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/tiangolo/fastapi

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FastAPI is a modern, fast (high-performance), web framework for building APIs with Python 3.8+ 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 on an internal development team, building production applications.

Sponsors

Other sponsors

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."

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

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

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"I’m over the moon excited about FastAPI. It’s so fun!"

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"Honestly, what you've built looks super solid and polished. In many ways, it's what I wanted Hug to be - it's really inspiring to see someone build that."

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"If you're looking to learn one modern framework for building REST APIs, check out FastAPI [...] It's fast, easy to use and easy to learn [...]"

"We've switched over to FastAPI for our APIs [...] I think you'll like it [...]"

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"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."

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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

Python 3.8+

FastAPI stands on the shoulders of giants:

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

Installation

You will also need an ASGI server, for production such as Uvicorn or Hypercorn.

Example

Create it

• Create a file main.py with:

```Python from typing import Union

from fastapi import FastAPI

app = FastAPI()

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

@app.get("/items/{itemid}") def readitem(itemid: int, q: Union[str, None] = None): return {"itemid": item_id, "q": q} ```

Or use async def...

If your code uses `async` / `await`, use `async def`: ```Python hl_lines="9 14" from typing import Union 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: Union[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:

About the command uvicorn main:app --reload...

The command `uvicorn main:app` refers to: * `main`: the file `main.py` (the Python "module"). * `app`: the object created inside of `main.py` with the line `app = FastAPI()`. * `--reload`: make the server restart after code changes. Only do this for development.

Check it

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

You will see the JSON response as:

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.

```Python hl_lines="4 9-12 25-27" from typing import Union

from fastapi import FastAPI from pydantic import BaseModel

app = FastAPI()

class Item(BaseModel): name: str price: float is_offer: Union[bool, None] = None

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

@app.get("/items/{itemid}") def readitem(itemid: int, q: Union[str, None] = None): return {"itemid": item_id, "q": q}

@app.put("/items/{itemid}") def updateitem(itemid: int, item: Item): return {"itemname": item.name, "itemid": itemid} ```

The server should reload automatically (because you added --reload to the uvicorn command above).

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 3.8+.

For example, for an int:

Python item_id: int

or for a more complex Item model:

Python 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.

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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:

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

...from:

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

...to:

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

...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 as: headers, cookies, form fields and files.
• How to set validation constraints 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) as:
  • WebSockets
  • extremely easy tests based on HTTPX and pytest
  • CORS
  • Cookie Sessions
  • ...and more.

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.

Optional Dependencies

Used by Pydantic:

• email_validator - for email validation.
• pydantic-settings - for settings management.
• pydantic-extra-types - for extra types to be used with Pydantic.

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().
• itsdangerous - Required for SessionMiddleware support.
• pyyaml - Required for Starlette's SchemaGenerator support (you probably don't need it with FastAPI).

Used by FastAPI / Starlette:

• uvicorn - for the server that loads and serves your application.
• orjson - Required if you want to use ORJSONResponse.

You can install all of these with pip install "fastapi[all]".

License

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