What happens when a python mock has both a return value and a list of side effects?

Question

I'm having trouble understanding what's happening in some test code. It looks like this:

import pytest
from unittest.mock import MagicMock
from my_module import MyClass

confusing_mock = MagicMock(
    return_value=b"",
    side_effect=[
        ConnectionError(),
        b"another_return_value?",
        b"another_another_return_value?"
    ])

mocked_class = MyClass()
monkeypatch.setattr(mocked_class, "method_to_call_thrice", confusing_mock)

I know that:

side_effect is a function to be called whenever the mock is called
but if side_effect is an iterable, then "each call to the mock will return the next value from the iterable" (thanks pytest docs)
the docs also say that if the function passed to side_effect returns DEFAULT, then the mock will return it's normal value from return_value

But here's what I don't get:

What happens when I provide both a list of side effects and a return value?
What should I expect to see on each call of MyClass.method_to_call_thrice?

The question "unittest.Mock - Combining return_value and side_effect" seems related, but I'm still not quite getting it. — tessafyi
– tessafyi, Commented May 17, 2019 at 17:42
You could try with a simple return valeu and a simple side effect and see what happens? — Tom Dalton
– Tom Dalton, Commented May 17, 2019 at 17:46

chepner · Accepted Answer · 2019-05-17 17:49:52Z

39

side_effect is used. A list value can contain mock.DEFAULT, and a function can return mock.DEFAULT, to indicate that the value of the return_value attribute be used.

>>> import unittest.mock
>>> m = unittest.mock.Mock(return_value="foo",
...                        side_effect=[1, 2, unittest.mock.DEFAULT, 4, 5])
>>> m()
1
>>> m()
2
>>> m()
'foo'
>>> m()
4
>>> m()
5
>>> unittest.mock.Mock(return_value="foo",
...                    side_effect=lambda: unittest.mock.DEFAULT)()
'foo'

answered May 17, 2019 at 17:49

chepner

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1 Comment

Intrastellar Explorer Over a year ago

To remove an already present side_effect, you can also set it to None (per the docs here)

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