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Update README with thread safety section by lysnikolaou · Pull Request #657 · msgpack/msgpack-python · GitHub

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Update README with thread safety section #657

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Update README with thread safety section #657

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Expand Up @@ -204,6 +204,41 @@ However, a tuple is lighter than a list.
You can use `use_list=False` while unpacking when performance is important.


### Thread safety (CPython 3.14+ free-threaded build)

msgpack is designed to be thread-safe when used with CPython 3.14's
free-threaded build (PEP 703).

#### Thread-Safety Guarantees

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

methane Jun 12, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

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.
You can safely call methods on the same `Packer` or `Unpacker` instance
from multiple threads concurrently. All public methods are protected by
critical sections that ensure atomic access to internal state.

* **Module-level functions are thread-safe**: Functions like `packb()` and
`unpackb()` create their own instances internally and are safe to call
from multiple threads.

#### Performance Considerations

While sharing a single `Packer` or `Unpacker` instance across threads is safe,
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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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

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.

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```python
```py

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import msgpack
from concurrent.futures import ThreadPoolExecutor

def worker(data):
# Each thread creates its own packer
packer = msgpack.Packer()
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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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

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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```

## Major breaking changes in the history

### msgpack 0.5
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