BanGUI/Docs/Backend-Development.md

Backend Development — Rules & Guidelines

Rules and conventions every backend developer must follow. Read this before writing your first line of code.

---

1. Language & Typing

• Python 3.12+ is the minimum version.
• Every function, method, and variable must have explicit type annotations — no exceptions.
• Use str, int, float, bool, None for primitives.
• Use list[T], dict[K, V], set[T], tuple[T, ...] (lowercase, built-in generics) — never typing.List, typing.Dict, etc.
• Use T | None instead of Optional[T].
• Use TypeAlias, TypeVar, Protocol, and NewType when they improve clarity.
• Return types are mandatory — including -> None.
• Never use Any unless there is no other option and a comment explains why.
• Run mypy --strict (or pyright in strict mode) — the codebase must pass with zero errors.

# Good
def get_jail_by_name(name: str) -> Jail | None:
    ...

# Bad — missing types
def get_jail_by_name(name):
    ...

---

2. Core Libraries

Purpose	Library	Notes
Web framework	FastAPI	Async endpoints only.
Data validation & settings	Pydantic v2	All request/response bodies and config models.
Async HTTP client	aiohttp (ClientSession)	For external calls (blocklists, IP lookups).
Scheduling	APScheduler 4.x (async)	Blocklist imports, periodic health checks.
Structured logging	structlog	Every log call must use structlog — never print() or logging directly.
Database	aiosqlite	Async SQLite access for the application database.
Testing	pytest + pytest-asyncio + httpx (AsyncClient)	Every feature needs tests.
Mocking	unittest.mock / pytest-mock	Isolate external dependencies.
Date & time	datetime (stdlib) — always timezone-aware	Use datetime.datetime.now(datetime.UTC). Never naive datetimes.
IP / Network	ipaddress (stdlib)	Validate and normalise IPs and CIDR ranges.
Environment / config	pydantic-settings	Load .env and environment variables into typed models.
fail2ban integration	fail2ban client (bundled)	Use the local copy at ./fail2ban-master. Import from ./fail2ban-master/fail2ban/client to communicate with the fail2ban socket. Do not install fail2ban as a pip package.

fail2ban Client Usage

The repository ships with a vendored copy of fail2ban located at ./fail2ban-master. All communication with the fail2ban daemon must go through the client classes found in ./fail2ban-master/fail2ban/client. Add the project root to sys.path (or configure it in pyproject.toml as a path dependency) so that from fail2ban.client ... resolves to the bundled copy.

import sys
from pathlib import Path

# Ensure the bundled fail2ban is importable
sys.path.insert(0, str(Path(__file__).resolve().parents[2] / "fail2ban-master"))

from fail2ban.client.csocket import CSSocket  # noqa: E402

Libraries you must NOT use

• requests — use aiohttp (async).
• flask — we use FastAPI.
• celery — we use APScheduler.
• print() for logging — use structlog.
• json.loads / json.dumps on Pydantic models — use .model_dump() / .model_validate().

Timestamp Handling

Timestamp consistency is critical for accurate ban history queries across the dashboard and history endpoints. Follow these rules:

Rule 1: Use consistent UTC timestamps

• All timestamps in the database are stored as Unix epochs (seconds since 1970-01-01 UTC).
• fail2ban stores timestamps using time.time(), which is always UTC epoch seconds.
• When querying fail2ban's SQLite database by timestamp, use app.utils.time_utils.since_unix() (not manual datetime calculations).

Rule 2: Time-range windows include a 60-second slack

• The since_unix() function includes a 60-second slack window (TIME_RANGE_SLACK_SECONDS in app.utils.constants).
• This slack accommodates:
  • Clock drift between the local system and fail2ban.
  • Test seeding delays when timestamps are manually set to exact boundaries.
• The slack ensures that dashboard and history queries return consistent row counts for the same time range.

Rule 3: Never duplicate timestamp calculation logic

• All services that query by time range must import and use since_unix().
• Do not recalculate timestamps locally using datetime or time modules in service code.
• If you need a timestamp for a time range, use since_unix().

