Python Cheatsheet 19 — Testing Patterns Beyond pytest Basics

from unittest.mock import MagicMock, AsyncMock, patch

m = MagicMock()
m.foo.return_value = 42
m.foo()                  # 42
m.foo.assert_called_once()
m.foo.assert_called_with()

# Async
am = AsyncMock()
am.foo.return_value = 42
await am.foo()

@patch("myapp.module.func")
def test_x(mock_func):
    mock_func.return_value = "patched"
    result = my_code()
    mock_func.assert_called()

# Or context manager
def test_y():
    with patch("myapp.module.func") as m:
        m.return_value = "x"
        ...

@patch.object(MyClass, "method")
def test_method(mock_method):
    mock_method.return_value = "x"

m.side_effect = [1, 2, 3]      # returns 1, 2, 3 on consecutive calls
m.side_effect = ValueError     # raises on call
m.side_effect = lambda x: x + 1

def test_env(monkeypatch):
    monkeypatch.setenv("API_KEY", "test")
    monkeypatch.setattr("module.func", lambda: "x")
    monkeypatch.delattr("module.attr", raising=False)

from freezegun import freeze_time

@freeze_time("2026-01-15")
def test_today():
    assert datetime.now().date() == date(2026, 1, 15)

# Tick time
@freeze_time("2026-01-15", tick=True)
def test_with_tick():
    # time advances normally; just starts at a known point
    ...

uv add --dev syrupy

def test_render(snapshot):
    result = render_page(...)
    assert result == snapshot

pytest --snapshot-update    # regenerate

uv add --dev hypothesis

from hypothesis import given, strategies as st

@given(st.integers(), st.integers())
def test_add_commutative(a, b):
    assert add(a, b) == add(b, a)

@given(st.text(min_size=1))
def test_uppercase_invertible(s):
    assert s.upper().lower().casefold() == s.casefold()

import factory
from src.app.models import User

class UserFactory(factory.Factory):
    class Meta:
        model = User
    
    id = factory.Sequence(lambda n: n)
    email = factory.Sequence(lambda n: f"user{n}@example.com")
    name = factory.Faker("name")

# Use
u = UserFactory.build()              # not saved
u = UserFactory()                    # may save (depends on Meta)

class PostFactory(factory.Factory):
    class Meta: model = Post
    title = factory.Faker("sentence")
    author = factory.SubFactory(UserFactory)

from polyfactory.factories.pydantic_factory import ModelFactory

class UserFactory(ModelFactory[User]):
    __model__ = User

u = UserFactory.build()

uv add --dev respx

import respx, httpx

@respx.mock
async def test_call():
    respx.get("https://api.example.com/users/1").mock(
        return_value=httpx.Response(200, json={"id": 1})
    )
    
    r = await my_code()
    assert r["id"] == 1

@pytest.fixture
def mock_api():
    with respx.mock(base_url="https://api.example.com") as m:
        m.get("/users").respond(json=[...])
        yield m

assert 0.1 + 0.2 == pytest.approx(0.3)
assert vector == pytest.approx([1.0, 2.0, 3.0], rel=1e-3)

@dataclass
class Point: x: int; y: int

assert Point(1, 2) == Point(1, 2)    # value equality

@freeze_time("2026-01-01")
def test_expires(db_session):
    user = UserFactory.build()
    db_session.add(user); db_session.commit()
    
    with freeze_time("2026-04-01"):
        assert user.is_expired

@pytest.mark.parametrize("payload, expected", [
    pytest.param({"a": 1}, True, id="happy"),
    pytest.param({"a": -1}, False, id="negative", marks=pytest.mark.xfail),
])
def test_(payload, expected): ...

tests/
├── conftest.py           # global fixtures
├── api/
│   ├── conftest.py       # api-specific fixtures
│   └── test_users.py

uv add --dev nox

# noxfile.py
import nox

@nox.session(python=["3.12", "3.13"])
def tests(session):
    session.install(".[dev]")
    session.run("pytest")

nox            # runs all sessions
nox -s tests   # one session

pytest --cov=src --cov-branch --cov-report=html