State in Blocks

We covered State in Interfaces, this guide takes a look at state in Blocks, which works mostly the same.

Global State

Global state in Blocks works the same as in Interface. Any variable created outside a function call is a reference shared between all users.

Session State

Gradio supports session state, where data persists across multiple submits within a page session, in Blocks apps as well. To reiterate, session data is not shared between different users of your model. To store data in a session state, you need to do three things:

1. Create a gr.State() object. If there is a default value to this stateful object, pass that into the constructor.
2. In the event listener, put the State object as an input and output as needed.
3. In the event listener function, add the variable to the input parameters and the return value.

Let's take a look at a simple example. We have a simple checkout app below where you add items to a cart. You can also see the size of the cart.

import gradio as gr

with gr.Blocks() as demo:
    cart = gr.State([])
    items_to_add = gr.CheckboxGroup(["Cereal", "Milk", "Orange Juice", "Water"])

    def add_items(new_items, previous_cart):
        cart = previous_cart + new_items
        return cart

    gr.Button("Add Items").click(add_items, [items_to_add, cart], cart)

    cart_size = gr.Number(label="Cart Size")
    cart.change(lambda cart: len(cart), cart, cart_size)

demo.launch()

Notice how we do this with state:

1. We store the cart items in a gr.State() object, initialized here to be an empty list.
2. When adding items to the cart, the event listener uses the cart as both input and output - it returns the updated cart with all the items inside.
3. We can attach a .change listener to cart, that uses the state variable as input as well.

You can think of gr.State as an invisible Component that can store any kind of value. Here, cart is not visible in the frontend but is used for calculations.

The .change listener for a state variable triggers after any event listener changes the value of a state variable. If the state variable holds a sequence (like a list, set, or dict), a change is triggered if any of the elements inside change. If it holds an object or primitive, a change is triggered if the hash of the value changes. So if you define a custom class and create a gr.State variable that is an instance of that class, make sure that the the class includes a sensible __hash__ implementation.

The value of a session State variable is cleared when the user refreshes the page. The value is stored on in the app backend for 60 minutes after the user closes the tab (this can be configured by the delete_cache parameter in gr.Blocks).

Learn more about State in the docs.