Skills/python-dev/SKILL.md

---
name: python-dev
description: |
  Apply this skill whenever writing, editing, generating, or reviewing Python code — including new .py files, modifications to existing Python modules, writing classes/functions/scripts, or any task that produces Python source. Enforces enterprise-grade Python: OOP, strict static typing, idiomatic/pythonic style, separation of concerns, DRY/KISS, security, testing, structured logging, and ruff+mypy-clean output.
version: 2.0.0
user-invocable: false
---

# Python Development — Enterprise Standards

Apply every rule below **every time** you write or touch Python code. These are non-negotiable defaults.

---

## 0 — Fetch Current Docs First

Before writing code that uses any third-party library or stdlib module you are not 100% certain about, use the **context7 MCP** to pull the current documentation:

```
mcp__plugin_context7_context7__resolve-library-id  →  mcp__plugin_context7_context7__query-docs
```

Always do this for:
- Third-party packages (pydantic, sqlalchemy, fastapi, httpx, click, typer, attrs, structlog, etc.)
- Any stdlib module whose API has changed between Python versions (e.g. `asyncio`, `pathlib`, `importlib`, `tomllib`)
- Any time you are unsure of the exact call signature, context-manager protocol, or configuration schema

---

## 1 — Static Typing (Strict)

- Every function, method, and class attribute **must** have type annotations — no exceptions.
- Use `from __future__ import annotations` at the top of every module (enables PEP 563 deferred evaluation, avoids forward-reference strings).
- Use `typing` / `collections.abc` generics: `Sequence`, `Mapping`, `Callable`, `Iterator`, `Generator`, `Awaitable`, `Coroutine`, etc. — **not** bare `list`, `dict`, `tuple` as annotations.
- Use `TypeVar`, `ParamSpec`, `TypeVarTuple`, `Protocol`, `TypedDict`, `NamedTuple`, `Literal`, `Final`, `ClassVar`, `Annotated` where appropriate.
- Use `X | Y` union syntax (PEP 604) instead of `Union[X, Y]`.
- Use `X | None` instead of `Optional[X]`.
- Never use `Any` except as a last resort; always with a `# type: ignore[...]` comment explaining why.
- `@override` (from `typing`) on all overriding methods (Python ≥ 3.12) or `typing_extensions`.
- `@dataclass(slots=True, frozen=True)` for immutable value objects; `@dataclass(slots=True)` for mutable ones.
- Move type-only imports under `TYPE_CHECKING` to avoid runtime overhead.

```python
from __future__ import annotations

from collections.abc import Sequence
from typing import TYPE_CHECKING, Final, TypeVar

if TYPE_CHECKING:
    from mypackage.domain.models import User

T = TypeVar("T")
MAX_RETRIES: Final = 3

def first(items: Sequence[T]) -> T | None:
    return items[0] if items else None
```

### Mypy (Strict Mode — Mandatory)

Run mypy after every change. Configure in `pyproject.toml`:

```toml
[tool.mypy]
python_version = "3.12"
strict = true
warn_return_any = true
warn_unused_ignores = true
no_implicit_reexport = true
plugins = ["pydantic.mypy"]   # if Pydantic is used
```

Iterate until `mypy` exits with code 0. Never suppress errors with `# type: ignore` without a comment explaining the reason.

---

## 2 — OOP Principles

- Model domain concepts as classes. Use inheritance only for genuine IS-A relationships; prefer composition.
- Apply SOLID:
  - **S** — one responsibility per class/function
  - **O** — extend via subclassing or strategy, not by modifying existing code
  - **L** — subtypes must be substitutable
  - **I** — small, focused protocols/ABCs; never fat interfaces
  - **D** — depend on abstractions (`Protocol` / ABC), not concretes
- Use `abc.ABC` + `@abc.abstractmethod` for explicit contracts when subclassing is intended.
- Use `Protocol` (structural typing) for duck-typed contracts — preferred over ABC for third-party types.
- Expose only what is needed; prefix internal names with `_`.
- `__slots__` on all classes instantiated many times (reduces per-instance memory ~40%).

```python
from __future__ import annotations

import abc
from typing import Protocol

class Serializable(Protocol):
    def to_dict(self) -> dict[str, object]: ...

class BaseRepository[T](abc.ABC):
    @abc.abstractmethod
    def get(self, id: int) -> T | None: ...

    @abc.abstractmethod
    def save(self, entity: T) -> None: ...
```

