Pydantic v2 Validators: Zero-Copy Paths for High-QPS API Boundaries

# ✅ FastAPI edge: validate JSON bytes once, dump once
from pydantic import BaseModel, TypeAdapter, Field, ConfigDict

class CreateOrder(BaseModel):
 model_config = ConfigDict(extra="forbid", str_strip_whitespace=True)
 sku: str = Field(min_length=1, max_length=64)
 qty: int = Field(ge=1, le=10_000)
 currency: str = Field(pattern=r"^[A-Z]{3}$")

adapter = TypeAdapter(CreateOrder)

def parse_order(raw: bytes) -> CreateOrder:
 # ✅ validate_json keeps work in pydantic-core; no intermediate dict
 return adapter.validate_json(raw)

def to_wire(order: CreateOrder) -> bytes:
 # ✅ serialize from model without Python-side field looping
 return adapter.dump_json(order, by_alias=True)

# ❌ Classic tax: decode → dict → validate → dump → encode
import json
from pydantic import TypeAdapter
# body = CreateOrder.model_validate(json.loads(raw)) # Python dict alloc
# return json.dumps(body.model_dump()).encode() # second copy storm

from pydantic import field_validator, model_validator

class Money(BaseModel):
 amount_cents: int = Field(ge=0)
 currency: str

 @field_validator("currency")
 @classmethod
 def upper_currency(cls, v: str) -> str:
 # ✅ cheap, pure, no I/O
 if len(v) != 3:
 raise ValueError("currency must be ISO-4217 length 3")
 return v.upper()

 @model_validator(mode="after")
 def reject_zero_usd_sentinel(self) -> "Money":
 # ✅ rare business rule — keep off the millions-QPS path if possible
 if self.currency == "USD" and self.amount_cents == 0:
 raise ValueError("zero USD amounts require explicit free-sku path")
 return self

from fastapi import FastAPI, Request, Response

app = FastAPI()
order_adapter = TypeAdapter(CreateOrder)

@app.post("/orders")
async def create_order(request: Request) -> Response:
 raw = await request.body()
 order = order_adapter.validate_json(raw) # ✅
 # persist...
 return Response(
 content=order_adapter.dump_json(order),
 media_type="application/json",
 )