fusion 0.8.2

Fusion is a modern ASGI web framework for Python with built-in dependency injection, OpenAPI schema generation, and MCP support.

Fusion

A modern, async-first ASGI web framework for Python with type-safe dependency injection.

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[!WARNING] This project is under active development and is not production-ready. APIs may change without notice between versions.

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Overview

Fusion is a lightweight ASGI web framework built on two pillars:

• msgspec — high-performance JSON serialization and validation
• Type-hint-driven DI — declare dependencies as annotated fields; Fusion resolves them automatically at request time

It is designed for Python 3.12+ and is async-first throughout.

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Installation

# From PyPI (once published)
pip install fusion

# From source
git clone https://github.com/okanakbulut/fusion.git
cd fusion
pip install -e .

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Quick Start

# app.py
from fusion import Fusion, Get, Handler, Object, Request, Response


class Greeting(Object):
    message: str


class HelloHandler(Handler):
    async def handle(self, request: Request) -> Response[Greeting]:
        return Response(Greeting(message="Hello, World!"))


app = Fusion(routes=[Get("/hello", handler=HelloHandler)])

Run it:

pip install uvicorn
uvicorn app:app

GET /hello  →  {"message": "Hello, World!"}

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Core Concepts

Objects (Data Models)

Object is a msgspec.Struct-backed base class for all serializable data. Define fields with standard type annotations:

from fusion import Object

class User(Object):
    id: int
    name: str
    email: str

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Handlers

Subclass Handler and implement async def handle(self, request: SomeRequest) -> SomeResponse.

• The request parameter must be type-annotated with Request or a subclass of it.
• The return type determines how the response is serialized.

from fusion import Handler, Object, Request, Response

class Output(Object):
    message: str

class MyHandler(Handler):
    async def handle(self, request: Request) -> Response[Output]:
        return Response(Output(message="ok"))

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Routing

Use Route for explicit method lists, or the shorthand helpers Get, Post, Put, Delete, Patch:

from fusion import Fusion, Get, Post, Route

app = Fusion(routes=[
    Get("/items", handler=ListItemsHandler),
    Post("/items", handler=CreateItemHandler),
    Route("/items/{id:int}", methods=["GET", "DELETE"], handler=ItemHandler),
])

Path parameter syntax:

Pattern	Matches
{name}	any string segment
{id:int}	integer
{id:uuid}	UUID

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Request Parameters

Request-scoped parameters (QueryParam, PathParam, Header, Cookie, RequestBody) must be declared on a Request subclass — not directly on a Handler. The handler's handle method receives that subclass as its request argument.

Query Parameters

from fusion import Fusion, Get, Handler, Object, QueryParam, Request, Response

class SearchRequest(Request):
    query: QueryParam[str]
    page: QueryParam[int]
    tags: QueryParam[list[str]]   # ?tags:list=a,b,c

class SearchResult(Object):
    query: str
    page: int

class SearchHandler(Handler):
    async def handle(self, request: SearchRequest) -> Response[SearchResult]:
        return Response(SearchResult(query=request.query, page=request.page))

Headers

from fusion import Handler, Header, Object, Request, Response

class AuthRequest(Request):
    authorization: Header[str]
    user_id: Header[int]          # header value is coerced to the declared type

class AuthHandler(Handler):
    async def handle(self, request: AuthRequest) -> Response[Object]:
        token = request.authorization  # "Bearer ..."
        ...

Request Body

from fusion import Body, Handler, Object, Request, Response

class User(Object):
    name: str
    email: str

class CreateUserRequest(Request):
    body: Body[User]

class CreateUserHandler(Handler):
    async def handle(self, request: CreateUserRequest) -> Response[User]:
        return Response(request.body)

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Dependency Injection

Use @factory to register a factory function for any type (e.g. a database connection). Declare the type as a field on an Injectable; Fusion calls the factory automatically.

from fusion import Fusion, Get, Handler, Injectable, Object, Request, Response, factory

class Database:
    def __init__(self, dsn: str) -> None:
        self.dsn = dsn

@factory
async def database_factory() -> Database:
    return Database("postgresql://localhost/mydb")

class StatusHandler(Handler):
    db: Database          # resolved from the factory above

    async def handle(self, request: Request) -> Response[Object]:
        # self.db is a Database instance
        ...

Lifecycle Management (async context managers)

Wrap a factory with @asynccontextmanager for per-request setup and teardown:

from collections.abc import AsyncIterator
from contextlib import asynccontextmanager
from fusion import factory

class Session:
    pass

@factory
@asynccontextmanager
async def session_factory() -> AsyncIterator[Session]:
    session = Session()
    try:
        yield session
    finally:
        await session.close()      # runs after the handler returns

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Middleware

Subclass BaseMiddleware, implement async def handle(self, request), and attach it to a route:

from fusion import Fusion, Get, Handler, Middleware, Object, Request, Response, Unauthorized
from fusion.middleware import BaseMiddleware

class AuthMiddleware(BaseMiddleware):
    async def handle(self, request: Request) -> Unauthorized | Response:
        token = request.headers.get("authorization", "")
        if not token.startswith("Bearer "):
            return Unauthorized(detail="Missing or invalid token")
        return await self.app.handle(request)

class ProtectedHandler(Handler):
    async def handle(self, request: Request) -> Response[Object]:
        ...

app = Fusion(routes=[
    Get("/protected", handler=ProtectedHandler, middlewares=[Middleware(AuthMiddleware)])
])

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Responses & Problem Details

Fusion follows RFC 9457 for error responses. All error types serialize to application/problem+json.

Class	Status	Use case
Response[T]	200	Success with body
Created[T]	201	Resource created
NoContent	204	Success, no body
BadRequest	400	Invalid input
Unauthorized	401	Authentication required
Forbidden	403	Permission denied
NotFound	404	Resource not found
MethodNotAllowed	405	Wrong HTTP method
InternalServerError	500	Unhandled error
ValidationError	400	Field-level validation errors

Returning errors from a handler

from fusion.responses import BadRequest, NotFound

class ItemHandler(Handler):
    async def handle(self, request: Request) -> NotFound | Response[Item]:
        item = db.get(item_id)
        if item is None:
            return NotFound(detail="Item not found")
        return Response(item)

Field-level validation errors

from fusion.responses import FieldError, ValidationError

return ValidationError(
    detail="Validation failed",
    errors=[
        FieldError(field="email", message="invalid format"),
        FieldError(field="name", message="required"),
    ],
)

Custom problem types

import typing
from fusion.responses import Problem

class OutOfStockProblem(Problem):
    type: typing.ClassVar[str] = "https://example.com/problems/out-of-stock"
    status: typing.ClassVar[int] = 409
    title: str = "Out of Stock"

# In a handler:
return OutOfStockProblem(detail="Item #42 is out of stock")

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Requirements

• Python 3.12+
• msgspec >= 0.21.1
• typedprotocol >= 0.1.0

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License

MIT — see LICENSE.md.