---

## 3 — Data Model: Pydantic vs dataclass vs attrs

Choose the right tool for the data's role:

| Scenario | Tool |
|---|---|
| Data crosses a trust boundary (API input, config file, user data) | **Pydantic v2** |
| Internal data structures (domain models, value objects, DTOs between layers) | **`@dataclass(slots=True, frozen=True)`** |
| Complex internal models needing composable validators, converters, and advanced slot support | **attrs** |

**Pydantic v2** at trust boundaries:
```python
from pydantic import BaseModel, field_validator

class CreateUserRequest(BaseModel):
    name: str
    email: str

    @field_validator("email")
    @classmethod
    def validate_email(cls, v: str) -> str:
        if "@" not in v:
            raise ValueError("invalid email")
        return v.lower()
```

**Dataclass** for internal models (no I/O):
```python
from dataclasses import dataclass

@dataclass(slots=True, frozen=True)
class UserId:
    value: int
```

---

## 4 — Separation of Concerns

Organise code into layers; never mix them in a single module or class:

| Layer | Responsibility |
|---|---|
| **Domain / Models** | Pure business logic, no I/O |
| **Repository / Data** | Persistence, queries |
| **Service / Use-case** | Orchestrates domain + repos |
| **Interface / API** | HTTP, CLI, events — thin adapters |
| **Infrastructure** | DB sessions, HTTP clients, config |

- No raw SQL / ORM calls in service or domain layers.
- No business logic in route handlers or CLI commands.
- Config (env vars, secrets) loaded once at startup, injected as typed Pydantic `BaseSettings` objects.

---

## 5 — Idiomatic & Pythonic Constructs

Always prefer:

```python
# Comprehensions over manual loops
squares = [x**2 for x in range(10) if x % 2 == 0]

# Generator expressions for lazy evaluation
total = sum(x**2 for x in big_sequence)

# Walrus operator for assign-and-test
if chunk := file.read(8192):
    process(chunk)

# Context managers for all resources
with open(path) as fh:
    data = fh.read()

# Unpacking
first, *rest = items
a, b = b, a

# enumerate / zip instead of index loops
for i, item in enumerate(items):
    ...

# dataclasses / NamedTuple over raw dicts for structured data
from dataclasses import dataclass

@dataclass(slots=True, frozen=True)
class Point:
    x: float
    y: float

# pathlib over os.path
from pathlib import Path
config = Path("~/.config/app.toml").expanduser()

# f-strings for all string formatting
msg = f"Processing {count} items in {duration:.2f}s"

# match/case (Python ≥ 3.10) for structural dispatch
match command:
    case "quit":
        raise SystemExit
    case "help":
        show_help()
    case _:
        handle(command)

# functools.cache / lru_cache for pure, expensive computations
from functools import cache

@cache
def fibonacci(n: int) -> int:
    if n < 2:
        return n
    return fibonacci(n - 1) + fibonacci(n - 2)
```

Never use:
- `%` or `.format()` for string formatting (use f-strings)
- `os.path` (use `pathlib`)
- Mutable default arguments (`def f(x=[])`) — use `None` sentinel
- Bare `except:` — always name the exception
- `type(x) == Foo` — use `isinstance(x, Foo)`
- `lambda` assigned to a name — use `def`

---

## 6 — DRY / KISS

- Extract any logic repeated ≥ 2 times into a named function or method.
- Functions do one thing; if you need "and" to describe it, split it.
- Prefer flat over nested; early return / guard clauses over deep indentation.
- No speculative abstractions — generalise only when you have ≥ 2 concrete use-cases.
- No feature flags, backwards-compat shims, or dead code — delete it.

