Implementation & Testing

Implementing the Feature Slice

Build one vertical slice end to end before widening:

# services/orders.py
import logging
from collections.abc import Callable
from datetime import datetime
from proj.domain import Item, Order, Pricing
from proj.repositories import OrderRepo

log = logging.getLogger(__name__)

class OrderService:
    def __init__(self, repo: OrderRepo, clock: Callable[[], datetime]) -> None:
        self._repo, self._clock = repo, clock

    def place(self, items: list[Item]) -> Order:
        order = Pricing.price(items, at=self._clock())
        self._repo.save(order)
        log.info("order.placed id=%s", order.id)
        return order

Discipline per slice: full type hints, domain errors separate from infrastructure errors, raise ... from exc when wrapping, logging only at boundaries. Tests construct the service with fakes and a fixed clock.

Comprehensive Tests

# tests/unit/test_pricing.py
import pytest
from proj.domain import pricing

@pytest.mark.parametrize("qty,unit,expected", [
    (1, 100, 100), (3, 100, 270), (0, 100, 0),
])
def test_bulk_discount(qty, unit, expected):
    assert pricing.total(qty, unit) == expected

def test_rejects_negative_qty():
    with pytest.raises(pricing.PricingError, match="negative"):
        pricing.total(-1, 100)

Layered suite:

• Unit: pure logic, monkeypatch for env, tmp_path for files — milliseconds
• Integration: one real dependency (SQLite file, HTTP against a stub) — seconds
• Contract: public API shape and error formats so refactors cannot silently break callers

Measure with pytest --cov=src --cov-fail-under=80; treat uncovered branches as the goal, not the percentage. Mutation tools (mutmut) audit whether assertions actually bite.

Docs and Release

Documentation ships with the code, not after it:

• README: install (pip install -e ".[dev]"), run, test — in that order, copy-pasteable
• Docstrings: Google or NumPy style on every public function; Args/Returns/Raises sections
• CHANGELOG: Keep a Changelog format — Added / Changed / Fixed per version
• Type hints + mypy are living documentation IDEs consume

Release hygiene: semantic versioning (breaking → major), python -m build + twine check for sdist/wheel correctness, tags trigger CI publish with trusted credentials, and pre-commit run --all-files gates the commit that starts a release. Interview answer: correctness comes from tests + types + review working together; onboarding speed comes from a README that answers "how do I run this" in under a minute.