Description

Reflex

Reflex is a library to build full-stack web apps in pure Python.

Key features:

Pure Python - Write your app's frontend and backend all in Python, no need to learn Javascript.
Full Flexibility - Reflex is easy to get started with, but can also scale to complex apps.
Deploy Instantly - After building, deploy your app with a single command or host it on your own server.

See our architecture page to learn how Reflex works under the hood.

⚙️ Installation

Open a terminal and run (Requires Python 3.8+):

pip install reflex

Create your first app

Installing reflex also installs the reflex command line tool.

Test that the install was successful by creating a new project. (Replace my_app_name with your project name):

mkdir my_app_name
cd my_app_name
reflex init

This command initializes a template app in your new directory.

You can run this app in development mode:

reflex run

You should see your app running at http://localhost:3000.

Now you can modify the source code in my_app_name/my_app_name.py. Reflex has fast refreshes so you can see your changes instantly when you save your code.

Example App

Let's go over an example: creating an image generation UI around DALL·E. For simplicity, we just call the OpenAI API, but you could replace this with an ML model run locally.

A frontend wrapper for DALL·E, shown in the process of generating an image.

Here is the complete code to create this. This is all done in one Python file!

import reflex as rx
import openai

openai_client = openai.OpenAI()


class State(rx.State):
    """The app state."""

    prompt = ""
    image_url = ""
    processing = False
    complete = False

    def get_image(self):
        """Get the image from the prompt."""
        if self.prompt == "":
            return rx.window_alert("Prompt Empty")

        self.processing, self.complete = True, False
        yield
        response = openai_client.images.generate(
            prompt=self.prompt, n=1, size="1024x1024"
        )
        self.image_url = response.data[0].url
        self.processing, self.complete = False, True


def index():
    return rx.center(
        rx.vstack(
            rx.heading("DALL-E", font_size="1.5em"),
            rx.input(
                placeholder="Enter a prompt..",
                on_blur=State.set_prompt,
                width="25em",
            ),
            rx.button(
                "Generate Image", 
                on_click=State.get_image,
                width="25em",
                loading=State.processing
            ),
            rx.cond(
                State.complete,
                rx.image(src=State.image_url, width="20em"),
            ),
            align="center",
        ),
        width="100%",
        height="100vh",
    )

# Add state and page to the app.
app = rx.App()
app.add_page(index, title="Reflex:DALL-E")

Let's break this down.

Reflex UI

Let's start with the UI.

def index():
    return rx.center(
        ...
    )

This index function defines the frontend of the app.

We use different components such as center, vstack, input, and button to build the frontend. Components can be nested within each other
to create complex layouts. And you can use keyword args to style them with the full power of CSS.

Reflex comes with 60+ built-in components to help you get started. We are actively adding more components, and it's easy to create your own components.

State

Reflex represents your UI as a function of your state.

class State(rx.State):
    """The app state."""
    prompt = ""
    image_url = ""
    processing = False
    complete = False

The state defines all the variables (called vars) in an app that can change and the functions that change them.

Here the state is comprised of a prompt and image_url. There are also the booleans processing and complete to indicate when to disable the button (during image generation) and when to show the resulting image.

Event Handlers

def get_image(self):
    """Get the image from the prompt."""
    if self.prompt == "":
        return rx.window_alert("Prompt Empty")

    self.processing, self.complete = True, False
    yield
    response = openai_client.images.generate(
        prompt=self.prompt, n=1, size="1024x1024"
    )
    self.image_url = response.data[0].url
    self.processing, self.complete = False, True

Within the state, we define functions called event handlers that change the state vars. Event handlers are the way that we can modify the state in Reflex. They can be called in response to user actions, such as clicking a button or typing in a text box. These actions are called events.

Our DALL·E. app has an event handler, get_image to which get this image from the OpenAI API. Using yield in the middle of an event handler will cause the UI to update. Otherwise the UI will update at the end of the event handler.