---

## 7 — Module & Project Layout

```
src/
  <package>/
    __init__.py          # public API only, explicit __all__
    py.typed             # PEP 561 marker
    domain/
      models.py
      exceptions.py
    repository/
      base.py
      <entity>.py
    service/
      <use_case>.py
    interface/
      api.py             # or cli.py
    infrastructure/
      db.py
      config.py
tests/
  unit/
  integration/
  conftest.py
pyproject.toml
.pre-commit-config.yaml
uv.lock
```

- `__init__.py` re-exports only the public API with explicit `__all__`.
- Every package has a `py.typed` marker (PEP 561).
- `pyproject.toml` is the single source of truth for metadata, deps, and tool config.
- `uv.lock` is committed for applications (reproducible builds); omitted for libraries.

### Dependency Management (uv)

Use `uv` as the **sole** package manager and tool runner. **`pip` and `pip3` are banned — never reference them directly.**

```bash
uv add <package>            # add production dependency
uv add --dev <package>      # add dev dependency
uv sync                     # install from lockfile
uv run pytest               # run in managed venv
uv run mypy src/            # run any tool via uv
uv run ruff check .         # always prefix tool invocations with uv run
```

Every script, CLI tool, and package binary **must** be invoked via `uv run <tool>`, never called directly. This ensures the correct venv is always used.

Separate production and dev dependencies in `pyproject.toml`:

```toml
[project]
dependencies = ["fastapi>=0.111", "pydantic>=2.7", "structlog>=24.1"]

[tool.uv]
dev-dependencies = [
    "pytest>=8",
    "hypothesis>=6",
    "mypy>=1.10",
    "ruff>=0.4",
    "bandit>=1.7",
    "pip-audit>=2.7",
]
```

---

## 8 — Ruff: Lint & Format (Mandatory)

After writing or modifying any Python file, run ruff and **iterate until the output is clean**:

### Step 1 — format
```bash
uv run ruff format <file_or_dir>
```

### Step 2 — lint (with auto-fix)
```bash
uv run ruff check --fix <file_or_dir>
```

### Step 3 — lint (final check, no auto-fix)
```bash
uv run ruff check <file_or_dir>
```

If Step 3 still reports errors:
- Read each diagnostic carefully.
- Fix the root cause in the source (do not suppress with `# noqa` unless there is a genuine reason).
- Repeat from Step 1 until `ruff check` exits with code 0 and no output.

### Recommended `pyproject.toml` ruff config:

```toml
[tool.ruff]
line-length = 100
target-version = "py312"

[tool.ruff.lint]
select = [
    "E",    # pycodestyle errors
    "W",    # pycodestyle warnings
    "F",    # pyflakes
    "I",    # isort
    "N",    # pep8-naming
    "UP",   # pyupgrade
    "ANN",  # flake8-annotations  (enforce type hints)
    "B",    # flake8-bugbear
    "C4",   # flake8-comprehensions
    "SIM",  # flake8-simplify
    "RUF",  # ruff-specific rules
    "TCH",  # flake8-type-checking (move type-only imports into TYPE_CHECKING)
    "PT",   # flake8-pytest-style
    "ERA",  # eradicate (commented-out code)
    "S",    # flake8-bandit (security)
    "D",    # pydocstyle (docstring enforcement)
    "PERF", # perflint (performance anti-patterns)
    "TRY",  # tryceratops (exception handling)
    "PIE",  # flake8-pie (misc. good practices)
]
ignore = [
    "D100",   # missing docstring in public module (optional at module level)
    "D104",   # missing docstring in public package
    "D203",   # conflicts with D211
    "D213",   # conflicts with D212
    "TRY003", # allow long messages in exceptions
]

[tool.ruff.lint.pydocstyle]
convention = "google"

[tool.ruff.lint.per-file-ignores]
"tests/**" = ["ANN", "S101", "D"]  # relax annotations/assert/docstrings in tests

[tool.ruff.format]
quote-style = "double"
indent-style = "space"
```

---

## 9 — Type Checking with Mypy (Mandatory)

After every change, run:

```bash
uv run mypy src/
```

Iterate until exit code 0. This is a **hard gate** — do not consider code complete until mypy is clean.

