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

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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/tiangolo/fastapi


FastAPI is a modern, fast (high-performance), web framework for building APIs with Python 3.6+ 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.

Gold 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."

Kabir Khan - Microsoft (ref)

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

Piero Molino, Yaroslav Dudin, and Sai Sumanth Miryala - Uber (ref)

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

Kevin Glisson, Marc Vilanova, Forest Monsen - Netflix (ref)

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

Brian Okken - Python Bytes podcast host (ref)

"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."

Timothy Crosley - Hug creator (ref)

"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 [...]"

Ines Montani - Matthew Honnibal - Explosion AI founders - spaCy creators (ref) - (ref)

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 3.6+

FastAPI stands on the shoulders of giants:

Installation

$ pip install fastapi

---> 100%

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

$ pip install uvicorn

---> 100%

Example

Create it

  • Create a file
    main.py
    with:
from typing import Optional

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: Optional[str] = None): return {"item_id": item_id, "q": q}

Or use async def...

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

from typing import Optional

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: Optional[str] = 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:

$ uvicorn main:app --reload

INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     Started reloader process [28720]
INFO:     Started server process [28722]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
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:

{"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.

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

from fastapi import FastAPI from pydantic import BaseModel

app = FastAPI()

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

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

@app.get("/items/{itemid}") def readitem(itemid: int, q: Optional[str] = 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:

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 3.6+.

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 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).
  • Many extra features (thanks to Starlette) as:
    • WebSockets
    • GraphQL
    • extremely easy tests based on
      requests
      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:

Used by Starlette:

  • requests - Required if you want to use the
    TestClient
    .
  • aiofiles - Required if you want to use
    FileResponse
    or
    StaticFiles
    .
  • 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).
  • graphene - Required for
    GraphQLApp
    support.
  • ujson - Required if you want to use
    UJSONResponse
    .

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.

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