Python Development

Python Development

Modern Python development covering project configuration, typing, testing, linting, async, web frameworks, ORM, and production patterns. Targets Python 3.10+ with focus on 3.12+ features.

"There should be one — and preferably only one — obvious way to do it." — The Zen of Python

1. Project Configuration (pyproject.toml)

Single source of truth for project metadata, dependencies, and tool config.

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

[project]
name = "my-app"
version = "0.1.0"
description = "My application"
requires-python = ">=3.12"
dependencies = [
    "fastapi>=0.115",
    "uvicorn[standard]>=0.30",
    "sqlalchemy[asyncio]>=2.0",
    "pydantic>=2.0",
    "pydantic-settings>=2.0",
    "structlog>=24.0",
]

[project.optional-dependencies]
dev = [
    "pytest>=8.0",
    "pytest-cov>=5.0",
    "pytest-asyncio>=0.24",
    "ruff>=0.5",
    "mypy>=1.10",
    "pre-commit>=3.7",
]
docs = ["mkdocs-material>=9.0"]

[project.scripts]
my-app = "my_app.cli:main"

[tool.hatch.build.targets.wheel]
packages = ["src/my_app"]

Build backends: hatchling (default, modern), setuptools (legacy), flit-core (simple), maturin (Rust extensions).

Src layout (recommended):

project/
├── src/
│   └── my_app/
│       ├── __init__.py
│       ├── __main__.py
│       ├── api/
│       ├── core/
│       ├── models/
│       └── services/
├── tests/
│   ├── conftest.py
│   ├── unit/
│   └── integration/
├── pyproject.toml
└── README.md

2. Dependency Management

uv (Recommended, 2024+)

Modern Python packaging and project management. Replaces pip, venv, pip-tools, and poetry in most workflows.

# Project initialization
uv init my-project               # new project with pyproject.toml
uv init --lib my-lib             # library package layout

# Dependencies
uv add fastapi sqlalchemy        # add runtime deps
uv add --dev pytest ruff mypy    # add dev deps
uv add --optional docs mkdocs    # optional dependency groups
uv remove fastapi                # remove a dep
uv tree                          # show dependency tree

# Running & locking
uv run python main.py            # run in project venv (auto-creates)
uv run --python 3.13 python -c "print('hi')"
uv lock                          # generate uv.lock
uv sync                          # install from uv.lock
uv export -o requirements.txt    # export to requirements

# Tools (replaces pipx)
uv tool install ruff             # install global CLI tool
uv tool run ruff check .         # run without installing
uvx ruff check .                 # shorthand

# Python versions
uv python install 3.13           # install a Python version
uv python pin 3.12               # pin in .python-version

uv vs Poetry vs pip-tools:

Poetry

poetry init
poetry add fastapi sqlalchemy
poetry add --group dev pytest ruff mypy
poetry install
poetry build

pip-tools

pip-compile pyproject.toml -o requirements.txt
pip install -r requirements.txt

Virtual Environments

python -m venv .venv             # stdlib
source .venv/bin/activate        # activate
uv venv --python 3.12            # uv with specific version

.python-version file for pyenv/asdf:

3.12.4

3. Modern Typing

Type Aliases & Syntax (PEP 604, 613, 695)

# Union syntax (3.10+)
def process(value: int | str | None) -> str: ...

# Type alias (3.12+)
type Vector = list[float]
type UserId = int

# TypeAlias (3.10+)
from typing import TypeAlias
Vector: TypeAlias = list[float]

Generics (3.12+)

# Modern syntax
def first[T](items: list[T]) -> T:
    return items[0]

class Repository[T]:
    def __init__(self) -> None:
        self._items: list[T] = []

    def add(self, item: T) -> None: ...
    def get(self, id: int) -> T | None: ...

