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GeezerVison is showing github.com — Update README with thread safety section by lysnikolaou · Pull Request #657 · msgpack/msgpack-python

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@lysnikolaou

@lysnikolaou lysnikolaou commented Oct 24, 2025

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Since the next version is going to be supporting the free-threaded build, I thought it might be nice to have a note about it in the README. If it's desirable, I can also update the docs.

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@lysnikolaou
Update README with thread safety section
e9f812c
@methane
methane requested a review from Copilot November 29, 2025 13:56
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Pull request overview

This PR adds documentation to the README explaining thread safety guarantees for msgpack when used with CPython 3.14's free-threaded build (PEP 703). The documentation clarifies that Packer and Unpacker instances are thread-safe through critical section locking, and provides performance guidance on creating separate instances per thread for optimal performance.

Key changes:

  • Documents thread safety guarantees for Packer and Unpacker instances
  • Explains thread safety of module-level functions like packb() and unpackb()
  • Provides a code example demonstrating the recommended pattern for multi-threaded usage

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Comment thread README.md
return packer.pack(data)

with ThreadPoolExecutor(max_workers=4) as executor:
results = executor.map(worker, <data>)

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The code example contains a syntax error with the placeholder <data>. This should be replaced with a concrete example like [1, 2, 3, 4] or data_items to make the example functional and executable.

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results = executor.map(worker, <data>)
results = executor.map(worker, [1, 2, 3, 4])

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Comment thread README.md
it may serialize operations due to critical section locking. For optimal
performance in multi-threaded applications, create separate instances per thread:

```python

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The code block uses python which is inconsistent with the existing README convention. Other non-interactive code examples in this README use py (see lines 60, 81). For consistency, this should be changed to ```py.

Suggested change
```python
```py

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Comment thread README.md
return packer.pack(data)

with ThreadPoolExecutor(max_workers=4) as executor:
results = executor.map(worker, <data>)

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The new thread safety documentation and recommended usage pattern lacks test coverage. Consider adding a test case that validates thread-safe usage of Packer and Unpacker instances across multiple threads, similar to the example provided in the documentation.

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@lysnikolaou

lysnikolaou commented Dec 23, 2025

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@methane A gentle ping on this. I don't think the Copilot comments are very relevant and I still feel this PR is ready to go.
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@clin1234

clin1234 commented Jun 11, 2026

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@methane A gentle ping on this. I don't think the Copilot comments are very relevant and I still feel this PR is ready to go.

Your wish is about to be fulfilled: https://github.com/msgpack/msgpack-python/releases/tag/v1.2.0
;-)
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methane
methane reviewed Jun 12, 2026
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Comment thread README.md

#### Thread-Safety Guarantees

* **Individual `Packer` and `Unpacker` instances are thread-safe**:

@methane methane Jun 12, 2026

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thread-safe is a very strong guarantee. Packer and Unpacker are designed to not crash when used from multiple threads, but the correctness of the output depends on the types and timing of the methods being called simultaneously.

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@lysnikolaou @clin1234 @methane
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