Mocking python function based on input arguments

Question

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

when(dummy).foo("something").thenReturn("my_cool_mock")

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

Any ideas?

Answer 1

I know its quite an old question, might help as an improvement using python lamdba

self.some_service.foo.side_effect = lambda *args:"Called with 42" \
            if args[0] == 42 \
            else "Called with 42" if args[0] == 43 \
            else "Called with 43" if args[0] == 43 \
            else "Called with 45" if args[0] == 45 \
            else "Called with 49" if args[0] == 49 else None

Answer 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 kwargs['foo'] == 7:
        return "Foo is seven"

mockobj.mockmethod.side_effect = my_side_effect

That does the trick

Answer 3

You can also use partial from functools if you want to use a function that takes parameters but the function you are mocking does not. E.g. like this:

def mock_year(year):
    return datetime.datetime(year, 11, 28, tzinfo=timezone.utc)

@patch('django.utils.timezone.now', side_effect=partial(mock_year, year=2020))

This will return a callable that doesn't accept parameters (like Django's timezone.now()), but my mock_year function does.

Answer 4

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))

Answer 5

I've ended up here looking for "how to mock a function based on input arguments" and I finally solved this creating a simple aux function:

def mock_responses(responses, default_response=None):
  return lambda input: responses[input] if input in responses else default_response

Now:

my_mock.foo.side_effect = mock_responses(
  {
    'x': 42, 
    'y': [1,2,3]
  })
my_mock.goo.side_effect = mock_responses(
  {
    'hello': 'world'
  }, 
  default_response='hi')
...

my_mock.foo('x') # => 42
my_mock.foo('y') # => [1,2,3]
my_mock.foo('unknown') # => None

my_mock.goo('hello') # => 'world'
my_mock.goo('ey') # => 'hi'

Hope this will help someone!

Answer 6

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