NestWatch

NestWatch is a Python package that allows applications to react to meaningful file changes instead of raw filesystem events.

Rather than repeatedly polling files and risking cache desynchronization, NestWatch instantly notifies your application whenever a file changes and provides both its previous and new state.

Why NestWatch?

Many applications update caches using polling tasks.

@tasks.loop(minutes=1)
async def update_cache():
        ...

While this works, it introduces a delay between when data changes and when applications become aware of those changes.

This becomes problematic when multiple applications depend on the same data source.

For example:

• Application A updates "config.json"
• Application B continues using outdated cached data
• Application B must wait for its polling task to run
• Both applications become temporarily out of sync

NestWatch solves this by allowing applications to react to changes instantly.

Features

• ⚡ Instant file change detection
• 🧠 Returns both old and new state
• ➕ Detects added keys
• ➖ Detects removed keys
• 🔄 Detects changed values
• 📍 Dot-path support
• 🧩 Extensible architecture
• 📄 JSON support built in
• 🏗️ Create custom watchers for your own file formats

Installation

pip install nestwatch

Quick Start

from nestwatch.watchers import JSONWatcher


class ConfigCache:

    def __init__(self):

        self.watcher = JSONWatcher("data/config.json")

        self.watcher.on_change(
            self.update_cache
        )

    async def update_cache(self, event):

        # Affected changes only
        print(event.added)
        print(event.removed)
        print(event.changed)

        # Reconstructed views)
        print(event.old)
        print(event.new)
        print(event.updated)

    async def startup(self):

        await self.watcher.start()

Event Object

NestWatch emits an Event object containing:

Event Examples

Suppose this file:

{
    "BOT_STATUS": {
        "presence": "online",
        "activity": {
            "type": "playing",
            "name": "Roblox"
        }
    }
}

becomes:

{
    "BOT_STATUS": {
        "presence": "idle",
        "custom": "Watching NestWatch 👀"
    },
    "API": {
        "enabled": true
    }
}

Then:

event.added

{
    "BOT_STATUS.custom": "Watching NestWatch 👀",
    "API.enabled": True
}

event.removed

{
    "BOT_STATUS.activity.type": "playing",
    "BOT_STATUS.activity.name": "Roblox"
}

event.changed

{
    "BOT_STATUS.presence": {
        "old": "online",
        "new": "idle"
    }
}

event.old

{
    "BOT_STATUS": {
        "presence": "online",
        "activity": {
            "type": "playing",
            "name": "Roblox"
        }
    }
}

event.new

{
    "BOT_STATUS": {
        "presence": "idle",
        "custom": "Watching NestWatch 👀"
    },
    
    "API": {
        "enabled": True
    }
}

event.updated_state

{
    "BOT_STATUS": {
        "presence": "idle",
        "custom": "Watching NestWatch 👀"
    },
    "API": {
        "enabled": True
    }
}

IMPORTANT NOTE: “These are partial reconstructions of only affected regions, not full file snapshots.”

Creating Custom Watchers

NestWatch is designed to be extensible.

You can support any file format by subclassing "Watcher".

from nestwatch.watchers import Watcher


class MyCustomWatcher(Watcher):

    def _serialize(self):

        return my_language.load(
            self.file_path
        )

The Watcher class supports asynchronous serializers as well.

This allows you to perform asynchronous file reads before NestWatch processes the data.

import aiofiles
from nestwatch.watchers import Watcher

class MyCustomWatcher(Watcher):

    async def _serialize(self):
        async with aiofiles.open(self.file_path) as f:
            content = await f.read()
            return my_language.load(content)

That's it.

NestWatch will automatically handle:

• Watching the file
• Detecting changes
• Comparing states
• Generating events
• Calling your listeners

Use Cases

NestWatch works especially well for:

• 🤖 Discord bots
• 🌐 FastAPI applications
• 📦 Shared caches
• ⚙️ Configuration files
• 🔄 Runtime reload systems
• 🧠 Multi-application projects

Philosophy

Traditional file watchers answer:

"Did the file change?"

NestWatch answers:

"What changed inside the file?"

NestWatch's Dependency