---

## 10 — Docstrings

Every **public** module, class, and function **must** have a docstring. Use Google style.

```python
def calculate_discount(price: float, rate: float) -> float:
    """Calculate the discounted price.

    Args:
        price: The original price in the base currency.
        rate: The discount rate as a fraction (e.g. 0.1 for 10%).

    Returns:
        The price after applying the discount.

    Raises:
        ValueError: If rate is not in [0, 1].
    """
    if not 0 <= rate <= 1:
        raise ValueError(f"rate must be in [0, 1], got {rate!r}")
    return price * (1 - rate)
```

Rules:
- Summary line: one sentence, imperative mood, ≤ 79 chars, ends with `.`.
- Omit docstrings on private methods (`_foo`) unless the logic is non-obvious.
- Never duplicate the signature in the docstring — document intent, not mechanics.

---

## 11 — Error Handling

- Define domain-specific exception hierarchies rooted at a project base class.
- Never silence exceptions with bare `except Exception: pass`.
- Use `contextlib.suppress` only for genuinely expected, ignorable errors.
- Attach context with `raise NewError("...") from original_exc`.
- Log at the boundary (service/interface layer), not deep inside domain logic.
- Keep `try` blocks minimal — only the line that can raise, not surrounding logic.

```python
class AppError(Exception):
    """Base for all application errors."""

class NotFoundError(AppError):
    def __init__(self, entity: str, id: int) -> None:
        super().__init__(f"{entity} with id={id} not found")
        self.entity = entity
        self.id = id
```

---

## 12 — Structured Logging (structlog)

Use `structlog` for all logging. Never use `print()` for operational output.

```python
import structlog

log = structlog.get_logger(__name__)

def process_order(order_id: int) -> None:
    log.info("processing_order", order_id=order_id)
    try:
        ...
        log.info("order_processed", order_id=order_id)
    except AppError as exc:
        log.error("order_failed", order_id=order_id, error=str(exc))
        raise
```

Configure structlog in `infrastructure/logging.py` to:
- Emit **JSON** in production (`JSONRenderer`).
- Emit human-readable output in development (`ConsoleRenderer`).
- Bind request-id / trace-id into context at the request boundary.
- Never log secrets, PII, or credentials.

```python
import structlog

def configure_logging(*, json_output: bool) -> None:
    processors: list[structlog.types.Processor] = [
        structlog.contextvars.merge_contextvars,
        structlog.processors.add_log_level,
        structlog.processors.TimeStamper(fmt="iso"),
        structlog.processors.StackInfoRenderer(),
        structlog.processors.ExceptionRenderer(),
    ]
    if json_output:
        processors.append(structlog.processors.JSONRenderer())
    else:
        processors.append(structlog.dev.ConsoleRenderer())

    structlog.configure(processors=processors, wrapper_class=structlog.BoundLogger)
```

Log levels:
- `debug` — detailed diagnostic info (dev only)
- `info` — normal operational events
- `warning` — unexpected but recoverable state
- `error` — failure requiring attention; always include exception context
- `critical` — system-level failure; triggers alerting

---

## 13 — Immutability & Safety

- Prefer immutable data: `@dataclass(frozen=True)`, `tuple` over `list` where mutation is not needed.
- Use `Final` for module-level constants.
- Avoid global mutable state; pass dependencies explicitly.
- Thread-safety: document concurrency assumptions; use `threading.Lock` / `asyncio.Lock` when sharing state.
- Use `functools.cached_property` for lazy-computed instance properties (not compatible with `__slots__` — choose one).

---

## 14 — Async

When writing async code:
- `async def` for all I/O-bound operations.
- Never call blocking I/O inside a coroutine — use `asyncio.to_thread` or an executor.
- Use `asyncio.TaskGroup` (Python ≥ 3.11) for concurrent tasks, not bare `asyncio.gather`.
- Annotate return types explicitly: `async def fetch(...) -> bytes:`.
- Never mix sync and async code in the same layer without an explicit boundary.

```python
async def fetch_all(urls: list[str]) -> list[bytes]:
    async with asyncio.TaskGroup() as tg:
        tasks = [tg.create_task(fetch(url)) for url in urls]
    return [t.result() for t in tasks]
```

---

## 15 — Security

These rules are mandatory for all enterprise code:

