Nest-Py: Experimenting with Decorators, DI, and Controllers in Python (FastAPI + NestJS style)

Nest-Py: A Tiny NestJS-Like Framework in Python Powered by FastAPI

I’ve been experimenting with metaprogramming in Python and decided to build a tiny prototype called Nest-Py.

It’s a small framework inspired by NestJS but built on top of FastAPI.

The goal?

To explore how we can bring concepts like controllers, decorators, and dependency injection into Python — similar to how NestJS structures applications in the Node.js ecosystem.

⚡ Disclaimer: This is just an experiment and learning project, not production-ready. But it shows how Python’s flexibility lets us play with framework design.

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Why Nest-Py?

FastAPI is already amazing, but it leaves architecture decisions up to you.

NestJS, on the other hand, enforces a modular structure (controllers, services, modules, DI).

Nest-Py tries to combine both worlds:

• Keep the speed and type hints of FastAPI.
• Add structure with decorators, controllers, and a minimal DI context.

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Example: Users and Employees

Here’s a small demo with two services (UserService, EmployeeService) and two controllers (UserController, EmployeeController).

from nest_py import get, controller, ctx

# --- Mock Data ---
users = {
    1: {"username": "alice", "email": "[e-mail]"},
    2: {"username": "bob", "email": "[e-mail]"},
    3: {"username": "charlie", "email": "[e-mail]"},
}

employees = {
    101: {"name": "John Doe", "role": "Manager", "salary": 55000},
    102: {"name": "Jane Smith", "role": "Developer", "salary": 48000},
    103: {"name": "Mark Lee", "role": "Designer", "salary": 45000},
}

# --- Services ---
class UserService:
    def get_users(self):
        return users

    def get_user(self, id: int):
        return users.get(id, None)


class EmployeeService:
    def get_employees(self):
        return employees

    def get_employee(self, id: int):
        return employees.get(id, None)


# --- Controllers ---
@controller("/users")
class UserController:
    service: UserService = None

    @get("/")
    async def get_users():
        return ctx.service.get_users()

    @get("/{id}")
    async def get_user(id: int):
        return ctx.service.get_user(id)


@controller("/employees")
class EmployeeController:
    service: EmployeeService = None

    @get("/")
    async def get_employees():
        return ctx.service.get_employees()

    @get("/{id}")
    async def get_employee(id: int):
        return ctx.service.get_employee(id)


from nest_py.core import factory  # minimal bootstrap