spellbind

spellbind

Reactive programming for Python with reactive variables and events.

spellbind is a reactive programming library that lets you create Variables that automatically update when their dependencies change, plus an event system for notifying observers.

Installation

pip install spellbind

Quick Start

from spellbind.int_values import IntVariable
from spellbind.str_values import StrVariable

# Create reactive variables
name = StrVariable("Alice")
age = IntVariable(25)

# Create computed values that automatically update
greeting = name + " is " + age.to_str() + " years old"

print(greeting)  # "Alice is 25 years old"

# Update source values - computed values update automatically!
name.value = "Bob"
age.value = 30

print(greeting)  # "Bob is 30 years old"

Core Concepts

Values, Variables and Events

The foundation of spellbind consists of three key components:

Values are read-only reactive data that can be observed for changes. Variables are mutable Values that can be changed and bound to other Values. Events provide a way to notify observers when something happens.

from spellbind.values import Constant
from spellbind.int_values import IntVariable
from spellbind.event import Event

# Variables can be changed
counter = IntVariable(0)
counter.value = 10

# Constants cannot be changed
pi = Constant(3.14159)

# Events notify observers
button_clicked = Event()
button_clicked.observe(lambda: print("Clicked!"))
button_clicked()  # Prints: "Clicked!"

Reactive Bindings

Variables can be bound to other Values, making them automatically update:

from spellbind.int_values import IntVariable

# Create computed values
base = IntVariable(10)
multiplier = IntVariable(3)
result = base * multiplier

# Bind variables to computed values
my_variable = IntVariable(0)
my_variable.bind_to(result)

print(my_variable)  # 30

# Updates propagate automatically
base.value = 20
print(my_variable)  # 60

# Unbind to break connections
my_variable.unbind()

Advanced Features

Weak vs Strong Binding

Control memory management with binding strength:

from spellbind.str_values import StrVariable

source = StrVariable("hello")
target = StrVariable("")

# Strong binding (default) - keeps source alive
target.bind_to(source, bind_weakly=False)

# Weak binding - allows source to be garbage collected
target.bind_to(source, bind_weakly=True)

Circular Dependency Detection

spellbind automatically prevents circular dependencies:

from spellbind.int_values import IntVariable

a = IntVariable(1)
b = IntVariable(2)

a.bind_to(b)
# b.bind_to(a)  # This would raise RecursionError

Observing Changes

React to value changes with observers:

from spellbind.int_values import IntVariable


def on_value_change(new_value):
    print(f"Value changed to: {new_value}")


my_var = IntVariable(42)
my_var.observe(on_value_change)

my_var.value = 100  # Prints: "Value changed to: 100"

Event System

spellbind includes an event system for notifying observers when things happen.

Basic Events

from spellbind.event import Event

# Create an event
button_clicked = Event()


# Add observers
def handle_click():
    print("Button was clicked!")


button_clicked.observe(handle_click)

# Trigger the event
button_clicked()  # Prints: "Button was clicked!"

Value Events

Events that pass data to observers:

from spellbind.event import ValueEvent

user_logged_in = ValueEvent[str]()


def welcome_user(username: str):
    print(f"Welcome, {username}!")


user_logged_in.observe(welcome_user)
user_logged_in("Alice")  # Prints: "Welcome, Alice!"

Multi-Parameter Events

Events with multiple parameters:

from spellbind.event import BiEvent, TriEvent

# Two parameters
position_changed = BiEvent[int, int]()
position_changed.observe(lambda x, y: print(f"Position: ({x}, {y})"))
position_changed(10, 20)  # Prints: "Position: (10, 20)"

# Three parameters
rgb_changed = TriEvent[int, int, int]()
rgb_changed.observe(lambda r, g, b: print(f"Color: rgb({r}, {g}, {b})"))
rgb_changed(255, 128, 0)  # Prints: "Color: rgb(255, 128, 0)"

Weak Observation

Prevent memory leaks with weak observers:

from spellbind.event import Event

event = Event()


def temporary_handler():
    print("Handling event")


# Weak observation - handler can be garbage collected
event.weak_observe(temporary_handler)

Example Application

Here's a practical example showing how to create automatically positioned windows:

from spellbind.int_values import IntVariable


class Window:
    def __init__(self, x: int, y: int, width: int, height: int):
        self.x = IntVariable(x)
        self.y = IntVariable(y)
        self.width = IntVariable(width)
        self.height = IntVariable(height)

    def __repr__(self):
        return f"Window(x={self.x.value}, y={self.y.value}, width={self.width.value}, height={self.height.value})"


# Create two windows
main_window = Window(100, 50, 800, 600)
sidebar_window = Window(0, 0, 200, 400)

# Automatically position sidebar to the right of main window
margin = IntVariable(10)
sidebar_window.x.bind_to(main_window.x + main_window.width + margin)
sidebar_window.y.bind_to(main_window.y)

print(main_window)  # Window(x=100, y=50, width=800, height=600)
print(sidebar_window)  # Window(x=910, y=50, width=200, height=400)

# Moving the main window automatically repositions the sidebar
main_window.x.value = 200
main_window.y.value = 100

print(main_window)  # Window(x=200, y=100, width=800, height=600)
print(sidebar_window)  # Window(x=1010, y=100, width=200, height=400)

# Changing margin updates sidebar position
margin.value = 20
print(sidebar_window)  # Window(x=1020, y=100, width=200, height=400)

API Reference

Core Classes

• Value[T] - Type for all reactive values, useful for typing function parameters
• Variable[T] - Type for mutable values, useful for typing function parameters
• Constant[T] - Immutable value

Type-Specific Classes

• IntValue, IntVariable - Integer values with arithmetic operations
• FloatValue, FloatVariable - Float values with arithmetic operations
• StrValue, StrVariable - String values with concatenation
• BoolValue - Boolean values with logical operations

Event Classes

• Event - Basic event with no parameters
• ValueEvent[T] - Event that passes one value
• BiEvent[S, T] - Event that passes two values
• TriEvent[S, T, U] - Event that passes three values

Development

Running Tests

pytest

Type Checking

mypy src

Linting

flake8 .

Author: Georg Plaz