### Secrets
- **Never hardcode** secrets, API keys, tokens, or credentials in source files or tests.
- Load secrets from environment variables or a secrets manager (AWS Secrets Manager, HashiCorp Vault, Azure Key Vault).
- Use Pydantic `BaseSettings` to load and validate config/secrets at startup:

```python
from pydantic_settings import BaseSettings

class Settings(BaseSettings):
    database_url: str
    api_key: str

    class Config:
        env_file = ".env"
        env_file_encoding = "utf-8"
```

### Input Validation
- Validate **all** external input at the trust boundary with Pydantic before it enters the domain.
- Never pass raw user input to shell commands, SQL queries, or file paths.

### Dependency Scanning
Run in CI:
```bash
uv run pip-audit           # check for known CVEs in dependencies
uv run bandit -r src/ -ll  # SAST: flag high/medium severity issues
```

### Static Security Analysis
The `"S"` ruff rule set (flake8-bandit) catches the most common SAST issues inline. Treat all `S` violations as errors.

---

## 16 — Testing (pytest)

Every piece of logic **must** have tests. Minimum requirements:

### Structure
```
tests/
  conftest.py          # shared fixtures
  unit/                # fast, no I/O, pure functions/classes
  integration/         # real DB, real HTTP (use fixtures for setup/teardown)
```

### Pytest conventions
```python
import pytest

# Parametrize to cover multiple cases without duplication
@pytest.mark.parametrize("input,expected", [
    (0, 0),
    (1, 1),
    (10, 100),
])
def test_square(input: int, expected: int) -> None:
    assert square(input) == expected

# Use fixtures for shared setup
@pytest.fixture
def user_repo(db_session: Session) -> UserRepository:
    return UserRepository(db_session)
```

### Property-based testing (Hypothesis)
Use Hypothesis for any function with a non-trivial input space:

```python
from hypothesis import given, strategies as st

@given(st.floats(min_value=0.0, max_value=1.0))
def test_discount_never_negative(rate: float) -> None:
    assert calculate_discount(100.0, rate) >= 0
```

### Rules
- Test names: `test_<what>_<condition>_<expected_outcome>`.
- One logical assertion per test (use `pytest.approx` for floats).
- Do **not** mock the database in integration tests — use a real test DB.
- Minimum 80% line coverage; 100% for domain logic.
- Run with: `uv run pytest --tb=short -q`

### Coverage config
```toml
[tool.pytest.ini_options]
addopts = "--strict-markers --tb=short"
testpaths = ["tests"]

[tool.coverage.run]
source = ["src"]
branch = true

[tool.coverage.report]
fail_under = 80
show_missing = true
```

---

## 17 — Pre-commit Hooks

Every project must have `.pre-commit-config.yaml`:

```yaml
repos:
  - repo: https://github.com/astral-sh/ruff-pre-commit
    rev: v0.4.0
    hooks:
      - id: ruff
        args: [--fix]
      - id: ruff-format

  - repo: https://github.com/pre-commit/mirrors-mypy
    rev: v1.10.0
    hooks:
      - id: mypy
        additional_dependencies: [pydantic, types-all]

  - repo: https://github.com/PyCQA/bandit
    rev: 1.7.8
    hooks:
      - id: bandit
        args: ["-ll", "-r", "src/"]
```

Install once per checkout: `uv run pre-commit install`

---

## 18 — Observability (OpenTelemetry)

For services (APIs, workers), instrument with OpenTelemetry:

```python
from opentelemetry import trace

tracer = trace.get_tracer(__name__)

def process_payment(order_id: int) -> None:
    with tracer.start_as_current_span("process_payment") as span:
        span.set_attribute("order.id", order_id)
        ...
```

- Use semantic conventions for attribute names (`http.method`, `db.system`, etc.).
- Auto-instrument frameworks (FastAPI, SQLAlchemy, httpx) via `opentelemetry-instrument`.
- Export to an OTel Collector; never export directly to a vendor from application code.

