AI Skills

name: standardizing-python-testing description: Python testing standards enforced across all skills. Reference skill for property-based testing, factories, harness patterns, and level-specific implementations. allowed-tools: Read

<objective> Define Python testing standards and patterns that other skills reference. Not invoked directly—invoke `/testing-python` (to write tests) or `/reviewing-python-tests` (to review tests) instead. These standards apply to ALL Python test code. </objective>

<quick_start> Reference this skill for:

Level routing - How /testing decisions map to Python implementations
Level tooling - Infrastructure requirements per level
Level patterns - Concrete patterns for Level 1, 2, 3 tests
Exception implementations - The 7 exception cases from /testing in Python
Property-based testing - MANDATORY for parsers, serializers, math, algorithms
Data factories - NO magic values, use factories with constants
File naming - .unit.py, .integration.py, .e2e.py (level indicated by filename, no markers needed)
DI patterns - Protocols and dataclass dependencies
Harness patterns - Docker, subprocess, binary harnesses
Test signatures - -> None, type annotations
Credential management - Fail loudly, not skip silently
Anti-patterns - What to avoid

</quick_start>

<router_to_python_mapping> After running through /testing router, use this mapping:

Router Decision	Python Implementation
Stage 2 → Level 1	pytest + temp dirs + dataclasses for DI
Stage 2 → Level 2	pytest fixtures + Docker/subprocess harnesses
Stage 2 → Level 3	pytest + skip decorators + credential loading
Stage 3A (Pure computation)	Pure functions, test directly
Stage 3B (Extract pure part)	Factor into pure functions + thin wrappers
Stage 5 Exception 1 (Failure modes)	Protocol + stub returning errors
Stage 5 Exception 2 (Interaction protocols)	Spy class recording calls
Stage 5 Exception 3 (Time/concurrency)	`patch("time.time")`, `patch("random.random")`
Stage 5 Exception 4 (Safety)	Stub that records but doesn't execute
Stage 5 Exception 6 (Observability)	Spy class capturing request details

</router_to_python_mapping>

<level_tooling>

Level	Infrastructure	Speed
1: Unit	Python stdlib + temp dirs + standard dev tools	<100ms
2: Integration	Docker containers + project-specific binaries	<1s
3: E2E	Network services + external APIs + test accounts	<10s

Standard dev tools (Level 1): git, cat, grep, curl—available in CI without setup. Project-specific tools (Level 2): Docker, PostgreSQL, Hugo, ffmpeg—require installation.

</level_tooling>

<level_1_patterns>

Pure Computation (Stage 3A)

When the router determines your code is pure computation, test it directly.

# ✅ Test pure functions directly, no doubles needed
def test_command_includes_checksum_flag() -> None:
    cmd = build_rclone_command("/source", "remote:dest", checksum=True)
    assert "--checksum" in cmd


def test_unicode_paths_preserved() -> None:
    cmd = build_rclone_command("/tank/фото", "remote:резервная")
    assert "/tank/фото" in cmd


def test_validates_empty_order() -> None:
    result = validate_order(Order(items=[]))
    assert result.ok is False
    assert "empty" in result.error.lower()

Temporary Directories

Temp dirs are NOT external dependencies—use freely at Level 1.

import tempfile
from pathlib import Path


def test_loads_yaml_config() -> None:
    with tempfile.TemporaryDirectory() as tmpdir:
        config_path = Path(tmpdir) / "config.yaml"
        config_path.write_text("""
site_dir: ./site
base_url: http://localhost:1313
""")
        config = load_config(config_path)
        assert config.site_dir == "./site"

Extracted Logic (Stage 3B)

When the router says "extract the pure part," factor your code.

