and it should changing after each call of a.update() (iterate over the different states).

I started with this set up for my unit test:

from mock import Mock, patch
import unittest

class TestClass(unittest.TestClass):

    @patch('get_a')
    def test_do_stuff(self, mock_get_a):
         mock_a = Mock(spec=A)
         mock_get_a.return_value = mock_a

         # do some assertions

Can I achieve that kind behaviour with Mock? I know that Mock has side_effect to return different value for consecutive function calls. But I can't figure out a way to change a value of an instance variable after a function call?

Is it possible to keep A as it is and mock the external function? That seems the easiest way to iterate over the states as you can indeed use side_effect to have it return each of the states. — Simeon Visser
– Simeon Visser, Commented Sep 13, 2014 at 0:52
@SimeonVisser: What you are proposing is possible, but goes against the recommendation Where to patch. — ByteNudger
– ByteNudger, Commented Sep 15, 2014 at 17:37
It doesn't go against that recommendation; in fact, it follows it as you need to patch the object in the location where it's used. That only changes depending on how external_func is imported (but we can't see that in your example). If it would be import mymodule and mymodule.external_func() then you'd need to change the patching to include mymodule. — Simeon Visser
– Simeon Visser, Commented Sep 15, 2014 at 22:35

wwii · Accepted Answer · 2014-09-13 17:53:58Z

1

Setup:

from mock import Mock, MagicMock, patch

sep = '***********************\n'

# non-mock mocks
def get_a():
    return A()
def extern_func():
    return 'foo'

def do_stuff(self):
    # get a new instance of A
    a = get_a()
    while True:
        a.update()
        print a.state
        if a.state == 'state':
            break

class A(object):
    def __init__(self):
        self.state = ''

    def update(self):
        # call to external system
        self.state = extern_func()

As @Simeon Viser mentioned, mocking `extern_func` would work:

print sep, 'patch extern'
mock = MagicMock(side_effect = [1,2,3,4, 'state'])
@patch('__main__.extern_func', mock)
def foo():
    do_stuff(3)
foo()

>>> 
***********************
patch extern
1
2
3
4
state

`side_effect` can be a function and you can mock the unbound method `A.update` using the `auto_spec = True` argument.

Using a context manager:

print sep, 'patch update context manager call do_stuff'
def update_mock(self):
    self.state = mock()
mock = MagicMock(side_effect = [1,2,3,4, 'state'])
with patch.object(A, 'update', autospec = True) as mock_update:
    mock_update.side_effect = update_mock
    do_stuff(3)

>>>
***********************
patch update context manager call do_stuff
1
2
3
4
state

Using a decorator:

print sep, 'patch update decorator test_do_stuff'
def update_mock(self):
    self.state = mock()
mock = MagicMock(side_effect = [1,2,3,4, 'state'])
@patch.object(A, 'update', autospec = True, side_effect = update_mock)
def test_do_stuff(self):
    do_stuff(3)
test_do_stuff()

>>>
***********************
patch update decorator test_do_stuff
1
2
3
4
state

Caveat: I've never written comprehensive unit tests and only recently started reading the mock docs, so even though I seemed to have made this work I can't comment on it's efficacy in your testing scheme. Edits welcome.

edited Sep 13, 2014 at 17:53

answered Sep 13, 2014 at 17:47

wwii

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2 Comments

ByteNudger Over a year ago

As mentioned in the comment to @SimeonVisser your solution goes against the recommendation Where to patch. But so far it looks like the only possible solution. I will try to get my test running and let you know about my findings.

wwii Over a year ago

... That isn't a recommendation, it is describing how you reference the object you are patching - you have to follow those instructions. In my examples, the objects being patched and the test/mocking code are in the same module.

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Mocking a python class which changes an instance variable after a method call?

1 Answer 1

As @Simeon Viser mentioned, mocking `extern_func` would work:

`side_effect` can be a function and you can mock the unbound method `A.update` using the `auto_spec = True` argument.

Using a context manager:

Using a decorator:

2 Comments

Your Answer

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Collectives™ on Stack Overflow

1 Answer 1

As @Simeon Viser mentioned, mocking extern_func would work:

side_effect can be a function and you can mock the unbound method A.update using the auto_spec = True argument.

Using a context manager:

Using a decorator:

2 Comments

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As @Simeon Viser mentioned, mocking `extern_func` would work:

`side_effect` can be a function and you can mock the unbound method `A.update` using the `auto_spec = True` argument.