---

## 19 — Configuration Management (pydantic-settings)

Load all configuration and secrets from the environment. Never read env vars ad-hoc with `os.environ.get()` scattered throughout code.

```python
from pydantic import SecretStr
from pydantic_settings import BaseSettings, SettingsConfigDict

class DatabaseSettings(BaseSettings):
    url: str
    pool_size: int = 10

class Settings(BaseSettings):
    model_config = SettingsConfigDict(env_file=".env", env_nested_delimiter="__")

    debug: bool = False
    database: DatabaseSettings
    api_key: SecretStr          # redacted in repr() — never logged

_settings: Settings | None = None

def get_settings() -> Settings:
    global _settings
    if _settings is None:
        _settings = Settings()
    return _settings
```

Rules:
- `SecretStr` for any sensitive field — it redacts itself in `repr()` and logs.
- Commit `.env.example` (placeholder values) to the repo; never commit `.env`.
- Fail fast: `Settings()` raises `ValidationError` at startup if required vars are missing.
- Inject `Settings` as a dependency — never call `get_settings()` inside domain logic.

---

## 20 — Concurrency Model Selection

Choose the right tool:

| Workload | Tool |
|---|---|
| I/O-bound (network, disk) | `asyncio` with `async/await` |
| I/O-bound + threading simplicity | `concurrent.futures.ThreadPoolExecutor` |
| CPU-bound (compute, data processing) | `concurrent.futures.ProcessPoolExecutor` |
| Fire-and-forget background tasks | `asyncio.TaskGroup` or `ThreadPoolExecutor` |

```python
from concurrent.futures import ProcessPoolExecutor

def cpu_intensive(data: bytes) -> bytes:
    ...  # pure computation

def process_all(items: list[bytes]) -> list[bytes]:
    with ProcessPoolExecutor() as pool:
        return list(pool.map(cpu_intensive, items))
```

- Prefer `concurrent.futures` over raw `threading.Thread` / `multiprocessing.Process`.
- Use `asyncio.timeout()` (Python ≥ 3.11) for coroutine deadlines — preferred over `asyncio.wait_for`.
- Use `asyncio.Semaphore` to rate-limit fan-out inside `TaskGroup`.
- Never rely on the GIL for thread-safety — Python 3.13 free-threaded mode removes it.

---

## 21 — ExceptionGroup and `except*` (Python ≥ 3.11)

`asyncio.TaskGroup` raises an `ExceptionGroup` when multiple tasks fail. Handle it correctly:

```python
import asyncio

async def main() -> None:
    try:
        async with asyncio.TaskGroup() as tg:
            tg.create_task(might_fail_a())
            tg.create_task(might_fail_b())
    except* ValueError as eg:
        for exc in eg.exceptions:
            log.error("validation_failed", error=str(exc))
    except* IOError as eg:
        for exc in eg.exceptions:
            log.error("io_failed", error=str(exc))
```

- `except*` catches matching exceptions from the group; unmatched exceptions re-raise.
- For Python < 3.11, use the `exceptiongroup` backport.
- Raising `ExceptionGroup` from a public API is a breaking change — version it.

---

## 22 — Naming Conventions

Follow these rules consistently (aligned with Google Style Guide and PEP 8):

| Thing | Convention | Example |
|---|---|---|
| Module | `snake_case`, short, no dashes | `user_repository.py` |
| Package | `snake_case`, short | `my_package/` |
| Class | `PascalCase` | `UserRepository` |
| Exception | `PascalCase` ending in `Error` | `NotFoundError` |
| Function / method | `snake_case` | `calculate_discount` |
| Constant | `UPPER_SNAKE_CASE` | `MAX_RETRIES` |
| Type alias | `PascalCase` | `UserId = NewType("UserId", int)` |
| `TypeVar` | Single capital or `T`-suffix | `T`, `EntityT`, `KT` |
| `Protocol` | Adjective or role noun | `Serializable`, `Closeable` |
| Private names | Single underscore prefix | `_internal_helper` |
| Internal module names | Single underscore prefix | `_utils.py` |

Additional rules:
- Avoid abbreviations except universally understood ones (`db`, `url`, `id`, `cfg`, `req`, `resp`).
- Boolean variables and functions: use `is_`, `has_`, `can_` prefix (`is_active`, `has_permission`).
- Never shadow builtins (`list`, `id`, `type`, `input`, `filter`, `map`).