Example:

from app.utils.time_utils import since_unix

# Get all bans from the last 24 hours (with 60-second slack)
since_ts: int = since_unix("24h")
rows = await db.execute(
    "SELECT * FROM bans WHERE timeofban >= ?",
    (since_ts,)
)

---

3. Project Structure

backend/
├── app/
│   ├── __init__.py
│   ├── main.py              # FastAPI app factory, lifespan
│   ├── config.py            # Pydantic settings
│   ├── dependencies.py      # FastAPI dependency providers
│   ├── models/              # Pydantic schemas (request, response, domain)
│   ├── routers/             # FastAPI routers grouped by feature
│   ├── services/            # Business logic — one service per domain
│   ├── repositories/        # Database access layer
│   ├── tasks/               # APScheduler jobs
│   └── utils/               # Helpers, constants, shared types
├── tests/
│   ├── conftest.py
│   ├── test_routers/
│   ├── test_services/
│   └── test_repositories/
├── pyproject.toml
└── .env.example

• Routers receive requests, validate input via Pydantic, and delegate to services.
• Services contain business logic and call repositories or external clients.
• Repositories handle raw database queries — nothing else.
• Never put business logic inside routers or repositories.

---

4. FastAPI Conventions

• Use async def for every endpoint — no sync endpoints.

• Every endpoint must declare explicit response models (response_model=...).

• Use Pydantic models for request bodies and query parameters — never raw dicts.

• Use Depends() for dependency injection (database sessions, services, auth).

• Group endpoints into routers by feature domain (routers/jails.py, routers/bans.py, …).

• Use appropriate HTTP status codes: 201 for creation, 204 for deletion with no body, 404 for not found, etc.

• Protected endpoints should return 401 Unauthorized or 403 Forbidden when the session is invalid or expired; the frontend treats these responses as a session-expiry event and redirects the user to /login.

• Use HTTPException or custom exception handlers — never return error dicts manually.

• GET endpoints are read-only — never call db.commit() or execute INSERT/UPDATE/DELETE inside a GET handler. If a GET path produces side-effects (e.g., caching resolved data), that write belongs in a background task, a scheduled flush, or a separate POST endpoint. Users and HTTP caches assume GET is idempotent and non-mutating.

  # Good — pass db=None on GET so geo_service never commits
  result = await geo_service.lookup_batch(ips, http_session, db=None)
  
  # Bad — triggers INSERT + COMMIT per IP inside a GET handler
  result = await geo_service.lookup_batch(ips, http_session, db=app_db)
  

from fastapi import APIRouter, Depends, HTTPException, status
from app.models.jail import JailResponse, JailListResponse
from app.services.jail_service import JailService

router: APIRouter = APIRouter(prefix="/api/jails", tags=["Jails"])

@router.get("/", response_model=JailListResponse)
async def list_jails(service: JailService = Depends()) -> JailListResponse:
    jails: list[JailResponse] = await service.get_all_jails()
    return JailListResponse(jails=jails)

---

5. Pydantic Models

• Every model inherits from pydantic.BaseModel.
• Use model_config = ConfigDict(strict=True) where appropriate.
• Field names use snake_case in Python, export as camelCase to the frontend via alias generators if needed.
• Validate at the boundary — once data enters a Pydantic model it is trusted.
• Use Field(...) with descriptions for every field to keep auto-generated docs useful.
• Separate request models, response models, and domain (internal) models — do not reuse one model for all three.

from pydantic import BaseModel, Field
from datetime import datetime

class BanResponse(BaseModel):
    ip: str = Field(..., description="Banned IP address")
    jail: str = Field(..., description="Jail that issued the ban")
    banned_at: datetime = Field(..., description="UTC timestamp of the ban")
    expires_at: datetime | None = Field(None, description="UTC expiry, None if permanent")
    ban_count: int = Field(..., ge=1, description="Number of times this IP was banned")

---

6. Async Rules

• Never call blocking / synchronous I/O in an async function — no time.sleep(), no synchronous file reads, no requests.get().

• Use aiohttp.ClientSession for HTTP calls, aiosqlite for database access.

• Use asyncio.TaskGroup (Python 3.11+) when you need to run independent coroutines concurrently.

• Long-running startup/shutdown logic goes into the FastAPI lifespan context manager.

• Never call db.commit() inside a loop. With aiosqlite, every commit serialises through a background thread and forces an fsync. N rows × 1 commit = N fsyncs. Accumulate all writes in the loop, then issue a single db.commit() once after the loop ends. The difference between 5,000 commits and 1 commit can be seconds vs milliseconds.

  # Good — one commit for the whole batch
  for ip, info in results.items():
      await db.execute(INSERT_SQL, (ip, info.country_code, ...))
  await db.commit()  # ← single fsync
  
  # Bad — one fsync per row
  for ip, info in results.items():
      await db.execute(INSERT_SQL, (ip, info.country_code, ...))
      await db.commit()  # ← fsync on every iteration
  

• Prefer executemany() over calling execute() in a loop when inserting or updating multiple rows with the same SQL template. aiosqlite passes the entire batch to SQLite in one call, reducing Python↔thread overhead on top of the single-commit saving.

