Python Expert

Python Expert Skill

This skill provides deep Python expertise including FastAPI/Django patterns, async handling, Pydantic v2 validation, SQLAlchemy 2.0, and security best practices.

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Pydantic v2 (IMPORTANT!)

Always Use v2 Syntax

from pydantic import BaseModel, Field, ConfigDict, EmailStr, field_validator

# ✅ Pydantic v2 syntax
class UserCreate(BaseModel):
    email: EmailStr
    name: str = Field(min_length=2, max_length=100)
    age: int | None = Field(default=None, ge=0, le=150)

class UserResponse(BaseModel):
    id: int
    email: str
    name: str

    model_config = ConfigDict(from_attributes=True)

# ❌ Pydantic v1 syntax (DON'T USE!)
class UserOld(BaseModel):
    class Config:           # Wrong! Use model_config
        orm_mode = True     # Wrong! Use from_attributes

Validation

from pydantic import field_validator, model_validator

class OrderCreate(BaseModel):
    items: list[OrderItem]
    discount_code: str | None = None

    @field_validator('items')
    @classmethod
    def validate_items(cls, v: list[OrderItem]) -> list[OrderItem]:
        if not v:
            raise ValueError('Order must have at least one item')
        return v

    @model_validator(mode='after')
    def validate_order(self) -> 'OrderCreate':
        if self.discount_code and len(self.items) < 3:
            raise ValueError('Discount requires at least 3 items')
        return self

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Async Patterns

Always await I/O Operations

# ✅ Correct - async for I/O
async def get_user(db: AsyncSession, user_id: int) -> User | None:
    result = await db.execute(select(User).where(User.id == user_id))
    return result.scalar_one_or_none()

# ✅ Parallel operations
async def get_dashboard_data(db: AsyncSession, user_id: int):
    user, posts, notifications = await asyncio.gather(
        get_user(db, user_id),
        get_user_posts(db, user_id),
        get_notifications(db, user_id),
    )
    return {"user": user, "posts": posts, "notifications": notifications}

# ❌ Sequential when could be parallel
async def get_dashboard_data_slow(db: AsyncSession, user_id: int):
    user = await get_user(db, user_id)
    posts = await get_user_posts(db, user_id)  # Doesn't depend on user
    notifications = await get_notifications(db, user_id)
    return {"user": user, "posts": posts, "notifications": notifications}