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First Steps

The simplest FastAPI file could look like this:

from fastapi import FastAPI

app = FastAPI()

async def root():
    return {"message": "Hello World"}

Copy that to a file

Run the live server:

uvicorn main:app --debug


The command uvicorn main:app refers to:

  • main: the file (the Python "module").
  • app: the object created inside of with the line app = FastAPI().
  • --debug: make the server restart after code changes. Only use for development.

You will see an output like:

INFO: Started reloader process [17961]
INFO: Started server process [17962]
INFO: Waiting for application startup.
INFO: Uvicorn running on (Press CTRL+C to quit)

That last line shows the URL where your app is being served, in your local machine.

Check it

Open your browser at

You will see the JSON response as:

{"hello": "world"}

Interactive API docs

Now go to

You will see the automatic interactive API documentation (provided by Swagger UI):

Swagger UI

Alternative API docs

And now, go to

You will see the alternative automatic documentation (provided by ReDoc):



FastAPI generates a "schema" with all your API using the OpenAPI standard for defining APIs.


A "schema" is a definition or description of something. Not the code that implements it, but just the abstract description.

API "schema"

In this case, OpenAPI is a specification that dictates how to define a schema of your API.

This OpenAPI schema would include your API paths, the posible parameters they take, etc.

Data "schema"

The term "schema" might also refer to the shape of some data, like a JSON content.

In that case, it would mean the JSON attributes, and data types they have, etc.

OpenAPI and JSON Schema

OpenAPI defines an API schema for your API. And that schema includes definitions (or "schemas") of the data sent and received by your API using JSON Schema, the standard for JSON data schemas.

Check it

If you are curious about how the raw OpenAPI schema looks like, it is just an automatically generated JSON with the descriptions of all your API.

You can see it directly at:

It will show a JSON starting with something like:

    "openapi": "3.0.2",
    "info": {
        "title": "Fast API",
        "version": "0.1.0"
    "paths": {
        "/items/": {
            "get": {
                "responses": {
                    "200": {
                        "description": "Successful Response",
                        "content": {
                            "application/json": {


What for?

This OpenAPI schema is what powers the 2 interactive documentation systems included.

And there are dozens of alternatives, all based on OpenAPI. You could easily add any of those alternatives to your application built with FastAPI.

You could also use it to generate code automatically, for clients that communicate with your API. For example, frontend, mobile or IoT applications.

Recap, step by step

Step 1: import FastAPI

from fastapi import FastAPI

app = FastAPI()

async def root():
    return {"message": "Hello World"}

FastAPI is a Python class that provides all the functionality for your API.

Technical Details

FastAPI is a class that inherits directly from Starlette.

You can use all the Starlette functionality with FastAPI too.

Step 2: create a FastAPI "instance"

from fastapi import FastAPI

app = FastAPI()

async def root():
    return {"message": "Hello World"}

Here the app variable will be an "instance" of the class FastAPI.

This will be the main point of interaction to create all your API.

This app is the same one referred by uvicorn in the command:

uvicorn main:app --debug

If you create your app like:

from fastapi import FastAPI

my_awesome_api = FastAPI()

async def root():
    return {"message": "Hello World"}

And put it in a file, then you would call uvicorn like:

uvicorn main:my_awesome_api --debug

Step 3: create a path operation


"Path" here refers to the last part of the URL starting from the first /.

So, in a URL like:

...the path would be:



A "path" is also commonly called an "endpoint" or a "route".

Building an API, the "path" is the main way to separate "concerns" and "resources".


"Operation" here refers to one of the HTTP "methods".

One of:

  • POST
  • GET
  • PUT

...and the more exotic ones:

  • HEAD

In the HTTP protocol, you can communicate to each path using one (or more) of these "methods".

When building APIs, you normally use these specific HTTP methods to perform a specific action.

Normally you use:

  • POST: to create data.
  • GET: to read data.
  • PUT: to update data.
  • DELETE: to delete data.

So, in OpenAPI, each of the HTTP methods is called an "operation".

We are going to call them "operations" too.

Define a path operation function

from fastapi import FastAPI

app = FastAPI()

async def root():
    return {"message": "Hello World"}

The @app.get("/") tells FastAPI that the function right below is in charge of handling requests that go to:

  • the path /
  • using a get operation

@decorator Info

That @something syntax in Python is called a "decorator".

You put it on top of a function. Like a pretty decorative hat (I guess that's where the term came from).

A "decorator" takes the function below and does something with it.

In our case, this decorator tells FastAPI that the function below corresponds to the path / with an operation get.

It is the "path operation decorator".

You can also use the other operations:

  • @app.put()
  • @app.delete()

And the more exotic ones:

  • @app.options()
  • @app.head()
  • @app.patch()
  • @app.trace()


You are free to use each operation (HTTP method) as you wish.

FastAPI doesn't enforce any specific meaning.

The information here is presented as a guideline, not a requirement.

For example, when using GraphQL you normally perform all the actions using only post.

Step 4: define the path operation function

This is our "path operation function":

  • path: is /.
  • operation: is get.
  • function: is the function below the "decorator" (below @app.get("/")).
from fastapi import FastAPI

app = FastAPI()

async def root():
    return {"message": "Hello World"}

This is a Python function.

It will be called by FastAPI whenever it receives a request to the URL "/" using GET.

In this case, it is an async function.

You could also define it as a normal function instead of async def:

from fastapi import FastAPI

app = FastAPI()

def root():
    return {"message": "Hello World"}


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

Step 5: return the content

from fastapi import FastAPI

app = FastAPI()

async def root():
    return {"message": "Hello World"}

You can return a dict, list, singular values as str, int, etc.

You can also return Pydantic models (you'll see more about that later).

There are many other objects and models that will be automatically converted to JSON (including ORMs, etc). Try using your favorite ones, it's highly probable that they are already supported.


  • Import FastAPI.
  • Create an app instance.
  • Write a path operation decorator (like @app.get("/")).
  • Write a path operation function (like def root(): ... above).
  • Run the debugging server (like uvicorn main:app --debug).