Reflex is an open-source framework for quickly building beautiful, interactive web applications in pure Python.

Here, we go over a simple counter app that lets the user count up or down.

Here is the full code for this example:

import reflex as rx


class State(rx.State):
    count: int = 0

    def increment(self):
        self.count += 1

    def decrement(self):
        self.count -= 1


def index():
    return rx.hstack(
        rx.button(
            "Decrement",
            color_scheme="ruby",
            on_click=State.decrement,
        ),
        rx.heading(State.count, font_size="2em"),
        rx.button(
            "Increment",
            color_scheme="grass",
            on_click=State.increment,
        ),
        spacing="4",
    )


app = rx.App()
app.add_page(index)

Let's break this example down.

import reflex as rx

We begin by importing the reflex package (aliased to rx). We reference Reflex objects as rx.* by convention.

class State(rx.State):
    count: int = 0

The state defines all the variables (called vars ) in an app that can change, as well as the functions (called event_handlers ) that change them.

Here our state has a single var, count, which holds the current value of the counter. We initialize it to 0.

def increment(self):
    self.count += 1


def decrement(self):
    self.count -= 1

Within the state, we define functions, called event handlers, that change the state vars.

Event handlers are the only 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 counter app has two event handlers, increment and decrement.

def index():
    return rx.hstack(
        rx.button(
            "Decrement",
            color_scheme="ruby",
            on_click=State.decrement,
        ),
        rx.heading(State.count, font_size="2em"),
        rx.button(
            "Increment",
            color_scheme="grass",
            on_click=State.increment,
        ),
        spacing="4",
    )

This function defines the app's user interface.

We use different components such as rx.hstack, rx.button, and rx.heading to build the frontend. Components can be nested to create complex layouts, and can be styled using the full power of CSS.

Reflex comes with 50+ built-in components to help you get started. We are actively adding more components. Also, it's easy to wrap your own React components .

rx.heading(State.count, font_size="2em"),

Components can reference the app's state vars. The rx.heading component displays the current value of the counter by referencing State.count. All components that reference state will reactively update whenever the state changes.

rx.button(
    "Decrement",
    color_scheme="ruby",
    on_click=State.decrement,
),

Components interact with the state by binding events triggers to event handlers. For example, on_click is an event that is triggered when a user clicks a component.

The first button in our app binds its on_click event to the State.decrement event handler. Similarly the second button binds on_click to State.increment.

In other words, the sequence goes like this:

• User clicks "increment" on the UI.
• on_click event is triggered.
• Event handler State.increment is called.
• State.count is incremented.
• UI updates to reflect the new value of State.count.

Next we define our app and add the counter component to the base route.

app = rx.App()
app.add_page(index)

🎉 And that's it!

We've created a simple, yet fully interactive web app in pure Python.

By continuing with our documentation, you will learn how to building awesome apps with Reflex.

For a glimpse of the possibilities, check out these resources:

• For a more real-world example, check out the tutorial .
• We have bots that can answer questions and generate Reflex code for you, check them out in #ask-ui and #ask-ai in our Discord !