Protocol (Structural Typing)

from typing import Protocol, runtime_checkable

@runtime_checkable
class Renderable(Protocol):
    def render(self) -> str: ...

class Report:
    def render(self) -> str:
        return "report data"

# Report satisfies Renderable without inheritance
def print_item(item: Renderable) -> None:
    print(item.render())

Self, TypeGuard, Annotated

from typing import Self, TypeGuard, Annotated

class Builder:
    def with_name(self, name: str) -> Self:
        self.name = name
        return self

def is_string_list(val: list[object]) -> TypeGuard[list[str]]:
    return all(isinstance(x, str) for x in val)

# Annotated for metadata
PositiveInt = Annotated[int, Field(gt=0)]

TypedDict & Literal

from typing import TypedDict, Literal

class ApiResponse(TypedDict):
    status: Literal["ok", "error"]
    data: dict[str, Any]
    message: str

When to Use What

4. Linting, Formatting & Static Analysis

Ruff (2024+ standard)

Replaces flake8, black, isort, pyupgrade, and more.

# pyproject.toml
[tool.ruff]
target-version = "py312"
line-length = 88

[tool.ruff.lint]
select = [
    "E",    # pycodestyle errors
    "F",    # pyflakes
    "W",    # pycodestyle warnings
    "I",    # isort
    "N",    # pep8-naming
    "UP",   # pyupgrade
    "B",    # flake8-bugbear
    "A",    # flake8-builtins
    "SIM",  # flake8-simplify
    "PT",   # flake8-pytest-style
    "RUF",  # ruff-specific
    "S",    # flake8-bandit (security)
    "LOG",  # flake8-logging
]
ignore = ["E501"]  # line length handled by formatter

[tool.ruff.lint.isort]
known-first-party = ["my_app"]

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

ruff check .           # lint
ruff check . --fix     # auto-fix
ruff format .          # format (replaces black)

mypy (Static Type Checking)

# pyproject.toml
[tool.mypy]
python_version = "3.12"
strict = true
warn_return_any = true
warn_unused_configs = true
disallow_untyped_defs = true

# Per-module overrides
[[tool.mypy.overrides]]
module = "tests.*"
disallow_untyped_defs = false

mypy src/

basedpyright (Alternative, 2025+)

Fork of pyright with stricter defaults and better DX. Good for teams finding mypy too slow or permissive.

# pyproject.toml
[tool.basedpyright]
pythonVersion = "3.12"
typeCheckingMode = "standard"  # or "strict", "all"
reportMissingTypeStubs = true
reportUnusedCallResult = false

basedpyright src/          # CLI check
# Or via pre-commit:
# repo: https://github.com/astral-sh/ruff-pre-commit (ruff handles import sorting)
# repo: https://github.com/RobertCraigie/pyright-python (for basedpyright)

When to choose what:

Pre-commit

# .pre-commit-config.yaml
repos:
  - repo: https://github.com/astral-sh/ruff-pre-commit
    rev: v0.11.0
    hooks:
      - id: ruff
        args: [--fix]
      - id: ruff-format

  - repo: https://github.com/pre-commit/mirrors-mypy
    rev: v1.15.0
    hooks:
      - id: mypy
        additional_dependencies: [pydantic, types-requests]

5. Pydantic v2

Validation + serialization at system boundaries (API, config, DB).

from pydantic import BaseModel, Field, field_validator, model_validator
from pydantic import ConfigDict
from decimal import Decimal

class OrderItem(BaseModel):
    model_config = ConfigDict(str_strip_whitespace=True, frozen=True)

    product_id: str = Field(..., min_length=1)
    quantity: int = Field(..., gt=0)
    unit_price: Decimal = Field(..., ge=0)

    @field_validator("product_id")
    @classmethod
    def validate_product_id(cls, v: str) -> str:
        if not v.startswith("PROD-"):
            raise ValueError("Product ID must start with PROD-")
        return v

class Order(BaseModel):
    customer_id: str
    items: list[OrderItem]

    @model_validator(mode="after")
    def validate_min_items(self) -> "Order":
        if len(self.items) == 0:
            raise ValueError("Order must have at least one item")
        return self

    @property
    def total(self) -> Decimal:
        return sum(item.unit_price * item.quantity for item in self.items)

Settings Management

from pydantic_settings import BaseSettings
from pydantic import Field

class Settings(BaseSettings):
    model_config = SettingsConfigDict(env_file=".env", env_prefix="APP_")

    debug: bool = False
    database_url: str = Field(..., description="PostgreSQL connection string")
    api_key: str = Field(..., min_length=10)
    log_level: str = "INFO"

settings = Settings()  # reads from env vars / .env

Serialization

order = Order(customer_id="C1", items=[...])