Before (tangled):

class OrderProcessor:
    def __init__(self, repository) -> None:
        self.repository = repository

    def process(self, order: Order) -> None:
        # Validation (pure) mixed with persistence (integration)
        if not order.items:
            raise ValidationError("Empty order")
        self.repository.save(order)

After (factored):

# Pure computation - test at Level 1, no doubles
def validate_order(order: Order) -> ValidationResult:
    if not order.items:
        return ValidationResult(ok=False, error="Empty order")
    return ValidationResult(ok=True)


# Thin wrapper - test at Level 2 with real database
class OrderProcessor:
    def __init__(self, repository) -> None:
        self.repository = repository

    def process(self, order: Order) -> None:
        result = validate_order(order)
        if not result.ok:
            raise ValidationError(result.error)
        self.repository.save(order)

Now test separately:

# Level 1: Test validation logic exhaustively
def test_validates_empty_order() -> None:
    result = validate_order(Order(items=[]))
    assert result.ok is False


# Level 2: Test persistence with real database (see Level 2 section)

</level_1_patterns>

<exception_implementations>

Exception Case Implementations

When the /testing router reaches Stage 5 and an exception applies, here's how to implement each in Python.

Exception 1: Failure Modes

Testing retry logic, error handling, circuit breakers.

from typing import Protocol


class HttpClient(Protocol):
    def fetch(self, url: str) -> dict: ...


def test_retries_on_timeout() -> None:
    attempts = 0

    class TimeoutingClient:
        def fetch(self, url: str) -> dict:
            nonlocal attempts
            attempts += 1
            if attempts < 3:
                raise TimeoutError("Request timed out")
            return {"status": 200, "body": "ok"}

    result = fetch_with_retry("https://api.example.com", TimeoutingClient())
    assert attempts == 3
    assert result["status"] == 200

Exception 2: Interaction Protocols

Testing call sequences, saga compensation, "no extra calls."

def test_saga_compensates_in_reverse_order() -> None:
    calls: list[str] = []

    class Step1:
        def execute(self) -> None:
            calls.append("step1-execute")

        def compensate(self) -> None:
            calls.append("step1-compensate")

    class Step2:
        def execute(self) -> None:
            calls.append("step2-execute")
            raise RuntimeError("Step 2 failed")

        def compensate(self) -> None:
            calls.append("step2-compensate")

    saga = Saga([Step1(), Step2()])

    with pytest.raises(RuntimeError):
        saga.run()

    assert calls == [
        "step1-execute",
        "step2-execute",
        "step2-compensate",
        "step1-compensate",
    ]

Exception 3: Time and Concurrency

Testing time-dependent behavior with controlled time.

from unittest.mock import patch


def test_lease_renews_before_expiry() -> None:
    with patch("time.time") as mock_time:
        mock_time.return_value = 1000.0

        renewed = False

        def on_renew() -> None:
            nonlocal renewed
            renewed = True

        lease = Lease(ttl=30, renew_at=25, on_renew=on_renew)

        # Before renewal threshold
        mock_time.return_value = 1024.0
        lease.tick()
        assert renewed is False

        # After renewal threshold
        mock_time.return_value = 1026.0
        lease.tick()
        assert renewed is True

Exception 4: Safety

Testing destructive operations without executing them.

def test_processes_refund_for_cancelled_order() -> None:
    refunds: list[dict] = []

    class FakePaymentProvider:
        def refund(self, charge_id: str, amount: float, reason: str) -> dict:
            refunds.append({"charge_id": charge_id, "amount": amount, "reason": reason})
            return {"refund_id": "refund_123", "status": "succeeded"}

    processor = OrderProcessor(payment=FakePaymentProvider())
    processor.cancel_order(order_with_charge)

    assert refunds == [
        {"charge_id": "ch_123", "amount": 99.99, "reason": "order_cancelled"}
    ]

Exception 6: Observability

Testing request details the real system can't expose.

def test_includes_idempotency_key() -> None:
    requests: list[dict] = []

    class SpyHttpClient:
        def post(self, url: str, headers: dict, body: dict) -> dict:
            requests.append({"url": url, "headers": headers, "body": body})
            return {"status": 200}

    client = PaymentClient(http=SpyHttpClient())
    client.charge(amount=100, card_token="tok_123")

    assert len(requests) == 1
    assert "Idempotency-Key" in requests[0]["headers"]

