conftest.py

def pytest_configure(config):
    config.addinivalue_line(
        "markers", "enable_inline_css: run css postprocessing over generated emails"
    )

@pytest.fixture(autouse=True)
def disable_inline_css(request, monkeypatch):
    if "enable_inline_css" in request.keywords:
        return

    monkeypatch.setattr("email_templates.models.inline_css", identity)

@pytest.fixture(autouse=True)
def enable_db_access_for_all_tests(db):
    pass

test.py

import unittest
import pytest

class EmailTests(unittest.TestCase):
    # disable the global mock!
    @pytest.mark.enable_inline_css
    def test_some_email_css(self):
        self.assertIn("<div style='...'>email</div>", generate_email())