unit testing - Using python's mock patch.object to change the return value of a method called within another method

Question

Is it possible to mock a return value of a function called within another function I am trying to test? I would like the mocked method (which will be called in many methods I'm testing) to returned my specified variables each time it is called. For example:

class Foo:
    def method_1():
       results = uses_some_other_method()
    def method_n():
       results = uses_some_other_method()

In the unit test, I would like to use mock to change the return value of uses_some_other_method() so that any time it is called in Foo, it will return what I defined in @patch.object(...)

Answer 1

There are two ways you can do this; with patch and with patch.object

Patch assumes that you are not directly importing the object but that it is being used by the object you are testing as in the following

#foo.py
def some_fn():
    return 'some_fn'

class Foo(object):
    def method_1(self):
        return some_fn()

#bar.py
import foo
class Bar(object):
    def method_2(self):
        tmp = foo.Foo()
        return tmp.method_1()

#test_case_1.py
import bar
from mock import patch

@patch('foo.some_fn')
def test_bar(mock_some_fn):
    mock_some_fn.return_value = 'test-val-1'
    tmp = bar.Bar()
    assert tmp.method_2() == 'test-val-1'
    mock_some_fn.return_value = 'test-val-2'
    assert tmp.method_2() == 'test-val-2'

If you are directly importing the module to be tested, you can use patch.object as follows:

#test_case_2.py
import foo
from mock import patch

@patch.object(foo, 'some_fn')
def test_foo(test_some_fn):
    test_some_fn.return_value = 'test-val-1'
    tmp = foo.Foo()
    assert tmp.method_1() == 'test-val-1'
    test_some_fn.return_value = 'test-val-2'
    assert tmp.method_1() == 'test-val-2'

In both cases some_fn will be 'un-mocked' after the test function is complete.

Edit: In order to mock multiple functions, just add more decorators to the function and add arguments to take in the extra parameters

@patch.object(foo, 'some_fn')
@patch.object(foo, 'other_fn')
def test_foo(test_other_fn, test_some_fn):
    ...

Note that the closer the decorator is to the function definition, the earlier it is in the parameter list.