chumicro-msgpack-experimental 0.2.2

Compact MessagePack serialization for CircuitPython, MicroPython, and CPython — delegates to the native C module when available.

chumicro-msgpack

Binary serialization that's typically 30–50 % smaller than JSON.

The subset of MessagePack that fits on a 256 KB board — 32-bit ints, single-precision floats, 16-bit lengths. Wire-compatible with the PyPI msgpack library when it's configured for the same subset (use_single_float=True); on CircuitPython firmware that ships the native msgpack C module, encoding and decoding delegate to the C path automatically.

Part of the ChuMicro family — small, focused Python libraries for microcontrollers and laptops. Browse all libraries.

Install

# CircuitPython (after `circup bundle-add ChuMicro/ChuMicro-Bundle-Experimental`)
circup install chumicro_msgpack

# MicroPython
mpremote mip install github:ChuMicro/ChuMicro-Bundle-Experimental/chumicro_msgpack

# CPython
pip install chumicro-msgpack-experimental

For bundle setup, pre-compiled .mpy bundles, the experimental channel, and details on PyPI naming, see the chumicro INSTALL guide.

Quick example

from chumicro_msgpack import packb, unpackb

settings = {0: "MyNetwork", 1: "secret", 2: True}

data = packb(settings)       # compact binary bytes
print(len(data))             # much smaller than JSON

restored = unpackb(data)
print(restored)              # {0: 'MyNetwork', 1: 'secret', 2: True}

What's included

Stream-based API (preferred on microcontrollers)

Symbol	Description
pack(obj, stream)	Pack an object to a writable stream (streams without an intermediate buffer on the native CircuitPython encoder; on the pure path it encodes to bytes first)
unpack(stream)	Unpack one object from a readable stream

Bytes-based API

Symbol	Description
packb(obj)	Pack a Python object to msgpack bytes (allocates a temporary buffer)
unpackb(data)	Unpack msgpack bytes (bytes, bytearray, or memoryview) to a Python object

Use pack/unpack when writing to files, sockets, or NVM. Use packb/unpackb when you need the encoded bytes in memory (e.g., to measure length before framing).

Supported types

Python type	msgpack format	Limit
None	nil	—
True / False	bool	—
int	fixint, int8/16/32, uint8/16/32	-2^31 ≤ value ≤ 2^32 − 1
float	float32	single precision (~7 decimal digits)
str	fixstr, str8, str16	up to 65 535 bytes UTF-8
bytes / bytearray	bin8, bin16	up to 65 535 bytes
list / tuple	fixarray, array16	up to 65 535 elements
dict	fixmap, map16	up to 65 535 entries

Values outside these limits raise OverflowError on encode. Tags outside the subset (float64 0xcb, int64 0xd3, uint64 0xcf, the *32-length variants) raise a descriptive ValueError on decode that names the offending tag.

tuple decodes back as list — msgpack has no tuple type. dict keys may be any supported type, including int (no strict_map_key enforcement). Ext types (timestamps, custom classes) are not supported in either direction.

Cross-runtime compatibility

Direction	Works?	Notes
chumicro device writes → host reads	✓ always	Bytes are spec-compliant; any standard reader decodes them
chumicro device writes → chumicro device reads	✓ always	The common case; full round-trip
Host writes with PyPI msgpack → chumicro device reads	✓ if host stays in subset	See recipe below
Host writes default PyPI msgpack → chumicro device reads	✗	PyPI defaults to float64, decodes raise on device

Host-side recipe — when a host script needs to produce bytes a chumicro device will read:

import msgpack  # standard PyPI library
data = msgpack.packb(obj, use_single_float=True)
# Caller's job: keep ints in [-2**31, 2**32-1] and lengths under 65 536.

use_single_float=True switches PyPI msgpack from float64 to float32, matching what chumicro reads. This is what chumicro-workspace uses to write runtime_config.msgpack for the device.

Where this fits

Leaf — no upstream ChuMicro deps. Used directly by chumicro-config to decode /runtime_config.msgpack on the device, and by host-side workspace tooling to write it.

Platform support

Runtime	Implementation
CircuitPython (hardware)	Delegates to the native C msgpack module; pure-Python code is never loaded
CircuitPython (unix port)	Pure-Python encoder/decoder (native module not compiled in)
MicroPython	Pure-Python encoder/decoder
CPython	Pure-Python encoder/decoder

Examples

Example	What it shows
packb_basic.py	Pack and unpack a settings dict
packb_size_comparison.py	Compare msgpack vs JSON size for the same dict
stream_roundtrip.py	Use the stream-based pack / unpack API with BytesIO
circuitpython_nvm_settings.py	Store and load settings in non-volatile memory (hardware)

Contributing

Working on chumicro-msgpack itself? Clone the mono-repo if you haven't already — the rest of the workflow assumes you're inside that workspace.

pip install -e .[test]
pytest tests/                  # host-side tests
pytest functional_tests/       # on-device tests (needs a board registered in devices.yml)

Register a board before running functional tests: chumicro-workspace add-device <id> --address <port>.

Docs

📖 Stable docs · Experimental docs

Find this library

• PyPI: chumicro-msgpack
• Bundle: ChuMicro-Bundle (CircuitPython & MicroPython)
• Experimental bundle: ChuMicro-Bundle-Experimental
• Source: libraries/msgpack

License

MIT