order.model_dump()                    # → dict
order.model_dump(mode="json")         # JSON-safe (Decimal → str)
order.model_dump(exclude_unset=True)  # only explicitly set fields
order.model_dump_json()               # → JSON string
Order.model_validate_json(json_str)   # parse JSON

6. Async / Await

Core Patterns

import asyncio

async def fetch_user(user_id: int) -> dict:
    async with httpx.AsyncClient() as client:
        response = await client.get(f"https://api.example.com/users/{user_id}")
        response.raise_for_status()
        return response.json()

# Concurrent execution
async def fetch_all(user_ids: list[int]) -> list[dict]:
    tasks = [fetch_user(uid) for uid in user_ids]
    return await asyncio.gather(*tasks)

# Structured concurrency (3.11+)
async def fetch_all_v2(user_ids: list[int]) -> list[dict]:
    async with asyncio.TaskGroup() as tg:
        tasks = [tg.create_task(fetch_user(uid)) for uid in user_ids]
    return [t.result() for t in tasks]

Semaphore for Rate Limiting

sem = asyncio.Semaphore(10)

async def limited_fetch(url: str) -> str:
    async with sem:
        async with httpx.AsyncClient() as client:
            resp = await client.get(url)
            return resp.text

Async Context Manager

from contextlib import asynccontextmanager

@asynccontextmanager
async def get_db_session():
    async with AsyncSession(engine) as session:
        try:
            yield session
            await session.commit()
        except Exception:
            await session.rollback()
            raise

Common Pitfalls

6.5. Python 3.13+ Features

typing.TypeIs (PEP 742)

Narrowing type guard with better inference than TypeGuard:

from typing import TypeIs

def is_str_list(val: list[object]) -> TypeIs[list[str]]:
    return all(isinstance(x, str) for x in val)

items: list[object] = ["a", "b"]
if is_str_list(items):
    reveal_type(items)  # list[str], not just bool narrowed

@warnings.deprecated

import warnings

@warnings.deprecated("Use new_function() instead", category=DeprecationWarning)
def old_function() -> None:
    ...

Free-threaded Python (no-GIL, experimental)

# Python 3.13t — free-threaded build (no GIL)
uv python install 3.13t
uv run --python 3.13t python -c "import sys; print(sys._is_gil_enabled())"
# For CPU-bound parallelism without multiprocessing overhead

6.6. Task Runners (just / task)

Replace Makefile with modern task runners:

just

# justfile
set dotenv-load

default:
    just --list

test:
    uv run pytest -q

lint:
    uv run ruff check . --fix
    uv run ruff format .
    uv run mypy src/

dev:
    uv run uvicorn my_app.main:app --reload

just test        # run tests
just lint        # run all linters
just dev         # start dev server

task (go-task)

# Taskfile.yml
version: '3'

tasks:
  test:
    cmds:
      - uv run pytest -q
  lint:
    cmds:
      - uv run ruff check . --fix
      - uv run ruff format .
      - uv run mypy src/

7. Testing

pytest Core

# tests/test_order.py
import pytest
from my_app.models import Order, OrderItem

class TestOrder:
    def test_total_calculation(self):
        order = Order(
            customer_id="C1",
            items=[
                OrderItem(product_id="PROD-1", quantity=2, unit_price=Decimal("10.00")),
                OrderItem(product_id="PROD-2", quantity=1, unit_price=Decimal("25.00")),
            ],
        )
        assert order.total == Decimal("45.00")

    def test_empty_order_raises(self):
        with pytest.raises(ValueError, match="at least one item"):
            Order(customer_id="C1", items=[])

Fixtures

# tests/conftest.py
import pytest
from sqlalchemy.ext.asyncio import create_async_engine, AsyncSession

@pytest.fixture
def sample_order() -> Order:
    return Order(
        customer_id="C1",
        items=[OrderItem(product_id="PROD-1", quantity=1, unit_price=Decimal("10"))],
    )

