Your agent guesses.
Ours has receipts.

// Use cases for branded types:
//
// Database IDs — never pass OrderId where UserId expected
// Financial types — USD vs EUR at the type level
// Validated strings — Email, URL, PhoneNumber
// Access tokens — AuthToken vs RefreshToken
//
// The __brand property is a phantom type:
// it only exists at compile time, zero runtime cost.
//
// Combine with factory functions for validation:

function createEmail(raw: string): Email {
  if (!raw.includes('@'))
    throw new Error('Invalid email');
  return raw as Email;
}

// Advanced: discriminated unions + branded types
type Result<T> =
  | { success: true; data: T }
  | { success: false; error: string };

// Template literal types for API routes
type ApiRoute = `/api/${string}`;
function fetchApi(route: ApiRoute) { /* ... */ }
fetchApi("/api/users");    // OK
fetchApi("/not/api/path"); // Error

# Pydantic v2 Migration Cheatsheet
#
# Method renames:
#   parse_obj()  -> model_validate()
#   dict()       -> model_dump()
#   json()       -> model_dump_json()
#
# Field constraints:
#   conint, constr (removed)
#   -> Annotated[int, Field(ge=0, le=100)]
#
# Config class -> ConfigDict:
#   class Config:           -> model_config = ConfigDict(
#       orm_mode = True     ->     from_attributes=True,
#                           -> )
#
# Validators:
#   @validator("field")     -> @field_validator("field")
#                              @classmethod
#
# Performance: 4-17x faster (Rust core)
#
# Async endpoint pattern:
from fastapi import FastAPI, Depends
from contextlib import asynccontextmanager

@asynccontextmanager
async def lifespan(app: FastAPI):
    # Startup
    await init_db()
    yield
    # Shutdown
    await close_db()

app = FastAPI(lifespan=lifespan)

@app.get("/users/{user_id}")
async def get_user(
    user_id: int,
    db: AsyncSession = Depends(get_db)
):
    return await db.get(User, user_id)

# Key principles:
#
# 1. Multi-stage: build tools stay in builder stage
#    Runtime image has NO gcc, make, etc.
#
# 2. Non-root user: appuser with minimal perms
#    Never run containers as root in production
#
# 3. Layer ordering (cache optimization):
#    base -> system pkgs -> deps -> code
#    Code changes don't rebuild dependencies
#
# 4. Health check: kubelet/ECS/Compose
#    can restart unhealthy containers
#
# 5. No cache in pip: --no-cache-dir
#    Smaller image, no stale packages
#
# uv-based variant (faster installs):
# FROM python:3.12-slim-bookworm AS builder
# COPY --from=ghcr.io/astral-sh/uv:latest \
#     /uv /usr/local/bin/uv
# RUN uv pip install --system -r requirements.txt
#
# Graceful shutdown:
# import signal, sys
# def handler(sig, frame):
#     cleanup()
#     sys.exit(0)
# signal.signal(signal.SIGTERM, handler)

// React 19 Key Changes:
//
// NEW HOOKS:
//   useActionState — form state + pending
//   useFormStatus  — track submission state
//   useOptimistic  — instant UI feedback
//   use()          — unwrap Promise/Context
//
// SERVER COMPONENTS (RSC):
//   - Direct database access (no API layer!)
//   - 40-60% JavaScript reduction
//   - Secure API keys (server-only)
//   - Automatic code splitting
//
// Server Component + Client Island:
async function ProductPage({ id }) {
  const product = await db.query(
    `SELECT * FROM products WHERE id = $1`, [id]
  );
  return (
    <main>
      <h1>{product.name}</h1>
      <AddToCart id={id} /> {/* Client */}
    </main>
  );
}

// BREAKING CHANGES:
//   PropTypes removed -> TypeScript
//   defaultProps removed -> param defaults
//   String refs removed -> useRef
//   Hydration errors (not warnings)