example.py

class Example:

    def __init__(self):
        self.var = 1

    def run(self):
        return self.var

requirements.txt

pytest
pytest-mock

test.py

import pytest

from example import Example

def test_run():
    e = Example()
    assert e.run() == 1

@pytest.fixture
def mock_example(mocker):
    m = mocker.patch('example.Example.run')
    m.return_value = 2

def test_run_mocked(mock_example):
    e = Example()
    assert e.run() == 2