@pytest.fixture(scope="session")
def event_loop():
    loop = asyncio.new_event_loop()
    yield loop
    loop.close()

@pytest.fixture
async def db_session():
    engine = create_async_engine("sqlite+aiosqlite:///:memory:")
    async with engine.begin() as conn:
        await conn.run_sync(Base.metadata.create_all)
    async with AsyncSession(engine) as session:
        yield session

Parametrize

@pytest.mark.parametrize(
    "quantity, price, expected",
    [
        (1, Decimal("10"), Decimal("10")),
        (0, Decimal("10"), Decimal("0")),
        (5, Decimal("3.50"), Decimal("17.50")),
    ],
)
def test_subtotal(quantity, price, expected):
    item = OrderItem(product_id="PROD-1", quantity=quantity, unit_price=price)
    assert item.subtotal == expected

Mocking

from unittest.mock import AsyncMock, patch

@pytest.fixture
def mock_email_service():
    return AsyncMock()

async test_send_notification(mock_email_service):
    service = NotificationService(email=mock_email_service)
    await service.notify_user("user-1", "Hello")
    mock_email_service.send.assert_awaited_once_with("user-1", "Hello")

# Patching
@patch("my_app.services.requests.get")
def test_external_api(mock_get):
    mock_get.return_value.status_code = 200
    mock_get.return_value.json.return_value = {"result": "ok"}
    result = call_external_api()
    assert result == {"result": "ok"}

HTTP Request Mocking (responses / respx)

import responses
import respx
from httpx import Response

# responses — for requests/urllib3
@responses.activate
def test_api_call():
    responses.get(
        "https://api.example.com/users/1",
        json={"id": 1, "name": "Alice"},
        status=200,
    )
    result = fetch_user(1)
    assert result["name"] == "Alice"

# respx — for httpx / async
@respx.mock
def test_async_api():
    route = respx.get("https://api.example.com/users/1").mock(
        return_value=Response(200, json={"id": 1, "name": "Alice"})
    )
    result = await fetch_user_async(1)
    assert route.called
    assert result["name"] == "Alice"

AWS Service Mocking (moto)

import moto
import boto3

@moto.mock_aws
def test_s3_upload():
    client = boto3.client("s3", region_name="us-east-1")
    client.create_bucket(Bucket="my-bucket")
    client.put_object(Bucket="my-bucket", Key="file.txt", Body=b"hello")
    obj = client.get_object(Bucket="my-bucket", Key="file.txt")
    assert obj["Body"].read() == b"hello"

Multi-Environment Testing (nox)

# noxfile.py
import nox

@nox.session(python=["3.10", "3.11", "3.12", "3.13"])
def tests(session):
    session.install("-e", ".[dev]")
    session.run("pytest", "-q")

@nox.session
def lint(session):
    session.install("ruff", "mypy")
    session.run("ruff", "check", "src")
    session.run("mypy", "src")

nox -s tests        # run tests across all Python versions
nox -s lint         # run linters
nox -s tests-3.12   # run on specific version

pytest Config

# pyproject.toml
[tool.pytest.ini_options]
testpaths = ["tests"]
asyncio_mode = "auto"
addopts = "-ra -q --strict-markers --strict-config"
markers = [
    "slow: marks tests as slow (deselect with '-m \"not slow\"')",
    "integration: integration tests requiring external services",
]

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

[tool.coverage.report]
show_missing = true
fail_under = 80
exclude_lines = [
    "pragma: no cover",
    "if TYPE_CHECKING:",
    "if __name__ == .__main__.",
]

8. FastAPI Patterns

Application Structure

# src/my_app/main.py
from contextlib import asynccontextmanager
from fastapi import FastAPI

@asynccontextmanager
async def lifespan(app: FastAPI):
    # startup
    await init_db()
    yield
    # shutdown
    await close_db()

app = FastAPI(title="My App", lifespan=lifespan)

Router with Dependency Injection

# src/my_app/api/orders.py
from fastapi import APIRouter, Depends, HTTPException

router = APIRouter(prefix="/orders", tags=["orders"])

async def get_current_user(token: str = Depends(oauth2_scheme)) -> User:
    user = await verify_token(token)
    if not user:
        raise HTTPException(status_code=401, detail="Invalid token")
    return user