</exception_implementations>

<level_2_patterns>

Integration Patterns

When the router determines Level 2 is appropriate, use real dependencies via harnesses.

pytest Fixtures for Harnesses

import pytest


@pytest.fixture(scope="module")
def database() -> PostgresHarness:
    harness = PostgresHarness()
    harness.start()
    yield harness
    harness.stop()


@pytest.fixture(autouse=True)
def reset_database(database: PostgresHarness) -> None:
    yield
    database.reset()


def test_user_repository_saves_and_retrieves(database: PostgresHarness) -> None:
    repo = UserRepository(database.connection_string)
    user = User(email="test@example.com", name="Test User")

    repo.save(user)
    retrieved = repo.find_by_email("test@example.com")

    assert retrieved is not None
    assert retrieved.name == "Test User"

Binary Harness

@dataclass
class HugoHarness:
    site_dir: Path
    output_dir: Path

    def build(self, args: list[str] | None = None) -> subprocess.CompletedProcess:
        args = args or []
        return subprocess.run(
            [
                "hugo",
                "--source",
                str(self.site_dir),
                "--destination",
                str(self.output_dir),
            ]
            + args,
            capture_output=True,
            text=True,
        )


def test_builds_site_successfully(hugo: HugoHarness) -> None:
    result = hugo.build()
    assert result.returncode == 0
    assert (hugo.output_dir / "index.html").exists()

Docker Harness

from dataclasses import dataclass
import subprocess


@dataclass
class DockerHarness:
    """Base class for Docker-based test harnesses."""

    container_name: str
    image: str
    port: int

    def start(self) -> None:
        subprocess.run(
            [
                "docker",
                "run",
                "-d",
                "--name",
                self.container_name,
                "-p",
                f"{self.port}:{self.port}",
                self.image,
            ],
            check=True,
        )
        self._wait_for_ready()

    def stop(self) -> None:
        subprocess.run(["docker", "rm", "-f", self.container_name])

    def _wait_for_ready(self, timeout: int = 30) -> None:
        raise NotImplementedError


@dataclass
class PostgresHarness(DockerHarness):
    """PostgreSQL test harness."""

    container_name: str = "test-postgres"
    image: str = "postgres:15"
    port: int = 5432
    password: str = "test"

    def start(self) -> None:
        subprocess.run(
            [
                "docker",
                "run",
                "-d",
                "--name",
                self.container_name,
                "-p",
                f"{self.port}:5432",
                "-e",
                f"POSTGRES_PASSWORD={self.password}",
                self.image,
            ],
            check=True,
        )
        self._wait_for_ready()

    @property
    def connection_string(self) -> str:
        return f"postgresql://postgres:{self.password}@localhost:{self.port}/postgres"

</level_2_patterns>

<level_3_patterns>

E2E Patterns

When the router determines Level 3 is required (real credentials, external services).

Skip If No Credentials (Optional Tests Only)

credentials = load_credentials()
skip_no_creds = pytest.mark.skipif(
    credentials is None, reason="E2E credentials not configured"
)


@skip_no_creds
def test_full_sync_workflow(dropbox_test_folder: str) -> None:
    result = sync_to_dropbox(local_path, dropbox_test_folder)
    assert result.success
    assert result.files_transferred > 0

Note: Use skipif only for optional E2E tests. Required tests must fail loudly—see credential management section.

