MessagePack for Python

What’s this

MessagePack is an efficient binary serialization format.
It lets you exchange data among multiple languages like JSON.
But it’s faster and smaller.
This package provides CPython bindings for reading and writing MessagePack data.

Install

$ pip install msgpack

Pure Python implementation

The extension module in msgpack (msgpack._cmsgpack) does not support PyPy.

But msgpack provides a pure Python implementation (msgpack.fallback) for PyPy.

Windows

When you can’t use a binary distribution, you need to install Visual Studio
or Windows SDK on Windows.
Without extension, using pure Python implementation on CPython runs slowly.

How to use

One-shot pack & unpack

Use packb for packing and unpackb for unpacking.
msgpack provides dumps and loads as an alias for compatibility with
json and pickle.

pack and dump packs to a file-like object.
unpack and load unpacks from a file-like object.

>>> import msgpack
>>> msgpack.packb([1, 2, 3])
'\x93\x01\x02\x03'
>>> msgpack.unpackb(_)
[1, 2, 3]

Read the docstring for options.

Streaming unpacking

Unpacker is a “streaming unpacker”. It unpacks multiple objects from one
stream (or from bytes provided through its feed method).

import msgpack
from io import BytesIO

buf = BytesIO()
for i in range(100):
   buf.write(msgpack.packb(i))

buf.seek(0)

unpacker = msgpack.Unpacker(buf)
for unpacked in unpacker:
    print(unpacked)

Packing/unpacking of custom data type

It is also possible to pack/unpack custom data types. Here is an example for
datetime.datetime.

import datetime
import msgpack

useful_dict = {
    "id": 1,
    "created": datetime.datetime.now(),
}

def decode_datetime(obj):
    if '__datetime__' in obj:
        obj = datetime.datetime.strptime(obj["as_str"], "%Y%m%dT%H:%M:%S.%f")
    return obj

def encode_datetime(obj):
    if isinstance(obj, datetime.datetime):
        return {'__datetime__': True, 'as_str': obj.strftime("%Y%m%dT%H:%M:%S.%f")}
    return obj


packed_dict = msgpack.packb(useful_dict, default=encode_datetime)
this_dict_again = msgpack.unpackb(packed_dict, object_hook=decode_datetime)

Unpacker’s object_hook callback receives a dict; the
object_pairs_hook callback may instead be used to receive a list of
key-value pairs.

NOTE: msgpack can encode datetime with tzinfo into standard ext type for now.
See datetime option in Packer docstring.

Extended types

It is also possible to pack/unpack custom data types using the ext type.

>>> import msgpack
>>> import array
>>> def default(obj):
...     if isinstance(obj, array.array) and obj.typecode == 'd':
...         return msgpack.ExtType(42, obj.tostring())
...     raise TypeError("Unknown type: %r" % (obj,))
...
>>> def ext_hook(code, data):
...     if code == 42:
...         a = array.array('d')
...         a.fromstring(data)
...         return a
...     return ExtType(code, data)
...
>>> data = array.array('d', [1.2, 3.4])
>>> packed = msgpack.packb(data, default=default)
>>> unpacked = msgpack.unpackb(packed, ext_hook=ext_hook)
>>> data == unpacked
True

Advanced unpacking control

As an alternative to iteration, Unpacker objects provide unpack,
skip, read_array_header and read_map_header methods. The former two
read an entire message from the stream, respectively de-serialising and returning
the result, or ignoring it. The latter two methods return the number of elements
in the upcoming container, so that each element in an array, or key-value pair
in a map, can be unpacked or skipped individually.

Notes

string and binary type in old msgpack spec

Early versions of msgpack didn’t distinguish string and binary types.
The type for representing both string and binary types was named raw.

You can pack into and unpack from this old spec using use_bin_type=False
and raw=True options.

>>> import msgpack
>>> msgpack.unpackb(msgpack.packb([b'spam', 'eggs'], use_bin_type=False), raw=True)
[b'spam', b'eggs']
>>> msgpack.unpackb(msgpack.packb([b'spam', 'eggs'], use_bin_type=True), raw=False)
[b'spam', 'eggs']

ext type

To use the ext type, pass msgpack.ExtType object to packer.

>>> import msgpack
>>> packed = msgpack.packb(msgpack.ExtType(42, b'xyzzy'))
>>> msgpack.unpackb(packed)
ExtType(code=42, data='xyzzy')

You can use it with default and ext_hook. See below.

Security

To unpacking data received from unreliable source, msgpack provides
two security options.

max_buffer_size (default: 100*1024*1024) limits the internal buffer size.
It is used to limit the preallocated list size too.

strict_map_key (default: True) limits the type of map keys to bytes and str.
While msgpack spec doesn’t limit the types of the map keys,
there is a risk of the hashdos.
If you need to support other types for map keys, use strict_map_key=False.

Performance tips

CPython’s GC starts when growing allocated object.
This means unpacking may cause useless GC.
You can use gc.disable() when unpacking large message.

List is the default sequence type of Python.
But tuple is lighter than list.
You can use use_list=False while unpacking when performance is important.

Major breaking changes in the history

msgpack 0.5

Package name on PyPI was changed from msgpack-python to msgpack from 0.5.

When upgrading from msgpack-0.4 or earlier, do pip uninstall msgpack-python before
pip install -U msgpack.

msgpack 1.0

• Python 2 support

  • The extension module does not support Python 2 anymore.
    The pure Python implementation (msgpack.fallback) is used for Python 2.

  • msgpack 1.0.6 drops official support of Python 2.7, as pip and
    GitHub Action (setup-python) no longer support Python 2.7.

• Packer

  • Packer uses use_bin_type=True by default.
    Bytes are encoded in bin type in msgpack.
  • The encoding option is removed. UTF-8 is used always.

• Unpacker

  • Unpacker uses raw=False by default. It assumes str types are valid UTF-8 string
    and decode them to Python str (unicode) object.
  • encoding option is removed. You can use raw=True to support old format (e.g. unpack into bytes, not str).
  • Default value of max_buffer_size is changed from 0 to 100 MiB to avoid DoS attack.
    You need to pass max_buffer_size=0 if you have large but safe data.
  • Default value of strict_map_key is changed to True to avoid hashdos.
    You need to pass strict_map_key=False if you have data which contain map keys
    which type is not bytes or str.