Signified

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Documentation: https://dougmercer.github.io/signified

Source Code: https://github.com/dougmercer/signified

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A fast, fully-typed Python library for reactive programming.

$ uv add signified

Why care?

signified is a reactive programming library built around four reactive data structures:

• Signal stores a mutable value.
• Computed calculates a read-only value.
• Effect runs an action when the values it reads change.
• Binding is a stable handle that lets you switch between reactive values without rebuilding calculations that use it.

This allows us to create a network of computation, where one value being modified can trigger other objects to update.

We can write more declarative code, like:

from signified import Signal

x = Signal(3)
x_squared = x ** 2  # currently equal to 9
x.value = 10  # x_squared.value is now 100

Here, x_squared is a Computed object whose value is always equal to x ** 2. Neat!

In the above example, we generated the Computed object on-the-fly using overloaded Python operators like **, but we could have also written it using the computed decorator,

from signified import computed

@computed
def power(x, n):
    return x**n

x_squared = power(x, 2)  # equivalent to the above

To make something happen when a value changes, we can use an effect:

from signified import effect

@effect
def show(n):
    print(n)

watcher = show(x_squared)  # prints 100
x.value = 4                # prints 16
watcher.dispose()

Effects run once right away, then again whenever something they read changes until they are explicitly disposed or the effect is garbage collected.

In the above example, x_squared is wired to x. To swap what feeds a calculation without rebuilding it, use a Binding instead of Signal:

from signified import Binding

y = Signal(5)
base = Binding(x)       # follows x, for now
cubed = power(base, 3)  # currently equal to 64

base.value = y  # cubed now follows y, and becomes 125
x.value = 10    # cubed ignores x now

A Signal can't do this. signal.value = y stores the y object itself, not its value.

Together, these data structures allow us to implement a wide variety of capabilities. In particular, I wrote this library to make my animation library keyed easier to maintain and more fun to work with.

Ready to learn more?

Check out the docs at https://dougmercer.github.io/signified, or try it in the Playground.