Documentation: https://fastapi.tiangolo.com

Source Code: https://github.com/fastapi/fastapi

FastAPI is a modern, fast (high-performance), web framework for building APIs with Python based on standard Python type hints.

The key features are:

Fast: Very high performance, on par with NodeJS and Go (thanks to Starlette and Pydantic). One of the fastest Python frameworks available.
Fast to code: Increase the speed to develop features by about 200% to 300%. *
Fewer bugs: Reduce about 40% of human (developer) induced errors. *
Intuitive: Great editor support. Completion everywhere. Less time debugging.
Easy: Designed to be easy to use and learn. Less time reading docs.
Short: Minimize code duplication. Multiple features from each parameter declaration. Fewer bugs.
Robust: Get production-ready code. With automatic interactive documentation.
Standards-based: Based on (and fully compatible with) the open standards for APIs: OpenAPI (previously known as Swagger) and JSON Schema.

* estimation based on tests conducted by an internal development team, building production applications.

Opinions

"[...] I'm using FastAPI a ton these days. [...] I'm actually planning to use it for all of my team's ML services at Microsoft. Some of them are getting integrated into the core Windows product and some Office products."

"We adopted the FastAPI library to spawn a REST server that can be queried to obtain predictions. [for Ludwig]"

"Netflix is pleased to announce the open-source release of our crisis management orchestration framework: Dispatch! [built with FastAPI]"

"If anyone is looking to build a production Python API, I would highly recommend FastAPI. It is beautifully designed, simple to use and highly scalable, it has become a key component in our API first development strategy and is driving many automations and services such as our Virtual TAC Engineer."

FastAPI Conf

FastAPI Conf '26 is happening on October 28, 2026 in Amsterdam, NL. All about FastAPI, right from the source. 🎤

FastAPI mini documentary

There's a FastAPI mini documentary released at the end of 2025, you can watch it online:

Typer, the FastAPI of CLIs

If you are building a CLI app to be used in the terminal instead of a web API, check out Typer.

Typer is FastAPI's little sibling. And it's intended to be the FastAPI of CLIs. ⌨️ 🚀

Requirements

FastAPI stands on the shoulders of giants:

Starlette for the web parts.
Pydantic for the data parts.

Installation

First, install uv, and then add FastAPI to your project:

$ uv add "fastapi[standard]"

---> 100%

Note: Make sure you put "fastapi[standard]" in quotes to ensure it works in all terminals.

If you prefer to use pip, install fastapi[standard] inside a virtual environment. See the installation guide for the alternative steps.

Example

Create it

Create a file main.py with:

from fastapi import FastAPI

app = FastAPI()


@app.get("/")
def read_root():
    return {"Hello": "World"}


@app.get("/items/{item_id}")
def read_item(item_id: int, q: str | None = None):
    return {"item_id": item_id, "q": q}

If your code uses async / await, use async def:

from fastapi import FastAPI

app = FastAPI()


@app.get("/")
async def read_root():
    return {"Hello": "World"}


@app.get("/items/{item_id}")
async def read_item(item_id: int, q: str | None = None):
    return {"item_id": item_id, "q": q}

Note:

If you don't know, check the "In a hurry?" section about async and await in the docs.

Run it

Run the server with:

$ uv run fastapi dev

 ╭────────── FastAPI CLI - Development mode ───────────╮
 │                                                     │
 │  Serving at: http://127.0.0.1:8000                  │
 │                                                     │
 │  API docs: http://127.0.0.1:8000/docs               │
 │                                                     │
 │  Running in development mode, for production use:   │
 │                                                     │
 │  fastapi run                                        │
 │                                                     │
 ╰─────────────────────────────────────────────────────╯

INFO:     Will watch for changes in these directories: ['/home/user/code/awesomeapp']
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     Started reloader process [2248755] using WatchFiles
INFO:     Started server process [2248757]
INFO:     Waiting for application startup.
INFO:     Application startup complete.

The command fastapi dev reads your main.py file automatically, detects the FastAPI app in it, and starts a server using Uvicorn.

By default, fastapi dev will start with auto-reload enabled for local development.

You can read more about it in the FastAPI CLI docs.

Check it

Open your browser at http://127.0.0.1:8000/items/5?q=somequery.

