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Mocking python function based on input arguments

Posted by: admin November 30, 2017 Leave a comment


We have been using Mock for python for a while.

Now, we have a situation in which we want to mock a function

def foo(self, my_param):
    #do something here, assign something to my_result
    return my_result

Normally, the way to mock this would be (assuming foo being part of an object)

self.foo = MagicMock(return_value="mocked!")

Even, if i call foo() a couple of times i can use

self.foo = MagicMock(side_effect=["mocked once", "mocked twice!"])

Now, I am facing a situation in which I want to return a fixed value when the input parameter has a particular value. So if let’s say “my_param” is equal to “something” then I want to return “my_cool_mock”

This seems to be available on mockito for python


I have been searching on how to achieve the same with Mock with no success?

Any ideas?


If side_effect is a function then whatever that function returns is
what calls to the mock return. The side_effect function is called with
the same arguments as the mock. This allows you to vary the return
value of the call dynamically, based on the input:

>>> def side_effect(value):
...     return value + 1
>>> m = MagicMock(side_effect=side_effect)
>>> m(1)
>>> m(2)
>>> m.mock_calls
[call(1), call(2)]



As indicated at Python Mock object with method called multiple times

A solution is to write my own side_effect

def my_side_effect(*args, **kwargs):
    if args[0] == 42:
        return "Called with 42"
    elif args[0] == 43:
        return "Called with 43"
    elif kwarg['foo'] == 7:
        return "Foo is seven"

mockobj.mockmethod.side_effect = my_side_effect

That does the trick


Side effect takes a function (which can also be a lambda function), so for simple cases you may use:

m = MagicMock(side_effect=(lambda x: x+1))


Just to show another way of doing it:

def mock_isdir(path):
    return path in ['/var/log', '/var/log/apache2', '/var/log/tomcat']

with mock.patch('os.path.isdir') as os_path_isdir:
    os_path_isdir.side_effect = mock_isdir