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Migrating from FastAPI

This guide helps you migrate an existing FastAPI application to HawkAPI. Most concepts map directly — the biggest changes are msgspec instead of Pydantic and a proper DI container instead of ad-hoc Depends().

Automatic Migration

Run:

hawkapi migrate ./my_fastapi_app/

This rewrites imports, the app constructor (FastAPI → HawkAPI), APIRouter → Router, and lifespan hooks (@app.on_event("startup") → @app.on_startup). Pass --convert-models to also rewrite Pydantic BaseModel → msgspec.Struct (classes that use @validator / @field_validator are skipped with a warning so you can convert them by hand).

Other flags:

  • --output ./my_hawkapi_app/ — write the migrated tree elsewhere instead of overwriting the source in place.
  • --dry-run — print unified diffs without modifying any file.

The codemod is idempotent — running it twice is a no-op.

API Mapping Table

FastAPI HawkAPI Notes
FastAPI() HawkAPI() Same constructor pattern
APIRouter() Router() Same include_router() API
Depends(func) Depends(func) Same pattern, plus DI container
HTTPException HTTPException Same status_code + detail
BaseModel msgspec.Struct Faster serialization
Query() Query() Same Annotated[] pattern
Path() Path() Same Annotated[] pattern
Header() Header() Same Annotated[] pattern
Body() Body() Same Annotated[] pattern
Cookie() Cookie() Same Annotated[] pattern
BackgroundTasks BackgroundTasks Same API
response_model=X response_model=X Same
status_code=201 status_code=201 Same
@app.middleware("http") class M(Middleware) Class-based
Request Request Same interface
Response Response Same interface
JSONResponse JSONResponse Same interface
TestClient TestClient Same interface (httpx-based)

Step 1: Replace Imports

Before (FastAPI):

from fastapi import FastAPI, APIRouter, Depends, HTTPException, Request
from fastapi import Query, Path, Header, Body, Cookie
from fastapi.responses import JSONResponse, HTMLResponse
from fastapi.testclient import TestClient
from pydantic import BaseModel

After (HawkAPI):

from hawkapi import HawkAPI, Router, Depends, HTTPException, Request
from hawkapi import Query, Path, Header, Body, Cookie
from hawkapi import JSONResponse, HTMLResponse
from hawkapi import TestClient
import msgspec

Step 2: Replace Pydantic Models with msgspec Structs

Before:

from pydantic import BaseModel, Field

class User(BaseModel):
    name: str
    email: str
    age: int = Field(ge=0, le=150)

After:

from typing import Annotated
import msgspec

class User(msgspec.Struct):
    name: str
    email: str
    age: Annotated[int, msgspec.Meta(ge=0, le=150)]

Key differences:

  • msgspec.Struct replaces BaseModel
  • msgspec.Meta() inside Annotated[] replaces Field() for validation
  • No .model_dump() needed — HawkAPI serializes Structs directly
  • Structs are faster to create and serialize than Pydantic models

Step 3: Replace app = FastAPI() with app = HawkAPI()

Before:

app = FastAPI(title="My API", version="1.0.0")

After:

app = HawkAPI(title="My API", version="1.0.0")

Route decorators work identically:

@app.get("/users/{user_id}")
async def get_user(user_id: int):
    return {"id": user_id}

@app.post("/users", status_code=201)
async def create_user(body: User):
    return body

Step 4: Replace APIRouter with Router

Before:

from fastapi import APIRouter

router = APIRouter(prefix="/api/v1", tags=["users"])

@router.get("/users")
async def list_users():
    ...

app.include_router(router)

After:

from hawkapi import Router

router = Router(prefix="/api/v1", tags=["users"])

@router.get("/users")
async def list_users():
    ...

app.include_router(router)

Step 5: Migrate Middleware

Before (FastAPI):

@app.middleware("http")
async def add_timing(request, call_next):
    start = time.time()
    response = await call_next(request)
    response.headers["X-Process-Time"] = str(time.time() - start)
    return response

After (HawkAPI):

from hawkapi import Middleware, Response

class TimingMiddleware(Middleware):
    async def before_request(self, request):
        request.state.start_time = time.time()
        return None  # continue processing

    async def after_response(self, request, response):
        duration = time.time() - request.state.start_time
        response.headers["X-Process-Time"] = str(duration)
        return response

app.add_middleware(TimingMiddleware)

Or use raw ASGI mode for maximum performance:

class TimingMiddleware(Middleware):
    async def __call__(self, scope, receive, send):
        start = time.time()
        await self.app(scope, receive, send)
        # Note: headers must be set before response starts

Step 6: Migrate Dependency Injection

FastAPI's Depends() works the same way in HawkAPI:

from hawkapi import Depends

async def get_db():
    db = await connect()
    try:
        yield db
    finally:
        await db.close()

@app.get("/users")
async def list_users(db = Depends(get_db)):
    return await db.fetch_all("SELECT * FROM users")

HawkAPI also offers a DI container for larger applications:

from hawkapi import HawkAPI, Container

container = Container()
container.register(Database, factory=create_database, lifecycle="singleton")
container.register(UserService, lifecycle="scoped")

app = HawkAPI(container=container)

@app.get("/users")
async def list_users(service: UserService):
    # UserService auto-injected from container — no Depends() needed
    return await service.list_all()

Step 7: Migrate Exception Handlers

Before:

@app.exception_handler(ValueError)
async def value_error_handler(request, exc):
    return JSONResponse(status_code=400, content={"detail": str(exc)})

After (same API):

@app.exception_handler(ValueError)
async def value_error_handler(request, exc):
    return JSONResponse(content={"detail": str(exc)}, status_code=400)

What's Different

Serialization

HawkAPI uses msgspec instead of Pydantic. msgspec is 5-20x faster for JSON encoding/decoding. If you need Pydantic support, install hawkapi[pydantic] — both are supported.

DI Container

HawkAPI includes a built-in DI container with proper lifecycle management (singleton, scoped, transient). This replaces ad-hoc Depends() chains for service injection.

Middleware

HawkAPI uses class-based middleware with before_request/after_response hooks instead of the call_next pattern. This avoids background task issues and provides better control flow.

Performance

HawkAPI pre-computes parameter resolution at route registration time. No inspect.signature() or get_type_hints() per request. Route matching uses a radix tree instead of linear search.

What's the Same

  • Route decorators (@app.get, @app.post, etc.)
  • Path parameters with type conversion ({user_id:int})
  • Annotated[] parameter markers (Query, Path, Header, Body, Cookie)
  • Depends() for function-based dependencies
  • BackgroundTasks for post-response work
  • HTTPException for error responses
  • TestClient for testing (httpx-based)
  • OpenAPI/Swagger UI auto-generation
  • CORS, GZip, and other built-in middleware