Object-oriented state management. The aim is to separate the state from the UI. Atoms manage the state. A form now has two jobs. It displays the state and hooks up events (like button clicks) to actions. The atom’s job is to manage the state. Each atom is a global object, which any form can import. By eliminating state from forms, there is no need to pass state up and down the form hierarchy. It also makes testing easier, since we can test atoms in isolation.


Some examples can be found at in this clone link.


# Create an atom that holds state
from anvil_labs.atomic import atom, action, selector

class CountAtom:
    value = 0

    def get_count(self):
        return self.value

    def update_count(self, increment):
        self.value += increment

count_atom = CountAtom()
# Create a form to display the count
from anvil_labs.atomic import render
from ..atoms.count import count_atom

class Count(CountTemplate):
    def __init__(self):

    def display_count(self):
        # I get called any time the get_count return value changes
        self.count_lbl.text = count_atom.get_count()

    def neg_btn_click(self, **event_args):

    def pos_btn_click(self, **event_args):

In this example, whenever a button is clicked:

  • the button event handler calls an action on the atom,

  • which updates the state of the atom,

  • which then updates any selectors that depend on that state change; and finally,

  • any render methods that depend on those updates are re-rendered.

ActionState changeRe-compute selectorsCall render methods



An action is an expression/statement that updates the state of an atom. Whenever the state changes, the atomic module invokes a render cycle. We probably don’t want each state change to invoke a render cycle; sometimes it makes sense to combine state updates into a single action. To combine actions into a single action, use the @action decorator. Using the @action decorator means that the render cycle will only be invoked after all actions within the decorated function have been completed. The @action decorator can be used on any function and does not necessarily need to be a method of an atom.


A selector is a method of an atom that returns a value based on the atom’s state. Essentially it is a getter method. When a selector needs to do something expensive, by say, combining various attributes of an atom, use the @selector decorator. The return value from a decorated selector method is cached. Whenever the atoms state changes, if the selector depends on that state, the selector’s return value will be re-computed.

You should never update the state (call an action) within a selector.


A render is any method/function that depends on the state of an atom, or that depends on the return value of a selector.

It’s most commonly used on methods within forms, but a render can be used outside of a form.

from anvil.js.window import document
from anvil_labs.atomic import render

def update_tab_title():
    document.title = count_atom.get_count()


Note we might want to do this with the autorun function. The above example is equivalent to.

from anvil_labs.atomic import autorun

def update_tab_title():
    document.title = count_atom.get_count()


To depend on the state of an atom, the render method must explicitly access that state.

# BAD Example
class Count(CountTemplate):
    def __init__(self):

    def display_count(self, count):
        self.count_lbl.text = count

In the above example, the display_count method does not explicitly access the count_atom.value attribute. This means it does not depend on this attribute. The code should look like this:

# GOOD Example
class Count(CountTemplate):
    def __init__(self):

    def display_count(self):
        self.count_lbl.text = count_atom.value

Accessing an attribute/selector implicitly subscribes the render method to changes in the state of those attributes/selectors. Any time one of these attributes changes, the render method is invoked (re-rendered).

You should never update the state (call an action) within a render method.

If the render method is called by a component, it will only execute when the form is on the screen. This prevents renders from happening for cached forms, or forms that are no longer active.


An atom is any object that knows how to register subscribers and request renders. To create an atom, use the @atom decorator.

Whenever an attribute of an atom is a list or dict, the attribute will be converted to a ListAtom or DictAtom. Each is a subclass of list/ dict and behave as you’d expect. The only difference is that these classes know how to register subscribers and request renders when their state changes.


A subscriber is an advanced feature. It’s the final part of the render cycle. After all renders have been completed any subscribers that were decorated with the @subscribe decorator will be called. A subscriber takes a single argument, a tuple of actions that were called to invoke the render cycle.

A reason to use a subscriber might be to update storage based on an action that was invoked.

