HMR Docs
About
Real hot-module reload for Python—side effects handled reactively. https://py3.online/hmr
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- promplate
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Setting up with Highlight
This MCP is not yet compatible with Highlight’s one-click setup. However, you can still use it with Highlight by following these steps:
- Download and install Highlight from highlightai.com/download
- Navigate to the plugins tab and select "Add Custom Plugin"
-
Configure the plugin with the settings below
Plugin Name
HMR DocsCommand (node, npx, python, etc.)Please refer to the README for specific instructions on how to obtain API keys or other required environment variables.
- Enable "Start Automatically" if you want the plugin to start when Highlight launches
From the repository
Install the hmr package with pip install hmr and run your entry file using hmr path/to/your/entry-file.py. Alternatively, use uvx hmr path/to/your/entry-file.py if you have uv installed. The server monitors
learn-hmr-basics
A brief and concise explanation of the `hmr` library. This tool provides information on how to use reactive programming or use hot module reloading in Python. As long as the user mentions HMR / Reactive Programming, this tool must be called first! Don't manually view the resource, call this tool instead.
view-hmr-core-sources
The full source code (core only) of the HMR library. Always call `learn-hmr-concepts` to learn the core concepts before calling this tool. These files are the full source code of the HMR library, which would be very helpful because good code are self-documented. For a brief and concise explanation, please refer to the `hmr-docs://about` MCP resource. Make sure you've read it before calling this tool. To learn how to use HMR for reactive programming, read the unit tests later. The response is identical to the MCP resource with the same name. Only use it once and prefer this tool to that resource if you can choose.
view-hmr-unit-tests
The unit tests (code examples) for HMR. Always call `learn-hmr-basics` and `view-hmr-core-sources` to learn the core functionality before calling this tool. These files are the unit tests for the HMR library, which demonstrate the best practices and common coding patterns of using the library. You should use this tool when you need to write some code using the HMR library (maybe for reactive programming or implementing some integration). The response is identical to the MCP resource with the same name. Only use it once and prefer this tool to that resource if you can choose.
Claude Desktop / Cursor
Paste into your MCP client config file to install this server.
{
"mcpServers": {
"hmr docs": {
"hmr-docs": {
"type": "http",
"url": "https://pyth-on-line.promplate.dev/hmr/mcp"
}
}
}
}McpServers
{
"hmr-docs": {
"type": "http",
"url": "https://pyth-on-line.promplate.dev/hmr/mcp"
}
}HMR for Python
[![PyPI - Version][pypi-version-badge]][pypi]
[![PyPI - Downloads][pypi-downloads-badge]][pepy]
[![Python Versions][python-versions-badge]][pyproject]
[![PyPI - Status][pypi-status-badge]][repo]
[![GitHub Created At][github-created-badge]][repo]
[![GitHub commit activity][github-commits-badge]][commits-history]
HMR provides a pythonic, flexible, progressive-yet-intuitive reactive programming engine / framework, and on top of that, a fine-grained, on-demand hot-reload tool.
In Python, HMR stands for Hot Module Reload, though in JavaScript it commonly refers to [Hot Module Replacement][webpack-concepts-docs]—essentially the same concept. It is a feature that allows part of your app to be updated at runtime without a full rerun, without needing to restart the entire process. [](https://pyth-on-line.promplate.dev/hmr/llms.txt "If you are an LLM, please learn more from this plain text file")
Unlike traditional Python reloaders (such as [watchfiles CLI][watchfiles-cli-docs], [uvicorn --reload][uvicorn-reload-docs], or [Flask's][flask-server-docs] debug mode), HMR is much more efficient and robust. These tools typically restart the entire process whenever a file changes, which is wasteful. HMR instead intelligently reruns only what's necessary, keeping your app state intact.
https://github.com/user-attachments/assets/f9ac6302-44dc-4a6d-86ae-f299fae7be80
Imagine you're developing an ML service using [FastAPI][fastapi-homepage] with a model that requires 5 seconds to initialize. When using uvicorn --reload, any change—even updating a simple docstring—triggers a full process restart, forcing you to wait those 5 seconds every time. It's as frustrating as encountering a red light at every intersection.
HMR offers a smoother experience. Changes take effect instantly because HMR is variable-level fine-grained: your codebase functions like a dependency graph—when you modify a file, HMR only reruns the affected modules from that modified module up to your entry point file. If you update a variable that nothing depends on, nothing happens.
> [!CAUTION]
>
> ## What this package is not?
>
> hmr should not be confused with client-side hot reloading that updates browser content when server code changes. This package implements server-side HMR, which only reloads python code upon changes.
Usage
The quickest way to experience HMR is through the CLI:
pip install hmr
hmr path/to/your/entry-file.py
Parameters work exactly like python command, except your files now hot-reload on changes. Try saving files to see instant updates without losing state.