You will see the JSON response as:

{"item_id": 5, "q": "somequery"}

You already created an API that:

Receives HTTP requests in the paths / and /items/{item_id}.
Both paths take GET operations (also known as HTTP methods).
The path /items/{item_id} has a path parameter item_id that should be an int.
The path /items/{item_id} has an optional str query parameter q.

Interactive API docs

Now go to http://127.0.0.1:8000/docs.

You will see the automatic interactive API documentation (provided by Swagger UI):

Swagger UI

Alternative API docs

And now, go to http://127.0.0.1:8000/redoc.

You will see the alternative automatic documentation (provided by ReDoc):

ReDoc

Example upgrade

Now modify the file main.py to receive a body from a PUT request.

Declare the body using standard Python types, thanks to Pydantic.

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()


class Item(BaseModel):
    name: str
    price: float
    is_offer: bool | None = None


@app.get("/")
def read_root():
    return {"Hello": "World"}


@app.get("/items/{item_id}")
def read_item(item_id: int, q: str | None = None):
    return {"item_id": item_id, "q": q}


@app.put("/items/{item_id}")
def update_item(item_id: int, item: Item):
    return {"item_name": item.name, "item_id": item_id}

The fastapi dev server should reload automatically.

Interactive API docs upgrade

Now go to http://127.0.0.1:8000/docs.

The interactive API documentation will be automatically updated, including the new body:

Swagger UI

Click on the button "Try it out", it allows you to fill the parameters and directly interact with the API:

Swagger UI interaction

Then click on the "Execute" button, the user interface will communicate with your API, send the parameters, get the results and show them on the screen:

Swagger UI interaction

Alternative API docs upgrade

And now, go to http://127.0.0.1:8000/redoc.

The alternative documentation will also reflect the new query parameter and body:

ReDoc

Recap

In summary, you declare once the types of parameters, body, etc. as function parameters.

You do that with standard modern Python types.

You don't have to learn a new syntax, the methods or classes of a specific library, etc.

Just standard Python.

For example, for an int:

item_id: int

or for a more complex Item model:

item: Item

...and with that single declaration you get:

Editor support, including:
- Completion.
- Type checks.
Validation of data:
- Automatic and clear errors when the data is invalid.
- Validation even for deeply nested JSON objects.
Conversion of input data: coming from the network to Python data and types. Reading from:
- JSON.
- Path parameters.
- Query parameters.
- Cookies.
- Headers.
- Forms.
- Files.
Conversion of output data: converting from Python data and types to network data (as JSON):
- Convert Python types (str, int, float, bool, list, etc).
- datetime objects.
- UUID objects.
- Database models.
- ...and many more.
Automatic interactive API documentation, including 2 alternative user interfaces:
- Swagger UI.
- ReDoc.

Coming back to the previous code example, FastAPI will:

Validate that there is an item_id in the path for GET and PUT requests.
Validate that the item_id is of type int for GET and PUT requests.
- If it is not, the client will see a useful, clear error.
Check if there is an optional query parameter named q (as in http://127.0.0.1:8000/items/foo?q=somequery) for GET requests.
- As the q parameter is declared with = None, it is optional.
- Without the None it would be required (as is the body in the case with PUT).
For PUT requests to /items/{item_id}, read the body as JSON:
- Check that it has a required attribute name that should be a str.
- Check that it has a required attribute price that has to be a float.
- Check that it has an optional attribute is_offer, that should be a bool, if present.
- All this would also work for deeply nested JSON objects.
Convert from and to JSON automatically.
Document everything with OpenAPI, that can be used by:
- Interactive documentation systems.
- Automatic client code generation systems, for many languages.
Provide 2 interactive documentation web interfaces directly.

We just scratched the surface, but you already get the idea of how it all works.

Try changing the line with:

    return {"item_name": item.name, "item_id": item_id}

...from:

        ... "item_name": item.name ...

...to:

        ... "item_price": item.price ...

...and see how your editor will auto-complete the attributes and know their types:

editor support

For a more complete example including more features, see the Tutorial - User Guide.

Spoiler alert: the tutorial - user guide includes:

Declaration of parameters from other different places such as: headers, cookies, form fields and files.
How to set validation constraints such as maximum_length or regex.
A very powerful and easy to use Dependency Injection system.
Security and authentication, including support for OAuth2 with JWT tokens and HTTP Basic auth.
More advanced (but equally easy) techniques for declaring deeply nested JSON models (thanks to Pydantic).
GraphQL integration with Strawberry and other libraries.
Many extra features (thanks to Starlette) such as:
- WebSockets
- extremely easy tests based on HTTPX and pytest
- CORS
- Cookie Sessions
- ...and more.