  # Good
  await db.executemany(INSERT_SQL, [(ip, cc, cn, asn, org) for ip, info in results.items()])
  await db.commit()
  

• Shared resources (DB connections, HTTP sessions) are created once during startup and closed during shutdown — never inside request handlers.

from contextlib import asynccontextmanager
from collections.abc import AsyncGenerator
from fastapi import FastAPI
import aiohttp
import aiosqlite

@asynccontextmanager
async def lifespan(app: FastAPI) -> AsyncGenerator[None]:
    # Startup
    app.state.http_session = aiohttp.ClientSession()
    app.state.db = await aiosqlite.connect("bangui.db")
    yield
    # Shutdown
    await app.state.http_session.close()
    await app.state.db.close()

---

7. Logging

• Use structlog for every log message.
• Bind contextual key-value pairs — never format strings manually.
• Log levels: debug for development detail, info for operational events, warning for recoverable issues, error for failures, critical for fatal problems.
• Never log sensitive data (passwords, tokens, session IDs).

import structlog

log: structlog.stdlib.BoundLogger = structlog.get_logger()

async def ban_ip(ip: str, jail: str) -> None:
    log.info("banning_ip", ip=ip, jail=jail)
    try:
        await _execute_ban(ip, jail)
        log.info("ip_banned", ip=ip, jail=jail)
    except BanError as exc:
        log.error("ban_failed", ip=ip, jail=jail, error=str(exc))
        raise

---

8. Error Handling

• Define custom exception classes for domain errors (e.g., JailNotFoundError, BanFailedError).
• Catch specific exceptions — never bare except: or except Exception: without re-raising.
• Map domain exceptions to HTTP status codes via FastAPI exception handlers registered on the app.
• Always log errors with context before raising.

class JailNotFoundError(Exception):
    def __init__(self, name: str) -> None:
        self.name: str = name
        super().__init__(f"Jail '{name}' not found")

# In main.py
@app.exception_handler(JailNotFoundError)
async def jail_not_found_handler(request: Request, exc: JailNotFoundError) -> JSONResponse:
    return JSONResponse(status_code=404, content={"detail": f"Jail '{exc.name}' not found"})

Routers and Exception Propagation

• Routers must NOT construct HTTPException for domain errors — let domain exceptions propagate.
• Routers should never have helper functions like _bad_gateway(), _not_found(), _conflict() etc. that convert domain exceptions to HTTPException.
• All domain exception types must have corresponding handlers registered in main.py via app.add_exception_handler().
• Exception handlers are registered in order from most specific to least specific — FastAPI evaluates them in registration order.

# ❌ BAD — routers constructing HTTPException for domain exceptions
@router.get("/{name}")
async def get_jail(name: str, socket_path: Fail2BanSocketDep) -> JailDetailResponse:
    try:
        return await jail_service.get_jail(socket_path, name)
    except JailNotFoundError:
        raise HTTPException(status_code=404, detail=f"Jail not found: {name!r}") from None

# ✅ GOOD — domain exception propagates to global handler
@router.get("/{name}")
async def get_jail(name: str, socket_path: Fail2BanSocketDep) -> JailDetailResponse:
    return await jail_service.get_jail(socket_path, name)

All domain exceptions raised by services propagate to handlers in main.py, ensuring:

1. Consistent error response format across the entire API.
2. No duplicated exception-to-HTTP-status mapping logic.
3. Easy to audit all error codes — they are all in one place.

---

9. Testing

• Every new feature or bug fix must include tests.
• Tests live in tests/ mirroring the app/ structure.
• Use pytest with pytest-asyncio for async tests.
• Use httpx.AsyncClient to test FastAPI endpoints (not TestClient which is sync).
• Mock external dependencies (fail2ban socket, aiohttp calls) — tests must never touch real infrastructure.
• Aim for >80 % line coverage — critical paths (auth, banning, scheduling) must be 100 %.
• Test names follow test_<unit>_<scenario>_<expected> pattern.

import pytest
from httpx import AsyncClient, ASGITransport
from app.main import create_app

@pytest.fixture
async def client() -> AsyncClient:
    app = create_app()
    transport: ASGITransport = ASGITransport(app=app)
    async with AsyncClient(transport=transport, base_url="http://test") as ac:
        yield ac

@pytest.mark.asyncio
async def test_list_jails_returns_200(client: AsyncClient) -> None:
    response = await client.get("/api/jails/")
    assert response.status_code == 200
    data: dict = response.json()
    assert "jails" in data

---

9.1 Background Tasks and Scheduler Architecture

BanGUI uses APScheduler 4.x (async mode) to manage background jobs that execute on a schedule without user interaction. This section documents how to write and register background tasks.

