FastAPI

Last updated
FastAPI
Developer(s) Sebastián Ramírez
Initial releaseDecember 5, 2018;6 years ago (2018-12-05) [1]
Stable release
0.116.1 [2]   OOjs UI icon edit-ltr-progressive.svg / 11 July 2025;2 months ago (11 July 2025)
Repository github.com/tiangolo/fastapi
Written in Python
Type Web framework
License MIT
Website fastapi.tiangolo.com

FastAPI is a high-performance web framework for building HTTP-based service APIs in Python 3.8+. [3] It uses Pydantic and type hints to validate, serialize and deserialize data. FastAPI also automatically generates OpenAPI documentation for APIs built with it. [4] It was first released in 2018.

Contents

Components

Pydantic

Pydantic is a data validation library for Python. While writing code in an IDE, Pydantic provides type hints based on annotations. [5] FastAPI extensively utilizes Pydantic models for data validation, serialization, and automatic API documentation. These models are using standard Python type hints, providing a declarative way to specify the structure and types of data for incoming requests (e.g., HTTP bodies) and outgoing responses. [6] 

fromfastapiimportFastAPIfrompydanticimportBaseModelapp=FastAPI()classItem(BaseModel):name:strprice:floatis_offer:bool|None=None@app.post("/items/")defcreate_item(item:Item):# The 'item' object is already validated and typedreturn{"message":"Item received","item_name":item.name}

Starlette

Starlette is a lightweight ASGI framework/toolkit, to support async functionality in Python. [7]

Uvicorn

Uvicorn is a minimal low-level server/application web server for async frameworks, following the ASGI specification. Technically, it implements a multi-process model with one main process, which is responsible for managing a pool of worker processes and distributing incoming HTTP requests to them. The number of worker processes is pre-configured, but can also be adjusted up or down at runtime. [8]

OpenAPI integration

FastAPI automatically generates OpenAPI documentation for APIs. This documentation includes both Swagger UI and ReDoc, which provide interactive API documentation that you can use to explore and test your endpoints in real time. This is particularly useful for developing, testing, and sharing APIs with other developers or users. Swagger UI is accessible by default at /docs and ReDoc at /redoc route. [9]

Features

Asynchronous operations

FastAPI's architecture inherently supports asynchronous programming. This design allows the single-threaded event loop to handle a large number of concurrent requests efficiently, particularly when dealing with I/O-bound operations like database queries or external API calls. For reference, see async/await pattern.

Dependency injection

FastAPI incorporates a Dependency Injection (DI) system to manage and provide services to HTTP endpoints. This mechanism allows developers to declare components such as database sessions or authentication logic as function parameters. FastAPI automatically resolves these dependencies for each request, injecting the necessary instances. [10] 

fromfastapiimportDepends,HTTPException,statusfromdbimportDbSession# --- Dependency for Database Session ---defget_db():db=DbSession()try:yielddbfinally:db.close()@app.post("/items/",status_code=status.HTTP_201_CREATED)defcreate_item(name:str,description:str,db:DbSession=Depends(get_db)):new_item=Item(name=name,description=description)db.add(new_item)db.commit()db.refresh(new_item)return{"message":"Item created successfully!","item":new_item}@app.get("/items/{item_id}")defread_item(item_id:int,db:DbSession=Depends(get_db)):item=db.query(Item).filter(Item.id==item_id).first()ifitemisNone:raiseHTTPException(status_code=status.HTTP_404_NOT_FOUND,detail="Item not found")returnitem

WebSockets support

WebSockets allow full-duplex communication between a client and the server. This capability is fundamental for applications requiring continuous data exchange, such as instant messaging platforms, live data dashboards, or multiplayer online games. FastAPI leverages the underlying Starlette implementation, allowing for efficient management of connections and message handling. [11] 

# You must have 'websockets' package installedfromfastapiimportWebSocket@app.websocket("/ws")asyncdefwebsocket_endpoint(websocket:WebSocket):awaitwebsocket.accept()whileTrue:data=awaitwebsocket.receive_text()awaitwebsocket.send_text(f"Message text was: {data}")

Background tasks

FastAPI enables the execution of background tasks after an HTTP response has been sent to the client. This allows the API to immediately respond to user requests while simultaneously processing non-critical or time-consuming operations in the background. Typical applications include sending email notifications, updating caches, or performing data post-processing. [12] 

importtimeimportshutilfromfastapiimportBackgroundTasks,UploadFile,Filefromutilsimportgenerate_thumbnail@app.post("/upload-image/")asyncdefupload_image(image:UploadFile=File(...),background_tasks:BackgroundTasks):file_location=f"uploaded_images/{image.filename}"# Save uploaded imagewithopen(image_path,"wb")asf:contents=awaitfile.read()f.write(contents)# Add thumbnail generation as a background tasktasks.add_task(generate_thumbnail,file_location,"200x200")return{"message":f"Image '{image.filename}' uploaded. Thumbnail generation started in background."}

Example

The following code shows a simple web application that displays "Hello, World!" when visited:

# Import FastAPI class from the fastapi packagefromfastapiimportFastAPI# Create an instance of the FastAPI appapp=FastAPI()# Define a GET route for the root URL ("/")@app.get("/")asyncdefread_root()->str:# Return a plain text responsereturn"Hello, World!"

See also

References

  1. ↑ "fastapi repo". GitHub . 2018-12-05.
  2. ↑ "Release 0.116.1". 11 July 2025. Retrieved 18 July 2025.
  3. ↑ "FastAPI". fastapi.tiangolo.com. Retrieved 2024-04-10.
  4. ↑ Lubanovic, Bill (2019-11-06). Introducing Python: Modern Computing in Simple Packages (2nd ed.). O'Reilly Media, Inc. pp. 397, 418. ISBN   9781492051367.
  5. ↑ "Why use Pydantic - Pydantic". docs.pydantic.dev. Retrieved 2023-09-21.
  6. ↑ "Request Body - FastAPI". fastapi.tiangolo.com. Retrieved 2025-06-30.
  7. ↑ "Starlette". www.starlette.io. Retrieved 2023-09-21.
  8. ↑ "Restarting 'uvicorn' Workers with the 'SIGHUP' Signal". bugfactory.io. Retrieved 2024-06-17.
  9. ↑ https://fastapi.tiangolo.com/reference/openapi/docs/
  10. ↑ "Dependencies" . Retrieved 30 June 2025.
  11. ↑ "WebSockets - FastAPI". fastapi.tiangolo.com. Retrieved 2025-06-30.
  12. ↑ "Background Tasks - FastAPI". fastapi.tiangolo.com. Retrieved 2025-06-30.
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