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

Test Coverage Package version Supported Python versions

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 3.7+ 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.


Kabir Khan - Microsoft (ref)

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

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

Brian Okken - Python Bytes podcast host (ref)

Timothy Crosley - Hug creator (ref)

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

Typer, the FastAPI of CLIs



$ pip install fastapi

---> 100%
$ pip install "uvicorn[standard]"

---> 100%


Create it

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

from fastapi import FastAPI

app = FastAPI()

def read_root():
    return {"Hello": "World"}

def read_item(item_id: int, q: Union[str, None] = None):
    return {"item_id": item_id, "q": q}
Or use async def...

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

from typing import Union

from fastapi import FastAPI

app = FastAPI()

async def read_root():
    return {"Hello": "World"}

async def read_item(item_id: int, q: Union[str, None] = None):
    return {"item_id": item_id, "q": q}


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

Run it

$ uvicorn main:app --reload

INFO:     Uvicorn running on (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

{"item_id": 5, "q": "somequery"}
  • 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

Alternative API docs

Example upgrade

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

def read_root():
    return {"Hello": "World"}

def read_item(item_id: int, q: Union[str, None] = None):
    return {"item_id": item_id, "q": q}

def update_item(item_id: int, item: Item):
    return {"item_name": item.name, "item_id": item_id}

Interactive API docs upgrade

  • 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

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


item_id: int
item: Item
  • 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.

  • 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 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.

    return {"item_name": item.name, "item_id": item_id}
        ... "item_name": item.name ...
        ... "item_price": item.price ...
  • 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 requests and pytest
    • CORS
    • Cookie Sessions
    • ...and more.


Optional Dependencies

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


© 2018 Sebastián Ramírez
Licensed under the MIT License.