fastapi/fastapi

FastAPI framework, high performance, easy to learn, fast to code, ready for production

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Documentation: https://fastapi.tiangolo.com

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

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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.

Sponsors

Keystone Sponsor

Gold Sponsors

Silver Sponsors

Other sponsors

Opinions

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}

Note:

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

JSON {"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):

Alternative API docs
And now, go to http://127.0.0.1:8000/redoc.
You will see the alternative automatic documentation (provided by 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.

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:


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


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:


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:


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:

or for a more complex Item model:

...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.

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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.

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We just scratched the surface, but you already get the idea of how it all works.
Try changing the line with:

...from:

...to:

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


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. 🚀

Deploying to FastAPI Cloud...

✅ Deployment successful!

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