</level_3_patterns>

<property_based_testing> Property-based testing is MANDATORY for these code types:

Code Type	Property to Test	Example
Parsers	`parse(format(x)) == x`	JSON, YAML, CLI args
Mathematical operations	Algebraic properties	Commutativity, associativity
Serialization	`decode(encode(x)) == x`	Protocol buffers, msgpack
Complex algorithms	Invariant preservation	Sorting, tree operations

Hypothesis Configuration

from hypothesis import given, settings, strategies as st


# ✅ REQUIRED: All property tests use @given decorator
@given(st.text())
def test_round_trips_through_encoding(text: str) -> None:
    encoded = encode(text)
    decoded = decode(encoded)
    assert decoded == text


# ✅ REQUIRED: Settings for slow generators
@settings(max_examples=500, deadline=None)
@given(st.binary(min_size=1, max_size=10_000))
def test_handles_arbitrary_binary(data: bytes) -> None:
    result = process(data)
    assert result.valid or result.error is not None


# ✅ REQUIRED: Composite strategies for complex data
@st.composite
def valid_orders(draw: st.DrawFn) -> Order:
    items = draw(st.lists(st.builds(OrderItem), min_size=1, max_size=10))
    return Order(items=items, total=sum(item.price for item in items))


@given(valid_orders())
def test_order_validation_accepts_valid(order: Order) -> None:
    result = validate_order(order)
    assert result.ok is True

Common Properties

# ✅ Idempotency: f(f(x)) == f(x)
@given(st.text())
def test_normalization_is_idempotent(text: str) -> None:
    once = normalize(text)
    twice = normalize(once)
    assert once == twice


# ✅ Commutativity: f(a, b) == f(b, a)
@given(st.integers(), st.integers())
def test_merge_is_commutative(a: int, b: int) -> None:
    assert merge(a, b) == merge(b, a)


# ✅ Invariant preservation: property holds before and after
@given(st.lists(st.integers()))
def test_sort_preserves_elements(items: list[int]) -> None:
    sorted_items = sort(items)
    assert sorted(items) == sorted(sorted_items)
    assert len(items) == len(sorted_items)

If testing a parser or serializer without property-based tests, the tests are INCOMPLETE.

</property_based_testing>

<data_factories> Test data MUST use factories with named constants. Never use arbitrary literals.

Dataclass Factory Pattern

from dataclasses import dataclass, field
from typing import Iterator
import itertools

# Module-level counter for unique IDs
_id_counter: Iterator[int] = itertools.count(1)

# Named constants at module level
DEFAULT_PERFORMANCE_SCORE = 90
DEFAULT_ACCESSIBILITY_SCORE = 100
FAILING_PERFORMANCE_THRESHOLD = 45


@dataclass
class AuditResultFactory:
    """Factory for creating test audit results."""

    url: str = field(default_factory=lambda: f"https://example.com/{next(_id_counter)}")
    performance: int = DEFAULT_PERFORMANCE_SCORE
    accessibility: int = DEFAULT_ACCESSIBILITY_SCORE

    def build(self) -> dict:
        return {
            "url": self.url,
            "scores": {
                "performance": self.performance,
                "accessibility": self.accessibility,
            },
        }


# ✅ CORRECT: Factory with named constant
def test_fails_on_low_performance() -> None:
    result = AuditResultFactory(performance=FAILING_PERFORMANCE_THRESHOLD).build()
    analysis = analyze_results([result])
    assert analysis.passed is False


# ❌ REJECTED: Magic value (PLR2004)
def test_fails_on_low_performance_bad() -> None:
    result = {"scores": {"performance": 45}}  # What is 45?
    analysis = analyze_results([result])
    assert analysis.passed is False

Builder Pattern

@dataclass
class UserBuilder:
    """Builder for test users with sensible defaults."""

    email: str = field(default_factory=lambda: f"user{next(_id_counter)}@test.com")
    name: str = "Test User"
    role: str = "member"
    active: bool = True

    def with_admin_role(self) -> "UserBuilder":
        self.role = "admin"
        return self

    def inactive(self) -> "UserBuilder":
        self.active = False
        return self

    def build(self) -> User:
        return User(
            email=self.email, name=self.name, role=self.role, active=self.active
        )


# ✅ CORRECT: Builder with fluent interface
def test_admin_can_delete_users() -> None:
    admin = UserBuilder().with_admin_role().build()
    target = UserBuilder().build()
    result = delete_user(admin, target)
    assert result.success is True

