FastAPI

Warnings

• breaking Pydantic v1 support fully removed in 0.128.0. from pydantic.v1 import BaseModel no longer works with FastAPI — raises FastAPIError at route decoration time. Minimum Pydantic is now >=2.7.0.
  Fix: Migrate all models to pydantic v2. Replace .dict() with .model_dump(), .parse_obj() with .model_validate(), class Config with model_config = ConfigDict(...).
• breaking Python 3.8 and 3.9 support dropped. fastapi>=0.125.0 requires Python >=3.10.
  Fix: Pin fastapi<=0.124.4 for Python 3.8/3.9 environments, or upgrade Python.
• breaking Strict Content-Type checking added. FastAPI now rejects JSON requests missing a valid Content-Type: application/json header by default.
  Fix: Ensure clients send Content-Type: application/json. To disable: app = FastAPI(strict_content_type=False)
• breaking bare pip install fastapi does NOT include uvicorn or python-multipart. LLM-generated setups frequently omit these, causing ImportError on first run or silent failure on form/file endpoints.
  Fix: Use pip install 'fastapi[standard]' for development. For production containers install uvicorn[standard] and python-multipart explicitly.
• deprecated @app.on_event('startup') and @app.on_event('shutdown') deprecated since 0.93. Still functional but emits FastAPIDeprecationWarning. Will be removed in a future version.
  Fix: Use the lifespan context manager: app = FastAPI(lifespan=lifespan) with @asynccontextmanager.
• gotcha python-multipart is required for Form() and File() parameters but is not installed by bare fastapi. The error message when it's missing is: 'Form data requires python-multipart to be installed'.
  Fix: pip install python-multipart or use fastapi[standard].
• gotcha response_model_exclude_unset, response_model_exclude_none etc. are route decorator params, not model config. LLMs frequently place them in the Pydantic model instead.
  Fix: @app.get('/items', response_model_exclude_unset=True) — on the decorator, not the model.

Install

• pip install "fastapi[standard]" Recommended (includes uvicorn, python-multipart, httpx)
• pip install fastapi Core only (no server, no form support)
• pip install "fastapi[all]" All optional dependencies

Imports

• FastAPI

  from fastapi import FastAPI
  from pydantic import BaseModel

  All Pydantic models must use pydantic v2. from pydantic.v1 import BaseModel raises FastAPIError at route registration time.
• lifespan

  from contextlib import asynccontextmanager
  from fastapi import FastAPI
  
  @asynccontextmanager
  async def lifespan(app: FastAPI):
      # startup
      yield
      # shutdown
  
  app = FastAPI(lifespan=lifespan)

  @app.on_event() is deprecated since 0.93. Use the lifespan context manager parameter instead. Still works but emits FastAPIDeprecationWarning.

Quickstart

Minimal FastAPI app with lifespan handler and Pydantic v2 model.

from contextlib import asynccontextmanager
from fastapi import FastAPI
from pydantic import BaseModel

@asynccontextmanager
async def lifespan(app: FastAPI):
    # startup logic here
    yield
    # shutdown logic here

app = FastAPI(lifespan=lifespan)

class Item(BaseModel):
    name: str
    price: float

@app.get('/')
def read_root():
    return {'status': 'ok'}

@app.post('/items')
def create_item(item: Item) -> Item:
    return item

# Run: fastapi dev main.py  (dev)
# Run: fastapi run main.py  (prod)