@router.post("/", response_model=OrderResponse, status_code=201)
async def create_order(
    body: OrderCreate,
    user: User = Depends(get_current_user),
    db: AsyncSession = Depends(get_db),
):
    order = OrderService(db)
    return await order.create(user_id=user.id, items=body.items)

@router.get("/{order_id}", response_model=OrderResponse)
async def get_order(
    order_id: str,
    user: User = Depends(get_current_user),
    db: AsyncSession = Depends(get_db),
):
    order = await db.get(Order, order_id)
    if not order or (order.user_id != user.id and not user.is_admin):
        raise HTTPException(status_code=404, detail="Order not found")
    return order

Background Tasks & WebSockets

from fastapi import BackgroundTasks

async def send_email(email: str, message: str):
    await asyncio.sleep(2)  # simulate
    logger.info("email_sent", email=email)

@router.post("/notify")
async def notify(
    email: str,
    background_tasks: BackgroundTasks,
):
    background_tasks.add_task(send_email, email, "Welcome!")
    return {"status": "queued"}

# WebSocket
from fastapi import WebSocket

@router.websocket("/ws")
async def websocket_endpoint(websocket: WebSocket):
    await websocket.accept()
    try:
        while True:
            data = await websocket.receive_text()
            await websocket.send_text(f"Echo: {data}")
    except WebSocketDisconnect:
        logger.info("client_disconnected")

Class-Based Dependencies

from fastapi import Depends

class Pagination:
    def __init__(self, skip: int = 0, limit: int = 100):
        self.skip = skip
        self.limit = limit

async def get_pagination(p: Pagination = Depends()) -> Pagination:
    return p

@router.get("/items")
async def list_items(p: Pagination = Depends(get_pagination)):
    return await fetch_items(offset=p.skip, limit=p.limit)

Error Handling

from fastapi import Request
from fastapi.responses import JSONResponse

class AppError(Exception):
    def __init__(self, status_code: int, detail: str):
        self.status_code = status_code
        self.detail = detail

@app.exception_handler(AppError)
async def app_error_handler(request: Request, exc: AppError):
    return JSONResponse(
        status_code=exc.status_code,
        content={"error": exc.detail},
    )

@app.exception_handler(Exception)
async def generic_error_handler(request: Request, exc: Exception):
    logger.error("Unhandled error", exc_info=True, path=request.url.path)
    return JSONResponse(
        status_code=500,
        content={"error": "Internal server error"},
    )

9. SQLAlchemy 2.0

Declarative Models

from sqlalchemy import ForeignKey, String, func
from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column, relationship
from datetime import datetime
from uuid import UUID, uuid4

class Base(DeclarativeBase):
    pass

class Order(Base):
    __tablename__ = "orders"

    id: Mapped[UUID] = mapped_column(primary_key=True, default=uuid4)
    customer_id: Mapped[UUID] = mapped_column(ForeignKey("customers.id"))
    status: Mapped[str] = mapped_column(String(20), default="PENDING")
    created_at: Mapped[datetime] = mapped_column(server_default=func.now())

    items: Mapped[list["OrderItem"]] = relationship(back_populates="order", cascade="all, delete-orphan")
    customer: Mapped["Customer"] = relationship(back_populates="orders")

class OrderItem(Base):
    __tablename__ = "order_items"

    id: Mapped[UUID] = mapped_column(primary_key=True, default=uuid4)
    order_id: Mapped[UUID] = mapped_column(ForeignKey("orders.id"))
    product_id: Mapped[str] = mapped_column(String(50))
    quantity: Mapped[int] = mapped_column()
    unit_price: Mapped[Decimal] = mapped_column(Numeric(10, 2))

    order: Mapped["Order"] = relationship(back_populates="items")

Async Session

from sqlalchemy.ext.asyncio import create_async_engine, AsyncSession, async_sessionmaker

engine = create_async_engine("postgresql+asyncpg://user:pass@localhost/db", echo=False)
async_session = async_sessionmaker(engine, class_=AsyncSession, expire_on_commit=False)

async def get_db() -> AsyncGenerator[AsyncSession, None]:
    async with async_session() as session:
        yield session