Task Location and Structure

All background tasks live in backend/app/tasks/ as separate modules. Each task:

• Exports a register(app: FastAPI) -> None or async def register(app: FastAPI) -> None function.
• Opens its own database connection using app.db.open_db() or the task_db() helper.
• Closes connections when work completes (use the async context manager pattern).
• Runs independently of the FastAPI request/response cycle.

Example Task

# backend/app/tasks/my_task.py
import structlog
from fastapi import FastAPI
from apscheduler.schedulers.asyncio import AsyncIOScheduler

log = structlog.get_logger()

async def my_background_job(app: FastAPI) -> None:
    """Do important work on a schedule."""
    log.info("my_background_job_started")
    try:
        db = await app.db.open_db(app.state.settings.database_path)
        try:
            # Do work...
            pass
        finally:
            await db.close()
    except Exception:
        log.error("my_background_job_failed", exc_info=True)

def register(app: FastAPI) -> None:
    """Register the job with the scheduler."""
    scheduler: AsyncIOScheduler = app.state.scheduler
    scheduler.add_job(
        my_background_job,
        args=(app,),
        trigger="interval",
        seconds=60,
        id="my_task",
        name="My Background Job",
    )

Accessing Shared Resources in Tasks

Since tasks do not have access to Depends(get_db) (no request scope), they must:

1. Open their own DB connection via app.state.db_factory.open_db(path).
2. Access app-level state — app.state.http_session, app.state.geo_cache, app.state.settings, etc.
3. Use structlog for all logging (never print()).

Single-Worker Requirement

The scheduler is bound to a single asyncio event loop and cannot be shared across multiple worker processes. BanGUI enforces single-worker mode to prevent duplicate task execution.

• Deployment constraint: Set BANGUI_WORKERS=1 (default).
• Startup validation: startup_shared_resources() raises RuntimeError if BANGUI_WORKERS > 1.
• See Architekture.md § 9.2 for full details.

---

10. Code Style & Tooling

Tool	Purpose
Ruff	Linter and formatter (replaces black, isort, flake8).
mypy or pyright	Static type checking in strict mode.
pre-commit	Run ruff + type checker before every commit.

• Line length: 120 characters max.
• Strings: use double quotes (").
• Imports: sorted by ruff — stdlib → third-party → local, one import per line.
• No unused imports, no unused variables, no # type: ignore without explanation.
• Docstrings in Google style on every public function, class, and module.

---

11. fail2ban Response Utilities

All services that interact with the fail2ban daemon must use the canonical response parsing utilities from app.utils.fail2ban_response. This ensures consistent error handling, type safety, and makes it easy to fix bugs in response handling across the entire codebase.

Available Functions

ok(response: object) -> object Extracts the payload from a fail2ban (return_code, data) response tuple.

• Raises ValueError if return code ≠ 0 or response shape is invalid.
• Use this on every response from Fail2BanClient.send().

to_dict(pairs: object) -> dict[str, object] Converts a list of (key, value) pairs (fail2ban's native response format) to a Python dict.

• Silently ignores malformed entries and non-list/tuple inputs.
• Always returns a dict (empty if input is invalid).

ensure_list(value: object | None) -> list[str] Coerces fail2ban response values (which may be None, a single string, or a list) to a normalized list of strings.

• Handles all three cases consistently.
• Returns empty list for None or empty strings.

is_not_found_error(exc: Exception) -> bool Checks if an exception indicates a jail does not exist.

• Checks for multiple error message patterns (case-insensitive).
• Use this to distinguish "jail not found" errors from other failures.

