Python Expert Skill Skill

Python Expert Skill

Python backend patterns: Django, FastAPI, Flask, async, SQLAlchemy.

1. FastAPI Patterns

Pydantic models: BaseModel with Field, EmailStr, field_validator. Config: str_strip_whitespace.

DI: Depends() for shared logic. Use Annotated type aliases:

CurrentUser = Annotated[User, Depends(get_current_user)]
DB = Annotated[AsyncSession, Depends(get_db)]

Background tasks: BackgroundTasks.add_task(fn, args) for non-blocking ops.

Exception handling: Custom AppException + @app.exception_handler(AppException).

Lifespan: @asynccontextmanager with yield for startup/shutdown.

2. Django Patterns

N+1: select_related('fk') for FK, prefetch_related('m2m') for M2M
Atomic updates: F('views') + 1 (not read-modify-write)
Complex queries: Q(a=True) | Q(b=True)
Existence: .exists() not len(qs) > 0
Bulk: bulk_create([...]) not create in loop
Transactions: @transaction.atomic

3. SQLAlchemy 2.0

Declarative: Mapped[type] + mapped_column() + relationship().

Async: select(User).where(User.id == id) + await db.execute(stmt).

Eager loading: selectinload (separate query) / joinedload (JOIN).

Pagination: func.count() for total + offset/limit for page.

4. Async Best Practices

asyncio.gather() for parallel ops
asyncio.timeout() for timeouts
asyncio.Semaphore(N) for rate limiting
asyncio.TaskGroup() (3.11+) for structured concurrency

5. Type Hints (3.10+)

def get_users(active: bool | None = None) -> list[User]: ...

class Repository(Protocol):
    async def get(self, id: int) -> Model | None: ...

Use TypeVar for generics, Protocol for structural typing.

6. Testing

Fixtures: @pytest.fixture with async client and session rollback
Parametrize: @pytest.mark.parametrize("input,expected", [...])
Factories: factory.Factory with Faker fields
Mocking: AsyncMock + patch()

7. Error Handling

Custom exception hierarchy: AppError base with code, NotFoundError, ValidationError. Result pattern: Ok[T] | Err[E] with match statement.

Quick Reference

checklist[12]{pattern,best_practice}:
  FastAPI,Pydantic models + Depends injection
  Django ORM,select_related/prefetch_related
  SQLAlchemy,Mapped types + selectinload
  Async,asyncio.gather for parallel
  Typing,Modern 3.10+ syntax X | None
  Testing,pytest fixtures + parametrize
  Validation,Pydantic or Django Forms
  Errors,Custom exception hierarchy
  Transactions,atomic or db.transaction
  Queries,F() objects for atomic updates
  Bulk,bulk_create over loops
  Cache,Redis or Django cache

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