# Usage in service
async def create_order(db: AsyncSession, customer_id: UUID, items: list[dict]) -> Order:
    order = Order(customer_id=customer_id)
    for item_data in items:
        order.items.append(OrderItem(**item_data))
    db.add(order)
    await db.commit()
    return order

Alembic Migrations

alembic init migrations
alembic revision --autogenerate -m "add orders table"
alembic upgrade head
alembic downgrade -1

# alembic.ini
[alembic]
script_location = migrations
sqlalchemy.url = postgresql+asyncpg://user:pass@localhost/db

10. Error Handling & Logging

Custom Exception Hierarchy

class AppError(Exception):
    """Base error for application."""
    def __init__(self, message: str, code: str = "APP_ERROR"):
        self.message = message
        self.code = code
        super().__init__(message)

class NotFoundError(AppError):
    def __init__(self, resource: str, id: str):
        super().__init__(f"{resource} {id} not found", "NOT_FOUND")

class ValidationError(AppError):
    def __init__(self, detail: str):
        super().__init__(detail, "VALIDATION_ERROR")

class ConflictError(AppError):
    def __init__(self, detail: str):
        super().__init__(detail, "CONFLICT")

Exception Chaining

try:
    result = await external_api.fetch(data)
except httpx.HTTPStatusError as e:
    raise AppError("External service unavailable") from e

Structured Logging

import structlog

structlog.configure(
    processors=[
        structlog.contextvars.merge_contextvars,
        structlog.processors.add_log_level,
        structlog.processors.TimeStamper(fmt="iso"),
        structlog.dev.ConsoleRenderer() if settings.debug else structlog.processors.JSONRenderer(),
    ],
    wrapper_class=structlog.make_filtering_bound_logger(
        logging.getLevelName(settings.log_level)
    ),
)

logger = structlog.get_logger()

# Usage
logger.info("order_created", order_id=str(order.id), total=str(order.total))
logger.error("payment_failed", order_id=str(order.id), reason=str(exc))

Request Context

import structlog
from contextvars import ContextVar

request_id_ctx: ContextVar[str] = ContextVar("request_id", default="")

@app.middleware("http")
async def add_request_id(request: Request, call_next):
    request_id = request.headers.get("X-Request-ID", str(uuid4()))
    request_id_ctx.set(request_id)
    structlog.contextvars.bind_contextvars(request_id=request_id)
    response = await call_next(request)
    response.headers["X-Request-ID"] = request_id
    return response

11. Performance & Optimization

Profiling

# cProfile
python -m cProfile -s cumulative my_script.py

# py-spy (sampling, no code changes)
py-spy top --pid 12345
py-spy record -o profile.svg -- python my_script.py

# memray (memory)
memray run my_script.py
memray flamegraph output.bin

Caching

from functools import lru_cache, cache

@lru_cache(maxsize=256)
def expensive_computation(n: int) -> int: ...

@cache  # unbounded (3.9+)
def get_config() -> dict: ...

Concurrency

from concurrent.futures import ThreadPoolExecutor, ProcessPoolExecutor
import asyncio

# CPU-bound → ProcessPoolExecutor
def cpu_task(data):
    return heavy_computation(data)

async def run_cpu_tasks(data_list):
    loop = asyncio.get_event_loop()
    with ProcessPoolExecutor() as pool:
        tasks = [loop.run_in_executor(pool, cpu_task, d) for d in data_list]
    return await asyncio.gather(*tasks)

# I/O-bound → asyncio (preferred) or ThreadPoolExecutor

Memory Optimization

# __slots__ for high-volume objects
class Point:
    __slots__ = ("x", "y")
    def __init__(self, x: float, y: float):
        self.x = x
        self.y = y

# Slots dataclass (3.10+)
@dataclass(slots=True)
class Point:
    x: float
    y: float

# Generators for large datasets
def read_large_file(path: str):
    with open(path) as f:
        for line in f:
            yield process(line)

11.5. CLI & TUI (Rich / Textual)

Rich — Beautiful Terminal Output

from rich.console import Console
from rich.table import Table
from rich.progress import Progress

console = Console()