Example Usage

from app.utils.fail2ban_response import ok, to_dict, ensure_list, is_not_found_error
from app.utils.fail2ban_client import Fail2BanClient

client = Fail2BanClient(socket_path="/var/run/fail2ban/fail2ban.sock")

try:
    # Get jail status
    response = await client.send(["status", "sshd", "short"])
    status_dict = to_dict(ok(response))  # Extract payload and convert to dict
    
    # Get list of banned IPs
    ban_response = await client.send(["get", "sshd", "banip"])
    banned_ips = ensure_list(ok(ban_response))  # Normalize to list of strings
    
except ValueError as exc:
    if is_not_found_error(exc):
        raise JailNotFoundError("sshd") from exc
    raise

Why This Matters

Before this utility module, every service implemented its own copy of these functions, leading to:

• Code duplication across 7+ service files.
• Subtle inconsistencies in error handling.
• Difficult maintenance — every bug fix required touching multiple files.

Now, all services import from a single authoritative source, making response handling consistent, maintainable, and type-safe.

---

12. Configuration & Secrets

• All configuration lives in environment variables loaded through pydantic-settings.
• Secrets (master password hash, session key) are never committed to the repository.
• Provide a .env.example with all keys and placeholder values.
• Validate config at startup — fail fast with a clear error if a required value is missing.

from pydantic_settings import BaseSettings
from pydantic import Field

class Settings(BaseSettings):
    database_path: str = Field("bangui.db", description="Path to SQLite database")
    fail2ban_socket: str = Field("/var/run/fail2ban/fail2ban.sock", description="fail2ban socket path")
    session_secret: str = Field(..., description="Secret key for session signing")
    log_level: str = Field("info", description="Logging level")

    model_config = {"env_prefix": "BANGUI_", "env_file": ".env"}

---

13. Git & Workflow

• Branch naming: feature/<short-description>, fix/<short-description>, chore/<short-description>.
• Commit messages: imperative tense, max 72 chars first line (Add jail reload endpoint, Fix ban history query).
• Every merge request must pass: ruff, type checker, all tests.
• Do not merge with failing CI.
• Keep pull requests small and focused — one feature or fix per PR.

---

14. Coding Principles

These principles are non-negotiable. Every backend contributor must internalise and apply them daily.

14.1 Clean Code

• Write code that reads like well-written prose — a new developer should understand intent without asking.
• Meaningful names — variables, functions, and classes must reveal their purpose. Avoid abbreviations (cnt, mgr, tmp) unless universally understood.
• Small functions — each function does exactly one thing. If you need a comment to explain a block inside a function, extract it into its own function.
• No magic numbers or strings — use named constants.
• Boy Scout Rule — leave every file cleaner than you found it.
• Avoid deep nesting — prefer early returns (guard clauses) to keep the happy path at the top indentation level.

# Good — guard clause, clear name, one job
async def get_active_ban(ip: str, jail: str) -> Ban:
    ban: Ban | None = await repo.find_ban(ip=ip, jail=jail)
    if ban is None:
        raise BanNotFoundError(ip=ip, jail=jail)
    if ban.is_expired():
        raise BanExpiredError(ip=ip, jail=jail)
    return ban

# Bad — nested, vague name
async def check(ip, j):
    b = await repo.find_ban(ip=ip, jail=j)
    if b:
        if not b.is_expired():
            return b
        else:
            raise Exception("expired")
    else:
        raise Exception("not found")

14.2 Separation of Concerns (SoC)

• Each module, class, and function must have a single, well-defined responsibility.
• Routers → HTTP layer only (parse requests, return responses).
• Services → business logic and orchestration.
• Repositories → data access and persistence.
• Models → data shapes and validation.
• Tasks → scheduled background jobs.
• Never mix layers — a router must not execute SQL, and a repository must not raise HTTPException.

14.3 Single Responsibility Principle (SRP)

• A class or module should have one and only one reason to change.
• If a service handles both ban management and email notifications, split it into BanService and NotificationService.

14.4 Don't Repeat Yourself (DRY)

• Extract shared logic into utility functions, base classes, or dependency providers.
• If the same block of code appears in more than one place, refactor it into a single source of truth.
• But don't over-abstract — premature DRY that couples unrelated features is worse than a little duplication (see Rule of Three: refactor when something appears a third time).

14.5 KISS — Keep It Simple, Stupid

• Choose the simplest solution that works correctly.
• Avoid clever tricks, premature optimisation, and over-engineering.
• If a standard library function does the job, prefer it over a custom implementation.

14.6 YAGNI — You Aren't Gonna Need It

• Do not build features, abstractions, or config options "just in case".
• Implement what is required now. Extend later when a real need emerges.

