tiangolo.fastapi/docs/tutorial/first-steps.md

The simplest FastAPI file could look like this:

{!./src/first_steps/tutorial001.py!}

Copy that to a file main.py.

Run the live server:

uvicorn main:app --reload

!!! note 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 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 http://127.0.0.1:8000 (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 http://127.0.0.1:8000.

You will see the JSON response as:

{"message": "Hello World"}

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):

Alternative API docs

And now, go to http://127.0.0.1:8000/redoc.

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

OpenAPI

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

"Schema"

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 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, it is just an automatically generated JSON with the descriptions of all your API.

You can see it directly at: http://127.0.0.1:8000/openapi.json.

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 is OpenAPI 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

{!./src/first_steps/tutorial001.py!}

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

!!! note "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"

{!./src/first_steps/tutorial001.py!}

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 --reload

If you create your app like:

{!./src/first_steps/tutorial002.py!}

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

uvicorn main:my_awesome_api --reload

Step 3: create a path operation

Path

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

So, in a URL like:

https://example.com/items/foo

...the path would be:

/items/foo

!!! info 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

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

One of:

• POST
• GET
• PUT
• DELETE

...and the more exotic ones:

• OPTIONS
• HEAD
• PATCH
• TRACE

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

{!./src/first_steps/tutorial001.py!}

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

!!! info "@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.post()
• @app.put()
• @app.delete()

And the more exotic ones:

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

!!! tip 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("/")).

{!./src/first_steps/tutorial001.py!}

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:

{!./src/first_steps/tutorial003.py!}

!!! note If you don't know the difference, check the Async: "In a hurry?"{.internal-link target=_blank}.

Step 5: return the content

{!./src/first_steps/tutorial001.py!}

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.

Recap

• 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 development server (like uvicorn main:app --reload).