Deploy your app (optional)

You can optionally deploy your FastAPI app to FastAPI Cloud with a single command. 🚀

$ uv run fastapi deploy

Deploying to FastAPI Cloud...

✅ Deployment successful!

🐔 Ready the chicken! Your app is ready at https://myapp.fastapicloud.dev

The CLI will automatically detect your FastAPI application and deploy it to the cloud. If you are not logged in, your browser will open to complete the authentication process.

That's it! Now you can access your app at that URL. ✨

About FastAPI Cloud

FastAPI Cloud is built by the same author and team behind FastAPI.

It streamlines the process of building, deploying, and accessing an API with minimal effort.

It brings the same developer experience of building apps with FastAPI to deploying them to the cloud. 🎉

FastAPI Cloud is the primary sponsor and funding provider for the FastAPI and friends open source projects. ✨

Deploy to other cloud providers

FastAPI is open source and based on standards. You can deploy FastAPI apps to any cloud provider you choose.

Follow your cloud provider's guides to deploy FastAPI apps with them. 🤓

Performance

Independent TechEmpower benchmarks show FastAPI applications running under Uvicorn as one of the fastest Python frameworks available, only below Starlette and Uvicorn themselves (used internally by FastAPI). (*)

To understand more about it, see the section Benchmarks.

Dependencies

FastAPI depends on Pydantic and Starlette.

`standard` Dependencies

When you install FastAPI with uv add "fastapi[standard]" it comes with the standard group of optional dependencies:

Used by Pydantic:

email-validator - for email validation.

Used by Starlette:

httpx - Required if you want to use the TestClient.
jinja2 - Required if you want to use the default template configuration.
python-multipart - Required if you want to support form "parsing", with request.form().

Used by FastAPI:

uvicorn - for the server that loads and serves your application. This includes uvicorn[standard], which includes some dependencies (e.g. uvloop) needed for high performance serving.
fastapi-cli[standard] - to provide the fastapi command.
- This includes fastapi-cloud-cli, which allows you to deploy your FastAPI application to FastAPI Cloud.

Without `standard` Dependencies

If you don't want to include the standard optional dependencies, you can install with uv add fastapi instead of uv add "fastapi[standard]".

Without `fastapi-cloud-cli`

If you want to install FastAPI with the standard dependencies but without the fastapi-cloud-cli, you can install with uv add "fastapi[standard-no-fastapi-cloud-cli]".

Additional Optional Dependencies

There are some additional dependencies you might want to install.

Additional optional Pydantic dependencies:

pydantic-settings - for settings management.
pydantic-extra-types - for extra types to be used with Pydantic.

Additional optional FastAPI dependencies:

orjson - Required if you want to use ORJSONResponse.
ujson - Required if you want to use UJSONResponse.

License

This project is licensed under the terms of the MIT license.

Sponsors

Keystone Sponsor

Gold Sponsors

Silver Sponsors

Opinions

FastAPI Conf

FastAPI mini documentary

Typer, the FastAPI of CLIs

Requirements

Installation

Example

Create it

Run it

Check it

Interactive API docs

Alternative API docs

Example upgrade

Interactive API docs upgrade

Alternative API docs upgrade

Recap

Deploy your app (optional)

About FastAPI Cloud

Deploy to other cloud providers

Performance

Dependencies

`standard` Dependencies

Without `standard` Dependencies

Without `fastapi-cloud-cli`

Additional Optional Dependencies

License

Sponsors

Keystone Sponsor

Gold Sponsors

Silver Sponsors

Opinions

FastAPI Conf

FastAPI mini documentary

Typer, the FastAPI of CLIs

Requirements

Installation

Example

Create it

Run it

Check it

Interactive API docs

Alternative API docs

Example upgrade

Interactive API docs upgrade

Alternative API docs upgrade

Recap

Deploy your app (optional)

About FastAPI Cloud

Deploy to other cloud providers

Performance

Dependencies

standard Dependencies

Without standard Dependencies

Without fastapi-cloud-cli

Additional Optional Dependencies

License

`standard` Dependencies

Without `standard` Dependencies

Without `fastapi-cloud-cli`

Version Diff