fastreactpy: Reactive Python State Management

The Mental Model

React changed how we think about state with its unidirectional data flow and reactive updates. I wanted that same mental model in Python—not necessarily for the browser, but for any Python logic. Whether you're building a CLI, a background service, or a lightweight dashboard, managing state shouldn't involve a mess of callbacks and global variables.

fastreactpy is a zero-dependency, lightweight library that implements the core concepts of "Signals" and "Effects" in pure Python.

Core Concepts

1. Signals (State)

Signals are reactive containers. When you update a signal, any code that depends on it is automatically notified.

from fastreactpy import signal

# Create a signal
count = signal(0)

# Update it
count(count() + 1)

2. Effects (Side Effects)

Effects are functions that run automatically whenever the signals they use change. This is the "magic" that keeps your system in sync.

from fastreactpy import effect

@effect
def logger():
    print(f"The current count is: {count()}")

Why it Matters

In complex Python applications, tracking which part of the code needs to run when a variable changes is a major source of bugs. By using a reactive model:

• Consistency: Your UI, logs, or downstream data are always in sync with your source of truth.
• Decoupling: Parts of your system can react to state changes without ever knowing who triggered them.
• Zero Dependencies: It's built to be dropped into any project without bloating your requirements.txt.

Technical Implementation

• Automatic Dependency Tracking: Utilizing Python's call stack to track which signals are accessed during an effect's execution.
• Cross-File Support: Registering side-effects and triggering updates is seamless across different Python modules.
• Minimal Overhead: Designed for high-frequency updates with negligible performance impact.

Conclusion

FastReactPy isn't just about web dashboards—it's about bringing modern reactivity to the Python ecosystem. It's the simplest way to manage complex state transitions in your backends and tools.