from typing import Optional
from pydantic import BaseModel, validator

class Pen(BaseModel):
    name: str
    color: str
    brand: str
    price: int
    stock: int

    @validator('name')
    def name_must_not_be_empty(cls, value):
        if not value.strip():
            raise ValueError('Name cannot be empty')
        return value

    @validator('color')
    def color_must_not_be_empty(cls, value):
        if not value.strip():
            raise ValueError('Color cannot be empty')
        return value

    @validator('brand')
    def brand_must_not_be_empty(cls, value):
        if not value.strip():
            raise ValueError('Brand cannot be empty')
        return value

    @validator('price')
    def price_must_be_positive(cls, value):
        if value <= 0:
            raise ValueError('Price must be a positive integer')
        return value

    @validator('stock')
    def stock_must_be_non_negative(cls, value):
        if value < 0:
            raise ValueError('Stock cannot be negative')
        return value

def create_pen(db, pen_data: Pen):
    pen = Pen(**pen_data.dict())
    db.add(pen)
    db.commit()
    db.refresh(pen)
    return pen

def get_pen(db, pen_id: int) -> Optional[Pen]:
    return db.query(Pen).filter(Pen.id == pen_id).first()

def update_pen(db, pen_id: int, pen_data: Pen):
    pen = get_pen(db, pen_id)
    if not pen:
        return None
    for field, value in pen_data.dict(exclude_unset=True).items():
        setattr(pen, field, value)
    db.commit()
    db.refresh(pen)
    return pen

def delete_pen(db, pen_id: int):
    pen = get_pen(db, pen_id)
    if not pen:
        return None
    db.delete(pen)
    db.commit()
    return pen