FastAPI-HTMX

Extension for FastAPI to make HTMX easier to use.

FastAPI-HTMX is an opinionated extension for FastAPI to speed up development of lightly interactive web applications. FastAPI-HTMX is implemented as a decorator, so it can be used on endpoints selectively. Furthermore it reduces boilerplate for Jinja2 template handling and allows for rapid prototyping by providing convenient helpers.

Install

install via pip:

$ pip install fastapi-htmx

install via poetry:

$ poetry add fastapi-htmx

Usage

Getting Started

Basic example using FastAPI with fastapi-htmx

my_app/api.py:

from pathlib import Path

from fastapi import FastAPI, Request
from fastapi.responses import HTMLResponse
from fastapi.templating import Jinja2Templates
from fastapi_htmx import htmx, htmx_init

app = FastAPI()
htmx_init(templates=Jinja2Templates(directory=Path("my_app") / "templates"))

@app.get("/", response_class=HTMLResponse)
@htmx("index", "index")
async def root_page(request: Request):
    return {"greeting": "Hello World"}

@app.get("/customers", response_class=HTMLResponse)
@htmx("customers")
async def get_customers(request: Request):
    return {"customers": ["John Doe", "Jane Doe"]}

Note that:

htmx() got parameters, specifying the Jinja2 template to use
htmx_init() is needed for FastAPI-HTMX to find the templates
There is no direct handling of the template needed, it only needs to be specified and the needed variables need to be returned. This way endpoints can be designed in a familiar way to standard REST endpoints in FastAPI.
- This simplifies modularizing the app later (see below) and also providing a REST API if needed. See the "Usage" section for further examples.
- get_customers does not respond with the whole web page, but only with a part of it. See the HTMX documentation on how HTMX merges partials into the current web page.
request: Request although not used in the endpoint directly, it is currently required for the decorator to work!

The Jinja2 templates to go along with the above code need to be placed like specified in htmx_init in my_app/templates/ in order for the example to work.

The root page my_app/templates/index.jinja2:

<!DOCTYPE html>
<html>
<head>
    <title>Hello FastAPI-HTMX</title>
</head>
<body>
    <h1>{{ greeting }}</h1>
    <button
        hx-get="/customers"
        hx-swap="innerHTML"
        hx-target="#customers_list"
    >
        Load Data
    </button>
    <div id="customers_list"></div>
    <script src="https://unpkg.com/htmx.org@1.8.5"></script>
</body>
</html>

The partial template to load with HTMX my_app/templates/customers.jinja2:

<ul>
    {% for customer in customers %}
        <li>{{ customer }}</li>
    {% endfor %}
</ul>

Main Concept

The decorator htmx provides the following helpers:

partial_template_name The partial template to use
full_template_name The full page template to use when URL rewriting + history is used
*_template_constructor For DRY code, in case the logic to gather all needed variables is needed multiple times

Seeing these arguments one might ask themselves: Why all these parameters? The answer is an opinionated take on how to design modular endpoints wit partials and url-rewriting support:

The idea behind FastAPI-HTMX is to maintain a modular structure in the app and with the endpoints. Similar to a REST API with a SPA. This way the frontend can be modular as well. This majorly helps with supporting URL rewriting and the history in the frontend:

A simple endpoint just answers with the partial.
Without it, if the URL is rewritten and a user navigates back, reloads the page or copies the URL and opens it in another tab or shares the URL, only the partial would be shown in the browser.

To enable SPA like functionality FastAPI-HTMX uses the concept of partials and fullpages as arguments for the decorator and requires to return a dict of the needed variables.

In order to support this in an app, see the following example:

my_app/api_with_constructors.py:

from pathlib import Path

from fastapi import FastAPI, Request
from fastapi.responses import HTMLResponse
from fastapi.templating import Jinja2Templates
from fastapi_htmx import htmx, htmx_init

app = FastAPI()
htmx_init(templates=Jinja2Templates(directory=Path("my_app") / "templates"))

def construct_customers():
    return {"customers": ["John Doe", "Jane Doe"]}

def construct_root_page():
    return {
        "greeting": "Hello World",
        **construct_customers()
    }

@app.get("/", response_class=HTMLResponse)
@htmx("index", "index")
async def root_page(request: Request):
    return construct_root_page()

@app.get("/customers", response_class=HTMLResponse)
@htmx("customers", "index", construct_customers, construct_root_page)
async def get_customers(request: Request):
    pass

Note that:

The construct_* functions are added, they now return the data
- construct_root_page gathers all variables specified needed for the root page, including those for partials
  - This also means you must avoid naming conflicts across endpoints, so dicts can be merged.
  - Costly operations can still be ignored, just use if statements in the template or similar
The decorators arguments are extended
- The second argument is the fullpage template which is used when the endpoint is called directly (new tab, navigation or reload)
  - E.g. since construct_root_page gathers all the data for the whole page, the whole page can be returned to the client
- The other arguments are just to save some boilerplate code handling the HX-Request header
  - There is no need to use the arguments for the constructor functions, they are just for convenience. If needed the endpoint can be used for the logic as well. Especially if no URL rewriting is needed.

For the above code to work the my_app/templates/index.jinja2 needs to be changed as well. The changes are in the button and target div. Changed root page my_app/templates/index.jinja2:

<!DOCTYPE html>
<html>
<head>
    <title>Hello FastAPI-HTMX</title>
</head>
<body>
    <h1>{{ greeting }}</h1>
    <button
        hx-get="/customers"
        hx-push-url="true"
        hx-swap="innerHTML"
        hx-target="#customers_list"
    >
        Load Data
    </button>
    <div id="customers_list">
        {% include 'customers.jinja2' %}
    </div>
    <script src="https://unpkg.com/htmx.org@1.8.6"></script>
</body>
</html>

Note that:

hx-push-url="true" was added to the button
The partial is now loaded by default requiring the main endpoint to also provide the needed variables like shown above

The unchanged partial my_app/templates/customers.jinja2:

<ul>
    {% for customer in customers %}
        <li>{{ customer }}</li>
    {% endfor %}
</ul>

To add additional partials and endpoints just repeat the same logic:

Include the partial in the parent Jinja2 template, like the main template. A hierarchy is possible as well.
Refactor the partials endpoints logic into a function
- Add it's return value to the parents constructor function like done above in construct_root_page
- Add the parents template and constructor function to the partials endpoints htmx decorator arguments

Advanced Usage

In case the htmx() arguments for partial and fullpage callables are not flexible enough, an endpoint can be used like usual. For a bit more convenience the HX-Request header is easily accessible via request.hx_request:

from fastapi_htmx import HXRequest, htmx, htmx_init

@htmx("email_detail", "index")
def get_email(request: HXRequest, email_id: int):
    if request.hx_request:
        return my_partial()
    else:
        return fullpage()

Filters

In order to use custom Jinja2 filters like the following, configure them like below.

<p>{{ customer.created|datetime_format }}</p>

Add custom filters for use in Jinja2 templates:

# ...
def datetime_format(value: datetime, format="%H:%M %d.%m.%Y"):
    return value.strftime(format) if value is not None else ""

templates = Jinja2Templates(directory=Path("my_app") / "templates")
templates.env.filters["datetime_format"] = datetime_format
htmx_init(templates=templates)
# ...

saolof/fastapi-htmx

FastAPI-HTMX

Install

Usage

Getting Started

Main Concept

Advanced Usage

Filters