Here’s an example.

from import indexed_db
from anvil_labs.atomic import atom, action, subscribe

class Todos:
    def __init__(self):
        self.todos = indexed_db.get("todos", [])

    def add_todo(self, todo):
        self.todos = self.todos + [todo]

todos_atom = Todos()

def update_db_subscriber(actions):
    if any(hasattr(action, "update_db") for action in actions):
        indexed_db["todos"] = todos_atom.todos

The @action decorator can be used on any function or method. If the decorator is used above a method then the atom used as the self argument can be caught within a subscribe function

def update_db_subscriber(actions):
    for action in actions:
        if hasattr(action, "update_db"):
            atom = action.atom

    # now use the atom to do something specific

Alternative Approaches to the subscriber

# ALTERNATIVE APPROACH 1 - use a render

is_first_run = True
def update_db_with_render():
    global is_first_run

    todos = [dict(todo) for todo in todos_atom.todos]
    # accessing the todos and each converting each todo to a dict
    # creates a dependency on the todos and each key of each todo
    # whenever these change this method is called

    if is_first_run:
        is_first_run = False
    indexed_db["todos"] = todos


# ALTERNATIVE APPROACH 2 - use autorun

def update_db_with_render():
    # same code as above

# ALTERNATIVE APPROACH 3 - use a reaction

def update_db_with_reaction(todos):
    indexed_db["todos"] = todos

reaction(lambda: [dict(todo) for todo in todos_atom.todos], update_db_with_reaction)
# the first function sets up the dependencies
# the return value of this function is passed to the reaction function
# the reaction function is called only after the first change to any dependency

Bindings and Writeback

It’s not recommended to use anvil writebacks and data bindings with atoms. This is because we can’t control the render cycle.

Instead, there are two helper functions to create bindings and writebacks in code.

from anvil_labs.atomic import bind

class Count(CountTemplate):
    def __init__(self):
        bind(self.count_lbl, "text", count_atom.get_count)
        # or bind it to an attribute of an atom
        bind(self.count_lbl, "text", count_atom, "value")

The bind method is equivalent to:

def bind(component, prop, atom_or_selector, attr=None):
    def render_bind():
        if callable(atom_or_selector):
            setattr(component, prop, atom_or_selector())
        elif isinstance(atom_or_selector, dict):
            setattr(component, prop, atom_or_selector[attr])
            setattr(component, prop, getattr(atom_or_selector, attr))


Note the render decorator can take a bound parameter. This means that the render won’t fire if the component is not on the screen. This is not necessary when using the render decorator on a form method.

A writeback is similar to a bind, but a list of events must be provided.

writeback(self.check_box, "checked", self.item, "completed", events=["change"])

Alternatively, the writeback can be called with a selector in place of the atom and an action in place of the atom attribute. If the selector/action call signature is used, the action must take a single argument (the updated property of the component).

writeback(component, prop, atom, attr, events)
writeback(component, prop, selector, action, events)



Show logging output for the module


Create an atom class. An atom class knows how to register subscribers and request re-renders when its state changes.


Create an atom class which is also a portable class. It is recommended to use the @portable_atom decorator over a combination of @atom and @portable_class.


Use the render decorator anytime you want a function to depend on atom attributes or selectors. In a render method the attributes must be accessed explicitly. Whenever one of the attributes of the atom changes, the render method will be invoked.


The action decorator should be used above any method that you want to combine actions into a single action. A base action changes the state of an atom. When calling a function decorated with the @action decorator, the render cycle will be invoked only after all actions within the function have been executed. It’s worth noting that the decorator doesn’t need to be used unless you want to combine state updates into a single action. In the counter example, the action decorator is unnecessary, since there is only a single state update within the function (updating the .value property)


The selector decorator can only be used on methods within an atom. Its utility is caching the return value and a selector subscribes to atom attributes in a similar way to renders. If any attribute changes, the cached value will be re-computed. It’s worth noting that the selector decorator is unnecessary on methods where accessing the attribute is cheap. In the counter example, the selector is unnecessary and adds little to the implementation.

autorun(fn, bound=None)

Similar to render(fn)(). Any atom attributes accessed within the body of the function will trigger a new call to the function when changed.

