Python mock multiple return values

Question

I am using pythons mock.patch and would like to change the return value for each call.
Here is the caveat:
the function being patched has no inputs, so I can not change the return value based on the input.

Here is my code for reference.

def get_boolean_response():
    response = io.prompt('y/n').lower()
    while response not in ('y', 'n', 'yes', 'no'):
        io.echo('Not a valid input. Try again'])
        response = io.prompt('y/n').lower()

    return response in ('y', 'yes')

My Test code:

@mock.patch('io')
def test_get_boolean_response(self, mock_io):
    #setup
    mock_io.prompt.return_value = ['x','y']
    result = operations.get_boolean_response()

    #test
    self.assertTrue(result)
    self.assertEqual(mock_io.prompt.call_count, 2)

io.prompt is just a platform independent (python 2 and 3) version of “input”. So ultimately I am trying to mock out the users input. I have tried using a list for the return value, but that doesn’t seam to work.

You can see that if the return value is something invalid, I will just get an infinite loop here. So I need a way to eventually change the return value, so that my test actually finishes.

(another possible way to answer this question could be to explain how I could mimic user input in a unit-test)

Not a dup of this question mainly because I do not have the ability to vary the inputs.

One of the comments of the Answer on this question is along the same lines, but no answer/comment has been provided.

Asked By: Nick Humrich

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Source

Answer 1

You can assign an iterable to side_effect, and the mock will return the next value in the sequence each time it is called:

>>> from unittest.mock import Mock
>>> m = Mock()
>>> m.side_effect = ['foo', 'bar', 'baz']
>>> m()
'foo'
>>> m()
'bar'
>>> m()
'baz'

Quoting the Mock() documentation:

If side_effect is an iterable then each call to the mock will return the next value from the iterable.

Answered By: Martijn Pieters

Answer 2

You can also use patch for multiple return values:

@patch('Function_to_be_patched', return_value=['a', 'b', 'c'])

Remember that if you are making use of more than one patch for a method then the order of it will look like this:

@patch('a')
@patch('b')
def test(mock_b, mock_a);
    pass

as you can see here, it will be reverted. First mentioned patch will be in the last position.

Answered By: avinash bhat

Answer 3

for multiple return values, we can use side_effect during patch initializing also and pass iterable to it

sample.py

def hello_world():
    pass

test_sample.py

from unittest.mock import patch
from sample import hello_world

@patch('sample.hello_world', side_effect=[{'a': 1, 'b': 2}, {'a': 4, 'b': 5}])
def test_first_test(self, hello_world_patched):
    assert hello_world() == {'a': 1, 'b': 2}
    assert hello_world() == {'a': 4, 'b': 5}
    assert hello_world_patched.call_count == 2

Answered By: shrhawk

Answer 4

I think the simplest solution is using a combination of iter(), patch(), side_effect.

from unittest.mock import patch
from return_inputs import *

def test_something_that_has_2_inputs(self):
        input_list = ['Foo', 'Bar']
        inputs = iter(input_list)
        with patch("builtins.input", side_effect=inputs):
            name1, name2 = return_inputs()
        assert name1 == "Foo"
        assert name2 == 'Bar'

Answered By: Ambassador Kosh

Python mock multiple return values

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