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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/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.

Requirements

Python 3.6+

FastAPI stands on the shoulders of giants:

Installation

$ pip install fastapi

You will also need an ASGI server, for production such as uvicorn.

$ pip install uvicorn

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):
    return {"item_id": item_id, "q": q}

Or use async def...

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):
    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

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.

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()


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


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


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


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

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 is 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 SchemaGenerator support.
  • 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.

You can install all of these with pip3 install fastapi[all].

License

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