</data_factories>

<file_naming> Test level is indicated by filename suffix:

Level	Suffix	Example
1	`.unit.py`	`test_validation.unit.py`
2	`.integration.py`	`test_database.integration.py`
3	`.e2e.py`	`test_checkout.e2e.py`
3	`.e2e.spec.py`	`checkout.e2e.spec.py` (Playwright)

Directory Structure

spx/
└── {capability}/
    └── {feature}/
        ├── {feature}.md
        └── tests/
            ├── test_{name}.unit.py
            ├── test_{name}.integration.py
            └── test_{name}.e2e.py

</file_naming>

<dependency_injection> When Stage 3 of /testing determines DI is appropriate, use Protocols.

Protocol Definition

from typing import Protocol


class CommandRunner(Protocol):
    """Protocol for running shell commands."""

    def run(self, cmd: list[str]) -> tuple[int, str, str]:
        """Run command, return (exit_code, stdout, stderr)."""
        ...


class HttpClient(Protocol):
    """Protocol for HTTP operations."""

    def fetch(self, url: str) -> dict:
        """Fetch URL, return response as dict."""
        ...

Dataclass Dependencies

from dataclasses import dataclass
from typing import Callable
import os


@dataclass
class SyncDependencies:
    """Dependencies for sync operation, injectable for testing."""

    run_command: CommandRunner
    get_env: Callable[[str], str | None] = os.environ.get


def sync_to_remote(source: str, dest: str, deps: SyncDependencies) -> SyncResult:
    """Sync files to remote, using injected dependencies."""
    cmd = build_command(source, dest)
    returncode, stdout, stderr = deps.run_command.run(cmd)
    return SyncResult(success=returncode == 0, output=stdout)

Test Double Implementation

# ✅ CORRECT: Test double via DI (Exception 1: Failure modes)
def test_handles_command_failure() -> None:
    class FailingRunner:
        def run(self, cmd: list[str]) -> tuple[int, str, str]:
            return (1, "", "Connection refused")

    deps = SyncDependencies(run_command=FailingRunner())
    result = sync_to_remote("/src", "remote:dest", deps)
    assert result.success is False


# ❌ REJECTED: Mocking via patch
@patch("subprocess.run")
def test_handles_command_failure_bad(mock_run: Mock) -> None:
    mock_run.return_value = Mock(returncode=1)
    result = sync_to_remote("/src", "remote:dest")  # No DI!
    assert result.success is False

</dependency_injection>

<test_signatures> All test functions MUST have complete type annotations.

import pytest
from pathlib import Path


# ✅ REQUIRED: -> None on all test functions
def test_validates_input() -> None:
    result = validate("test")
    assert result.valid


# ✅ REQUIRED: Type annotations on fixture parameters
def test_creates_file(tmp_path: Path) -> None:
    file = tmp_path / "test.txt"
    file.write_text("content")
    assert file.exists()


# ✅ REQUIRED: Return type on fixtures
@pytest.fixture
def config(tmp_path: Path) -> Config:
    return Config(path=tmp_path)


# ❌ REJECTED: Missing -> None (ANN201)
def test_something(self):
    pass


# ❌ REJECTED: Missing parameter type (ANN001)
def test_with_fixture(self, tmp_path) -> None:
    pass

</test_signatures>

<credential_management> E2E tests requiring credentials MUST fail loudly, not skip silently.