---

## 23 — Dangerous Calls — Banned Without Justification

These calls are **banned** by default; any use requires an explicit comment explaining why it is safe:

| Call | Risk | Safe alternative |
|---|---|---|
| `eval()` / `exec()` | Code injection | Parse structured data instead |
| `pickle.loads()` | Arbitrary code execution | `json`, `msgpack`, `protobuf` |
| `subprocess.run(..., shell=True)` | Shell injection | Pass a list of args instead |
| `yaml.load()` | Arbitrary code execution | `yaml.safe_load()` |
| `__import__()` | Dynamic import abuse | `importlib.import_module()` with validation |
| `os.system()` | Shell injection | `subprocess.run([...])` |

The `"S"` ruff rule set flags most of these automatically.

---

## 24 — `pyproject.toml` Reference Template

Use this as a starting point for new projects:

```toml
[build-system]
requires = ["hatchling"]
build-backend = "hatchling.build"

[project]
name = "my-package"
version = "0.1.0"
requires-python = ">=3.12"
dependencies = [
    "pydantic>=2.7",
    "pydantic-settings>=2.3",
    "structlog>=24.1",
]

[tool.uv]
dev-dependencies = [
    "pytest>=8",
    "pytest-cov>=5",
    "hypothesis>=6",
    "mypy>=1.10",
    "ruff>=0.4",
    "bandit>=1.7",
    "pip-audit>=2.7",
    "pre-commit>=3.7",
]

[tool.ruff]
line-length = 100
target-version = "py312"

[tool.ruff.lint]
select = ["E","W","F","I","N","UP","ANN","B","C4","SIM","RUF","TCH","PT","ERA","S","D","PERF","TRY","PIE"]
ignore = ["D100","D104","D203","D213","TRY003"]

[tool.ruff.lint.pydocstyle]
convention = "google"

[tool.ruff.lint.per-file-ignores]
"tests/**" = ["ANN","S101","D"]

[tool.ruff.format]
quote-style = "double"
indent-style = "space"

[tool.mypy]
python_version = "3.12"
strict = true
warn_return_any = true
warn_unused_ignores = true
no_implicit_reexport = true

[tool.pytest.ini_options]
addopts = "--strict-markers --tb=short"
testpaths = ["tests"]

[tool.coverage.run]
source = ["src"]
branch = true

[tool.coverage.report]
fail_under = 80
show_missing = true
```

---

## Checklist Before Finalising Any Python File

**Typing**
- [ ] `from __future__ import annotations` at top of every module
- [ ] All functions/methods/attributes fully typed — no bare `Any`
- [ ] Type-only imports under `TYPE_CHECKING`
- [ ] `mypy src/` passes with zero errors (strict mode)

**Design**
- [ ] Classes model real concepts; single responsibility (SOLID)
- [ ] Pydantic used at trust boundaries; dataclass/attrs for internals
- [ ] No business logic leaking across layers
- [ ] Pythonic constructs throughout; no repeated logic (DRY)
- [ ] No dangerous calls (`eval`, `pickle.loads`, `shell=True`, etc.) without justification
- [ ] Naming follows conventions (§22)

**Documentation**
- [ ] All public modules, classes, and functions have Google-style docstrings
- [ ] Generator functions use `Yields:`, not `Returns:`

**Code quality**
- [ ] `ruff format` + `ruff check` both pass with zero diagnostics
- [ ] No `# noqa` / `# type: ignore` without an explanatory comment

**Security**
- [ ] No hardcoded secrets or credentials
- [ ] All external input validated with Pydantic at the boundary
- [ ] `bandit -r src/ -ll` clean
- [ ] `pip-audit` clean

**Testing**
- [ ] Tests written — unit for logic, integration for persistence/HTTP
- [ ] Coverage ≥ 80% (100% for domain logic)
- [ ] Hypothesis used for non-trivial input spaces

**Observability**
- [ ] Structured logging via `structlog` (no `print()`)
- [ ] OTel spans on service-layer operations (for service code)

**Dependencies & tooling**
- [ ] `uv.lock` committed and up to date
- [ ] context7 docs consulted for any library APIs used