# Styled printing
console.print("[bold green]Success![/bold green]", "Deployment complete.")

# Tables
table = Table(title="Services")
table.add_column("Name", style="cyan")
table.add_column("Status", style="magenta")
table.add_row("api", "✓ running")
table.add_row("worker", "✓ running")
console.print(table)

# Progress bars
with Progress() as progress:
    task = progress.add_task("[cyan]Processing...", total=100)
    for i in range(100):
        progress.update(task, advance=1)

Textual — Terminal User Interface

from textual.app import App, ComposeResult
from textual.widgets import Header, Footer, Static, Button

class MyApp(App):
    CSS_PATH = "app.tcss"
    BINDINGS = [("q", "quit", "Quit")]

    def compose(self) -> ComposeResult:
        yield Header()
        yield Static("Hello, Textual!")
        yield Button("Click me", id="btn")
        yield Footer()

    def on_button_pressed(self, event: Button.Pressed) -> None:
        self.query_one(Static).update("Button clicked!")

if __name__ == "__main__":
    MyApp().run()

11.6. Observability (OpenTelemetry)

from opentelemetry import trace
from opentelemetry.exporter.otlp.proto.grpc.trace_exporter import OTLPSpanExporter
from opentelemetry.sdk.trace import TracerProvider
from opentelemetry.sdk.trace.export import BatchSpanProcessor
from opentelemetry.instrumentation.fastapi import FastAPIInstrumentor
from opentelemetry.instrumentation.sqlalchemy import SQLAlchemyInstrumentor

# Setup
provider = TracerProvider()
processor = BatchSpanProcessor(OTLPSpanExporter(endpoint="http://otel-collector:4317"))
provider.add_span_processor(processor)
trace.set_tracer_provider(provider)

tracer = trace.get_tracer(__name__)

# Manual spans
with tracer.start_as_current_span("process_order") as span:
    span.set_attribute("order.id", order_id)
    span.set_attribute("order.total", float(total))
    result = await process(order_id)
    span.set_attribute("order.status", result.status)

# Auto-instrumentation
FastAPIInstrumentor.instrument_app(app)
SQLAlchemyInstrumentor().instrument()

Key exporters: Jaeger, Zipkin, OTLP (OpenTelemetry Collector), Prometheus (metrics).

12. Docker for Python

FROM python:3.12-slim AS base

ENV PYTHONDONTWRITEBYTECODE=1 \
    PYTHONUNBUFFERED=1 \
    PIP_NO_CACHE_DIR=1

WORKDIR /app

FROM base AS deps
COPY pyproject.toml .
RUN pip install --no-cache-dir .

FROM deps AS runtime
COPY src/ src/
RUN useradd --create-home appuser
USER appuser
EXPOSE 8000
CMD ["uvicorn", "my_app.main:app", "--host", "0.0.0.0", "--port", "8000"]

.dockerignore:

__pycache__
*.pyc
.venv
.mypy_cache
.ruff_cache
.pytest_cache
.git
tests/
docs/

Production: Use gunicorn with uvicorn workers:

gunicorn my_app.main:app -w 4 -k uvicorn.workers.UvicornWorker

13. CLI Tools

Typer (Recommended)

import typer
from typing import Optional

app = typer.Typer(help="My CLI tool")

@app.command()
def greet(name: str, uppercase: bool = False):
    """Greet someone."""
    msg = f"Hello, {name}!"
    if uppercase:
        msg = msg.upper()
    typer.echo(msg)

@app.command()
def process(
    input_file: typer.FileText,
    output: Optional[typer.FileTextWrite] = None,
    verbose: bool = typer.Option(False, "--verbose", "-v"),
):
    """Process input file."""
    data = input_file.read()
    result = transform(data)
    if output:
        output.write(result)
    else:
        typer.echo(result)

if __name__ == "__main__":
    app()

14. Integration with Other Skills

References

• Python Documentation
• PEP 8 Style Guide
• FastAPI Documentation
• SQLAlchemy 2.0 Documentation
• Pydantic v2 Documentation
• Ruff Documentation
• mypy Documentation
• pytest Documentation
• uv Documentation
• typing module