Python Services & CLI

Modern Tooling

• uv init --package myproject for distributable packages, uv init for apps
• uv add <pkg>, uv add --group dev <pkg>, never edit pyproject.toml deps manually
• uv run <cmd> instead of activating venvs
• uv.lock goes in version control
• Use [dependency-groups] (PEP 735) for dev/test/docs, not [project.optional-dependencies]
• PEP 723 inline metadata for standalone scripts with deps
• ruff check --fix . && ruff format . for lint+format in one pass

CLI Tools

Entry points in pyproject.toml:

[project.scripts]
my-tool = "my_package.cli:main"

Click (recommended for complex CLIs):

import click

@click.group()
@click.version_option()
def cli(): ...

@cli.command()
@click.argument("name")
@click.option("--count", default=1, type=int)
def greet(name: str, count: int):
    for _ in range(count):
        click.echo(f"Hello, {name}!")

def main():
    cli()

argparse for simple CLIs — subparsers for subcommands, parser.add_argument("--output", "-o").

Use src/ layout. Include py.typed for type hints. importlib.resources.files() for package data access.

Parallelism

Key rule: Stay fully sync or fully async within a call path.

asyncio patterns:

• asyncio.gather(*tasks) for concurrent I/O — use return_exceptions=True for partial failure tolerance
• asyncio.Semaphore(n) to limit concurrency (rate limiting external APIs)
• asyncio.wait_for(coro, timeout=N) for timeouts
• asyncio.Queue for producer-consumer
• asyncio.Lock when coroutines share mutable state
• Never block the event loop: asyncio.to_thread(sync_fn) for sync libs, aiohttp/httpx.AsyncClient for HTTP
• Handle CancelledError — always re-raise after cleanup
• Async generators (async for) for streaming/pagination

multiprocessing for CPU-bound:

from concurrent.futures import ProcessPoolExecutor
with ProcessPoolExecutor(max_workers=4) as pool:
    results = list(pool.map(cpu_task, items))

FastAPI Services

Project structure:

app/
├── api/v1/endpoints/    # Route handlers
├── core/                # config.py, security.py, database.py
├── models/              # SQLAlchemy models
├── schemas/             # Pydantic request/response
├── services/            # Business logic
├── repositories/        # Data access (generic CRUD base)
└── main.py              # Lifespan, middleware, router includes

Lifespan for startup/shutdown: @asynccontextmanager async def lifespan(app):

Configuration — pydantic_settings.BaseSettings with model_config = {"env_file": ".env"}. Required fields = no default (fails fast at boot). env_nested_delimiter = "__" for grouped config. secrets_dir for Docker/K8s mounted secrets.

Dependency injection — Depends(get_db) for sessions, Depends(get_current_user) for auth. Override in tests: app.dependency_overrides[get_db] = mock_db.

Async DB — SQLAlchemy AsyncSession with asyncpg. Session-per-request via async with AsyncSessionLocal() as session: yield session.

Repository pattern — Generic BaseRepository[ModelType, CreateSchema, UpdateSchema] with get/get_multi/create/update/delete. Service layer holds business logic, routes stay thin.

Background Jobs

• Return job ID immediately, process async. Client polls /jobs/{id} for status
• Celery: @app.task(bind=True, max_retries=3, autoretry_for=(ConnectionError,)) — exponential backoff: raise self.retry(countdown=2**self.request.retries * 60)
• Alternatives: Dramatiq (modern Celery), RQ (simple Redis), cloud-native (SQS+Lambda, Cloud Tasks)
• Idempotency is mandatory — tasks may retry. Use idempotency keys for external calls, check-before-write, upsert patterns
• Dead letter queue for permanently failed tasks after max retries
• Task workflows: chain(a.s(), b.s()) for sequential, group(...) for parallel, chord(group, callback) for fan-out/fan-in

Resilience

Retries with tenacity:

from tenacity import retry, stop_after_attempt, wait_exponential_jitter, retry_if_exception_type

@retry(
    retry=retry_if_exception_type((ConnectionError, TimeoutError)),
    stop=stop_after_attempt(5) | stop_after_delay(60),
    wait=wait_exponential_jitter(initial=1, max=30),
    before_sleep=log_retry_attempt,
)
def call_api(url: str) -> dict: ...

• Retry only transient errors: network, 429/502/503/504. Never retry 4xx (except 429), auth errors, validation errors
• Every network call needs a timeout
• @fail_safe(default=[]) decorator for non-critical paths — return cached/default on failure
• Stack decorators: @traced @with_timeout(30) @retry(...) — separate infra from business logic

Observability

• structlog for JSON structured logging. Configure once at startup with JSONRenderer, TimeStamper, merge_contextvars
• Correlation IDs — generate at ingress (X-Correlation-ID header), bind to contextvars, propagate to downstream calls
• Log levels: DEBUG=diagnostics, INFO=operations, WARNING=anomalies handled, ERROR=failures needing attention. Never log expected behavior at ERROR
• Prometheus metrics — track latency (Histogram), traffic (Counter), errors (Counter), saturation (Gauge). Keep label cardinality bounded (no user IDs)
• OpenTelemetry for distributed tracing across services

Discipline

• For non-trivial changes, pause and ask: "is there a more elegant way?" Skip for obvious fixes.
• Simplicity first — every change as simple as possible, impact minimal code
• Only touch what's necessary — avoid introducing unrelated changes
• No hacky workarounds — if a fix feels wrong, step back and implement the clean solution

Error Handling

• Validate inputs at boundaries before expensive ops. Report all errors at once when possible
• Use specific exceptions: ValueError, TypeError, KeyError, not bare Exception
• raise ServiceError("upload failed") from e — always chain to preserve debug trail
• Convert external data to domain types (enums, Pydantic models) at system boundaries
• Batch processing: BatchResult(succeeded={}, failed={}) — don't let one item abort the batch
• Pydantic BaseModel with field_validator for complex input validation