Calling autorun returns a dispose function. When the dispose function is called it stops any future renders of this autorun function.

autorun can be used as a decorator - but note that the returned function is not the original function but the dispose function.

reaction(depends_on_fn, then_react_fn, *, fire_immediately=False, include_previous=False)

a reaction is similar to a render. Changes in the depends_on_fn will force the then_react_fn to be called. The depends_on_fn is a function that takes no args. It should access any attributes that, when changed, should result in the call to the then_react_fn.

If depends_on_fn returns a value that is not None, this value will be passed to the then_react_fn. If you need the previous result returned from depends_on_fn set include_previous=True. If incldue_previous is True then the call signature for then_react_fn should take 2 argments, the current return value and the previous return value from the depnds_on_fn.

depends_on_fn will fire immediately. But the then_react_fn is only called the next time a dependency changes. To call the then_react_fn immediately set fire_immediately=True.

It would be rare to need to use this function.

However, in cases where you want to react to a change in an atom’s state that may result in a subsequent change, in another atom’s state a reaction may be useful. It can also be used as an alternative to autorun or render.

See the example above for alternative approaches to updating indexed_db

The reaction method returns a dispose function that can be called when you want to stop reactions.


A subscriber is called after all re-renders resulting from a series of actions a subscriber takes a single argument - the tuple of actions that caused the re-render. See examples for use cases.


Stop a subscriber from running.

class DictAtom

A subclass of dict. Any attribute within an atom that is a dict will be converted to a DictAtom. This allows render methods to depend on keys of dicts within the atom’s state.

class ListAtom

A subclass of list. Any attribute within an atom that is a list will be converted to a ListAtom. Renders that depend on the ListAtom will only be invoked if the ListAtom changes through methods like remove(), clear() etc.

class Atom(**kws)

A portable atom class that can be called with kwargs. Each kwarg will become an attribute of the atom. Useful if you prefer to access attributes rather than keys of a DictAtom.

e.g. todo_atom = Atom(done=False, description='walk the dog')


This can be used as a context manager (using with) to update an atom without invoking a render cycle. A reason to use this decorator is to lazy load an atom property. Use with caution.

Gotchas and advanced concepts

My component isn’t updating

Make sure that you have used the render decorator and that you have called this method from the __init__ function.

Why don’t you use self.init_components(**properties) in the example?

The primary job of init_components is to set up data bindings. But since we don’t have any data bindings, we don’t need to use this method. Note that init_components does more work when used within a custom component.

How do I lazy load an attribute?

You can use the ignore_updates decorator to prevent actions invoking render cycles. And since calling an action within a render or selector is not allowed it becomes necessary.

import anvil.server
from anvil_labs.atomic import atom, ignore_updates, selector

class Todos:
    def __init__(self):
        self._todos = None

    def todos(self):
        if self._todos is None:
            with ignore_updates:
                self._todos ="get_todos")
        return self._todos

Alternatively, you can call an action, ensuring that the action is not called inside a render/selector

from atoms import todos_atom

class Form1(Form1Template):
    def __init__(self, **properties):
        # fetch_todos is an action that calls the server if it needs to

My UI is taking a long time to update

That might be because you are calling a server function within an action. The fetch example is a good example of how to update the UI while you make a call.

class Fetch:
    value = None
    loading = False

    def set_status(self, value, loading=False):
        self.value = value
        self.loading = loading

    def do_fetch(self):
        self.set_status(None, loading=True)
        ret = anvil.server.call_s("do_fetch")
        self.set_status(ret, loading=False)

    def get_info(self):
        return self.value, self.loading

fetch_atom = Fetch()

do_fetch is not an action, but set_status is an action. set_status is cheap and so the UI updates quickly. Each call to set_status invokes a render cycle. When loading is True the UI can disable a button while we call the server function.

How do I work with anvil data tables?

We’re working on it.