Pira cara nulis validasi khusus ing Pydantic?

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Accepted Answer

Suwene watesan sing wis ana, Pydantic ngbolehake sampeyan nulis **logika validasi khusus** karo field validator (validasi siji field) lan model validator (validasi ing macem-macem field) — kanggo aturan sing watesan prasaja ora bisa nduweni. (Sintaks sing ditampilake yaiku Pydantic v2.)

## Field validator — validasi/ngowahi siji field

```python
from pydantic import BaseModel, field_validator

class User(BaseModel):
    username: str
    password: str

@field_validator("username")
    @classmethod
    def username_alphanumeric(cls, v: str) -> str:
        if not v.isalnum():
            raise ValueError("username must be alphanumeric")   # → a 422 error
        return v.lower()                                         # can also TRANSFORM the value
```

A `@field_validator` njalanake logika khusus ing field: angkat `ValueError` kanggo nolak input sing ora valid (FastAPI ngowah dadi 422 sing cetha), utawa bali nilai sing ngowah (conto: normalisasi dadi lowercase).

## Model validator — validasi lintas-field

```python
from pydantic import model_validator

class SignupForm(BaseModel):
    password: str
    confirm_password: str

@model_validator(mode="after")
    def passwords_match(self):
        if self.password != self.confirm_password:    # compare MULTIPLE fields
            raise ValueError("passwords do not match")
        return self
```

A `@model_validator(mode="after")` njalanake sawisé kabèh field divalidasi, kanthi akses menyang model kabèh — dianggo kanggo aturan sing melibatake **macem-macem field** (conto: ngonfirmasi password cocok, utawa tanggal wiwit sadurunge tanggal akhir).

## Validasi sing wis ana asring cukup pisanan

```python
class Product(BaseModel):
    price: float = Field(gt=0)          # use Field constraints for simple rules
    sku: str = Field(pattern=r"^[A-Z]{3}-\d+$")
    email: EmailStr                      # specialized types for formats
# reach for custom validators only when constraints/types can't express the rule
```

Lébih sukani `Field` watesan lan jinis khusus kanggo aturan prasaja; gunaake validator khusus kanggo logika bisnis sejati.

## Kapa iki penting

Validasi Pydantic khusus penting kanggo ngukuhake aturan bisnis sing ngluwihi pemeriksaan tipe lan watesan prasaja — lan amarga validasi penting kanggo **keamanan lan integritas data**, bisa ngandika aturan apa-apa yaiku berharga. **Field validator** nangani logika field-single (pemeriksaan format khusus, normalisasi/transformasi), dene **model validator** nangani kabutuhan umum kanggo **validasi lintas-field** (password cocok, rentang tanggal, persyaratan bersyarat) sing aturan level-field ora bisa nduweni.

Validator ngandhek kanthi mulus karo FastAPI — `ValueError` sing diunggah dadi 422 respon cetha kanthi otomatis.

Paham nalika nganggo watesan/jinis sing wis ana (kanggo aturan prasaja) versus validator khusus (kanggo logika bisnis nyata), lan kepiye field lawan validator model beda, ngbolehake sampeyan njamin data masuk ngrampungi persyaratan aplikasi sampeyan, gawé iki pengetahuan penting kanggo mbangun API sing kuwat lan bener.