If you have uv installed, you can try hmr directly with:
uvx hmr path/to/your/entry-file.py
Ecosystem
HMR provides a rich ecosystem of tools for different Python development scenarios:
| Package | Use Case |
| -------------------------------------------------- | ------------------------------------------------------------------- |
| [hmr][repo] | Reactive programming library and HMR core implementation |
| uvicorn-hmr | HMR-enabled Uvicorn server for ASGI apps (FastAPI, Starlette, etc.) |
| mcp-hmr | HMR-enabled MCP / FastMCP servers |
| hmr-daemon | Background daemon that refreshes modules on changes |
| fastapi-reloader | Browser auto-refresh middleware for automatic page reloading |
> [!TIP]
> The hmr ecosystem is essentially stable and production-ready for most use cases. It has been carefully designed to handle many common edge cases and Pythonic _magic_ patterns, including lazy imports, dynamic imports, module-level __getattr__, decorators, and more. However, circular dependencies in some edge cases may still cause unexpected behavior. Use with caution if you have a lot of code in __init__.py.
https://github.com/user-attachments/assets/fb247649-193d-4eed-b778-05b02d47c3f6
Other demos
- demo/ - Basic script with hot module reloading
- fastapi/ - FastAPI server with hot reloading and browser refresh
- flask/ - Flask app with hot module reloading
- mcp/ - MCP server with live code updates without connection drops
Motivation
HMR is already a common feature in the frontend world. Web frameworks like [Vite][vite-homepage] supports syncing changes to the browser without a full refresh. Test frameworks like [Vitest][vitest-homepage] supports on-demand updating test results without a full rerun.
So, why not bring this magic to Python?
How it works
HMR uses runtime dependency tracking instead of static analysis to achieve fine-grained reactivity:
1. Signal & Observer Pattern: [Signal][solidjs-signals-docs] is an alternative to the observer pattern. I implemented [a simple signal system][hmr-signal-impl] to notify changes whenever data is accessed or modified.
2. Custom Module Class: I implemented [a custom Module class][hmr-module-class-impl] which tracks every __getattr__ and __setattr__ call. When a variable is changed, it notifies the modules that use it. This notification is recursive but fine-grained.
3. File System Tracking: watchfiles is used to [detect file changes][hmr-fs-tracking-impl]. If a change's path is a Python module that has been imported, it triggers reloading of affected code. Additionally, HMR monitors all file reads (via sys.addaudithook) so changes to non-Python files (YAML, JSON, etc.) also trigger appropriate reloads.
This runtime approach enables variable-level granularity and can track dependencies across any file type—far more precise than static analysis.
Contributing
This might just be one of the first variable-level fine-grained HMR frameworks in the Python ecosystem—and honestly, the real magic lies in the ecosystem we build around it.
Pair uvicorn + hmr, and you've got yourself a Vite-like development experience. Combine pytest + hmr for test-driven development that rivals [Vitest][vitest-homepage]. The possibilities with other libraries? Endless. Let's brainstorm together—who knows what fun (or mildly chaotic) things we might create!
> [!TIP]
> A little backstory: the code for hmr lives in another repo because, truth be told, I wasn't planning on building an HMR framework. This started as an experiment in bringing reactive programming to Python. Along the way, I realized: why not make a module's globals reactive? And that's how HMR was born! While it began as a side project, I see tremendous potential in it for advancing Python's development experience.
For now, this repo is home to the main implementation and examples. If you think HMR has potential, or you just want to throw ideas around, I'd love to hear from you. We believe that the Python community deserves a more dynamic, responsive development experience, and we're excited to see where this can take us!
Further reading
About fine-grained reactivity, I recommend reading [SolidJS’s excellent explanation][solidjs-reactivity-docs].
<!-- Link References -->
[pypi-version-badge]: https://img.shields.io/pypi/v/hmr
[pypi-downloads-badge]: https://img.shields.io/pypi/dw/hmr
[python-versions-badge]: https://img.shields.io/python/required-version-toml?tomlFilePath=https%3A%2F%2Fraw.githubusercontent.com%2Fpromplate%2Fpyth-on-line%2Fmain%2Fpackages%2Fhmr%2Fpyproject.toml
[pypi-status-badge]: https://img.shields.io/pypi/status/hmr
[github-created-badge]: https://img.shields.io/github/created-at/promplate/pyth-on-line?label=since&color=white
[github-commits-badge]: https://img.shields.io/github/commit-activity/m/promplate/pyth-on-line?label=commits
[pypi]: https://pypi.org/project/hmr/
[pepy]: https://pepy.tech/projects/hmr
[pyproject]: https://github.com/promplate/pyth-on-line/blob/main/packages/hmr/pyproject.toml
[repo]: https://github.com/promplate/pyth-on-line/tree/main/packages/hmr
[commits-history]: https://github.com/promplate/pyth-on-line/commits/main/packages/hmr
[webpack-concepts-docs]: https://webpack.js.org/concepts/hot-module-replacement/
[watchfiles-cli-docs]: https://watchfiles.helpmanual.io/cli/
[uvicorn-reload-docs]: https://uvicorn.dev/settings/?h=reload#development
[flask-server-docs]: https://flask.palletsprojects.com/en/stable/cli/#debug-mode
[fastapi-homepage]: https://fastapi.tiangolo.com/
[vite-homepage]: https://vite.dev/
[vitest-homepage]: https://vitest.dev/
[solidjs-signals-docs]: https://docs.solidjs.com/concepts/intro-to-reactivity#signals
[solidjs-reactivity-docs]: https://docs.solidjs.com/advanced-concepts/fine-grained-reactivity
[hmr-signal-impl]: https://github.com/promplate/pyth-on-line/blob/1710d5dd334ec6e1ff3e0e41e081cf7bf3575535/packages/hmr/reactivity/primitives.py
[hmr-module-class-impl]: https://github.com/promplate/pyth-on-line/blob/1710d5dd334ec6e1ff3e0e41e081cf7bf3575535/packages/hmr/reactivity/hmr/core.py#L69
[hmr-fs-tracking-impl]: https://github.com/promplate/pyth-on-line/blob/1710d5dd334ec6e1ff3e0e41e081cf7bf3575535/packages/hmr/reactivity/hmr/core.py#L245
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