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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:

$ <font color="#4E9A06">fastapi</font> dev <u style="text-decoration-style:single"></u>
<font color="#3465A4">INFO    </font> Using path <font color="#3465A4"></font>
<font color="#3465A4">INFO    </font> Resolved absolute path <font color="#75507B">/home/user/code/awesomeapp/</font><font color="#AD7FA8"></font>
<font color="#3465A4">INFO    </font> Searching for package file structure from directories with <font color="#3465A4"></font> files
<font color="#3465A4">INFO    </font> Importing from <font color="#75507B">/home/user/code/</font><font color="#AD7FA8">awesomeapp</font>

 ╭─ <font color="#8AE234"><b>Python module file</b></font> ─╮
 │                      │
 │  🐍          │
 │                      │

<font color="#3465A4">INFO    </font> Importing module <font color="#4E9A06">main</font>
<font color="#3465A4">INFO    </font> Found importable FastAPI app

 ╭─ <font color="#8AE234"><b>Importable FastAPI app</b></font> ─╮
 │                          │
 │  <span style="background-color:#272822"><font color="#FF4689">from</font></span><span style="background-color:#272822"><font color="#F8F8F2"> main </font></span><span style="background-color:#272822"><font color="#FF4689">import</font></span><span style="background-color:#272822"><font color="#F8F8F2"> app</font></span><span style="background-color:#272822">  </span>  │
 │                          │

<font color="#3465A4">INFO    </font> Using import string <font color="#8AE234"><b>main:app</b></font>

 <span style="background-color:#C4A000"><font color="#2E3436">╭────────── FastAPI CLI - Development mode ───────────╮</font></span>
 <span style="background-color:#C4A000"><font color="#2E3436">│                                                     │</font></span>
 <span style="background-color:#C4A000"><font color="#2E3436">│  Serving at:                  │</font></span>
 <span style="background-color:#C4A000"><font color="#2E3436">│                                                     │</font></span>
 <span style="background-color:#C4A000"><font color="#2E3436">│  API docs:               │</font></span>
 <span style="background-color:#C4A000"><font color="#2E3436">│                                                     │</font></span>
 <span style="background-color:#C4A000"><font color="#2E3436">│  Running in development mode, for production use:   │</font></span>
 <span style="background-color:#C4A000"><font color="#2E3436">│                                                     │</font></span>
 <span style="background-color:#C4A000"><font color="#2E3436">│  </font></span><span style="background-color:#C4A000"><font color="#555753"><b>fastapi run</b></font></span><span style="background-color:#C4A000"><font color="#2E3436">                                        │</font></span>
 <span style="background-color:#C4A000"><font color="#2E3436">│                                                     │</font></span>
 <span style="background-color:#C4A000"><font color="#2E3436">╰─────────────────────────────────────────────────────╯</font></span>

<font color="#4E9A06">INFO</font>:     Will watch for changes in these directories: [&apos;/home/user/code/awesomeapp&apos;]
<font color="#4E9A06">INFO</font>:     Uvicorn running on <b></b> (Press CTRL+C to quit)
<font color="#4E9A06">INFO</font>:     Started reloader process [<font color="#34E2E2"><b>2265862</b></font>] using <font color="#34E2E2"><b>WatchFiles</b></font>
<font color="#4E9A06">INFO</font>:     Started server process [<font color="#06989A">2265873</font>]
<font color="#4E9A06">INFO</font>:     Waiting for application startup.
<font color="#4E9A06">INFO</font>:     Application startup complete.

In the output, there's a line with something like:

INFO:     Uvicorn running on (Press CTRL+C to quit)

That 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:

{"message": "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 an abstract description.

API "schema"

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

This schema definition includes your API paths, the possible 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 the openapi.json

If you are curious about how the raw OpenAPI schema looks like, FastAPI automatically generates a JSON (schema) with the descriptions of all your API.

You can see it directly at:

It will show a JSON starting with something like:

    "openapi": "3.1.0",
    "info": {
        "title": "FastAPI",
        "version": "0.1.0"
    "paths": {
        "/items/": {
            "get": {
                "responses": {
                    "200": {
                        "description": "Successful Response",
                        "content": {
                            "application/json": {


What is OpenAPI for

The OpenAPI schema is what powers the two 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.

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".

While 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 decorator

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 operations.

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 a GET operation.

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 Async: "In a hurry?".

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 using decorators like @app.get("/").
  • Define a path operation function; for example, def root(): ....
  • Run the development server using the command fastapi dev.