TestingOveride

foo

def _create_config():
    return {"foo_key":0}   

def create_foo_thing():
    config = _create_config()
    return config

test_foo

from unittest.mock import patch
import foo
import pytest

@pytest.fixture()
def setup_test_A():
    config = foo._create_config()
    config['foo_key'] = 10
    with patch.object(foo, '_create_config', return_value=config): 
        foo_thing = foo.create_foo_thing()
    return foo_thing
    
def test_A_fixed(setup_test_A):
    foo_thing = setup_test_A
    # We have edited the config in this test to 10, so assert its 10
    assert foo_thing['foo_key'] == 10
    
def test_B():
    # As we are not editing the config, we expect foo_key to be 0, its default
    foo_thing = foo.create_foo_thing()
    assert foo_thing['foo_key'] == 0 
 
   

How about:

from importlib import reload

import pytest

import foo


@pytest.fixture(autouse=True)
def setup():
    reload(foo)


def test_A_fixed():

    foo._create_config = lambda: {"foo_key": 10}

    foo_thing = foo.create_foo_thing()

    assert foo_thing['foo_key'] == 10


def test_B_fixed():

    foo._create_config = lambda: {"foo_key": 5}

    foo_thing = foo.create_foo_thing()

    assert foo_thing['foo_key'] == 5

Equivalent to above but in style of original gist entry

from unittest.mock import patch
import foo
import pytest

@pytest.fixture()
def setup_test_A():
    with patch.object(foo, '_create_config', return_value={"foo_key": 10}): 
        foo_thing = foo.create_foo_thing()
    return foo_thing

@pytest.fixture()
def setup_test_B():
    with patch.object(foo, '_create_config', return_value={"foo_key": 5}): 
        foo_thing = foo.create_foo_thing()
    return foo_thing
    
def test_A_fixed(setup_test_A):
    foo_thing = setup_test_A
    assert foo_thing['foo_key'] == 10
    
def test_B_fixed(setup_test_B):
    foo_thing = setup_test_B
    assert foo_thing['foo_key'] == 5