CREDENTIALS_DOC = """
Level 3 tests require these environment variables:

Required:
  STRIPE_TEST_KEY    - From 1Password: "Engineering/Test Credentials"

Setup:
  cp .env.test.example .env.test
  # Fill in values from 1Password
"""


def load_credentials() -> dict | None:
    key = os.environ.get("STRIPE_TEST_KEY")
    if not key:
        return None
    return {"key": key}


def require_credentials() -> dict:
    creds = load_credentials()
    if not creds:
        raise RuntimeError(f"Missing required credentials.\n\n{CREDENTIALS_DOC}")
    return creds


# ✅ CORRECT: Fail loudly if credentials required but missing
# File: test_payment.e2e.py
def test_charges_card() -> None:
    creds = require_credentials()
    client = StripeClient(creds["key"])
    result = client.charge(amount=100)
    assert result.success


# ❌ REJECTED: Silent skip on required credentials
# File: test_payment.e2e.py
@pytest.mark.skipif(not load_credentials(), reason="No credentials")
def test_charges_card_bad() -> (
    None
): ...  # CI goes green with zero payment verification!

</credential_management>

<anti_patterns>

Python-Specific Anti-Patterns

Using unittest.mock.patch on External Services

# ❌ WRONG: Mocking external service
@patch("httpx.Client.get")
def test_fetches_user(mock_get: Mock) -> None:
    mock_get.return_value = Mock(json=lambda: {"id": 1})
    user = api_client.get_user(1)
    assert user.id == 1  # Proves nothing about real API


# ✅ RIGHT: Use DI for Level 1, real service for Level 2/3
def test_parses_user_response() -> None:
    # Level 1: Test parsing logic with known data
    response = {"id": 1, "name": "Test", "email": "test@example.com"}
    user = parse_user_response(response)
    assert user.id == 1


# File: test_api.e2e.py
def test_fetches_real_user(test_credentials: dict) -> None:
    # Level 3: Test against real API
    client = ApiClient(credentials=test_credentials)
    user = client.get_user(known_test_user_id)
    assert user is not None

Overusing pytest-mock

# ❌ WRONG: mocker fixture everywhere
def test_sync(mocker: MockerFixture) -> None:
    mocker.patch("os.path.exists", return_value=True)
    mocker.patch("subprocess.run", return_value=Mock(returncode=0))
    result = sync_files(src, dest)
    assert result.success  # What did we prove? Nothing.


# ✅ RIGHT: DI for controllable behavior
def test_sync_returns_success_on_zero_exit() -> None:
    class FakeRunner:
        def run(self, cmd: list[str]) -> tuple[int, str, str]:
            return (0, "Done", "")

    deps = SyncDeps(runner=FakeRunner())
    result = sync_files(src, dest, deps)
    assert result.success

Testing Library Behavior

# ❌ WRONG: Testing that argparse works
def test_parses_verbose_flag() -> None:
    parser = create_parser()
    args = parser.parse_args(["--verbose"])
    assert args.verbose is True  # Testing argparse, not your code


# ✅ RIGHT: Test YOUR behavior that uses parsed args
def test_verbose_mode_produces_detailed_output() -> None:
    output = run_command(Config(verbose=True))
    assert "DEBUG:" in output

</anti_patterns>

<rejection_criteria>

Issue	Example	Rule/Reason
Missing property tests for parser	`test_parse_json` without `@given`	Mandatory for parsers
Magic values in assertions	`assert score == 45`	PLR2004
Missing `-> None` on test	`def test_foo(self):`	ANN201
Mocking	`@patch("module.func")`	Use DI instead
Silent skip on required dependency	`@pytest.mark.skipif(not has_postgres())`	Must fail, not skip
Wrong filename suffix	`test_db.py` for integration test	Use `.integration.py`
Untyped fixture parameter	`def test_foo(tmp_path) -> None:`	ANN001

</rejection_criteria>

<success_criteria> Code follows these standards when:

Parsers, serializers, math operations have property-based tests with @given
Test data uses factories with named constants (no magic values)
File names indicate level (.unit.py, .integration.py, .e2e.py)
DI uses Protocols, not mocking
Integration tests use harnesses with real dependencies
All test functions have -> None return type
All fixture parameters have type annotations
Required credentials fail loudly, not skip silently

</success_criteria>

Name	standardizing-python-testing
Description	Python testing standards enforced across all skills. Reference skill for property-based testing, factories, harness patterns, and level-specific implementations.

standardizing-python-testing

SKILL.md