14.7 Dependency Inversion Principle (DIP)

• High-level modules (services) must not depend on low-level modules (repositories) directly. Both should depend on abstractions (protocols / interfaces).
• Use FastAPI's Depends() to inject implementations — this makes swapping and testing trivial.

from typing import Protocol

class BanRepository(Protocol):
    async def find_ban(self, ip: str, jail: str) -> Ban | None: ...
    async def save_ban(self, ban: Ban) -> None: ...

class SqliteBanRepository:
    """Concrete implementation — depends on aiosqlite."""
    async def find_ban(self, ip: str, jail: str) -> Ban | None: ...
    async def save_ban(self, ban: Ban) -> None: ...

13.7.1 Repository Module Pattern — Module-as-Protocol Structural Compatibility

BanGUI uses module-level functions for repository implementations, not classes. Each repository module (e.g., session_repo.py, blocklist_repo.py) exports async functions that match the signatures defined in the Protocol interface in protocols.py. This is a structural typing pattern — mypy accepts the module as a valid Protocol implementation because the function signatures match, even though the module is not explicitly annotated as implementing the Protocol.

This approach works correctly with FastAPI's dependency injection via cast():

# In app/repositories/session_repo.py
async def create_session(db: aiosqlite.Connection, token: str, created_at: str, expires_at: str) -> Session:
    """Insert a new session row."""
    ...

# In app/repositories/protocols.py
class SessionRepository(Protocol):
    async def create_session(
        self,
        db: aiosqlite.Connection,
        token: str,
        created_at: str,
        expires_at: str,
    ) -> Session:
        ...

# In app/dependencies.py
async def get_session_repo() -> SessionRepository:
    """Provide the concrete session repository implementation."""
    from app.repositories import session_repo
    return session_repo  # ← mypy accepts this because the module has matching functions

Why this pattern is used:

• Simplicity — no boilerplate class/instance wrapping.
• Compatibility — Python's structural typing (PEP 544) means the module automatically satisfies the Protocol interface if function signatures match.
• Testability — the same DIP principle applies; services depend on the Protocol, not the module directly, so tests can mock the Protocol.

Risks and mitigations:

• Silent breakage if function signatures change — If a parameter is added or removed from a module function, the module no longer satisfies the Protocol, but mypy does not flag this as an error because the module is loosely coupled. To prevent this, Protocol signatures in protocols.py are the source of truth. Always check that module functions match the Protocol definitions before merging changes. The CI/CD pipeline validates this compatibility at build time.

How the validation works (CI check):

• Before each deployment, run mypy --strict to ensure all dependency providers return values compatible with their Protocol types.
• The cast() calls in dependencies.py are a documented signal that structural compatibility is being verified externally, not via explicit class inheritance.

14.8 Composition over Inheritance

• Favour composing small, focused objects over deep inheritance hierarchies.
• Use mixins or protocols only when a clear "is-a" relationship exists; otherwise, pass collaborators as constructor arguments.

14.9 Fail Fast

• Validate inputs as early as possible — at the API boundary with Pydantic, at service entry with assertions or domain checks.
• Raise specific exceptions immediately rather than letting bad data propagate silently.

14.10 Law of Demeter (Principle of Least Knowledge)

• A function should only call methods on:
  1. Its own object (self).
  2. Objects passed as parameters.
  3. Objects it creates.
• Avoid long accessor chains like request.state.db.cursor().execute(...) — wrap them in a meaningful method.

14.11 Defensive Programming

• Never trust external input — validate and sanitise everything that crosses a boundary (HTTP request, file, socket, environment variable).
• Handle edge cases explicitly: empty lists, None values, negative numbers, empty strings.
• Use type narrowing and exhaustive pattern matching (match / case) to eliminate impossible states.

---

15. Quick Reference — Do / Don't

Do	Don't
Type every function, variable, return	Leave types implicit
Use async def for I/O	Use sync functions for I/O
Validate with Pydantic at the boundary	Pass raw dicts through the codebase
Log with structlog + context keys	Use print() or format strings in logs
Write tests for every feature	Ship untested code
Use aiohttp for HTTP calls	Use requests
Handle errors with custom exceptions	Use bare except:
Keep routers thin, logic in services	Put business logic in routers
Use datetime.now(datetime.UTC)	Use naive datetimes
Run ruff + mypy before committing	Push code that doesn't pass linting
Keep GET endpoints read-only (no db.commit())	Call db.commit() / INSERT inside GET handlers
Batch DB writes; issue one db.commit() after the loop	Commit inside a loop (1 fsync per row)
Use executemany() for bulk inserts	Call execute() + commit() per row in a loop