MessagePack for Python

What is 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

If you can't use a binary distribution, you need to install Visual Studio or the Windows SDK on Windows. Without the extension, the 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 aliases for compatibility with json and pickle.

pack and dump pack to a file-like object. unpack and load unpack 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 types

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 deserializing 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 types in the old MessagePack 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 a msgpack.ExtType object to the 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

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

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

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

Performance tips

CPython's GC starts when the number of allocated objects grows. This means unpacking may trigger unnecessary GC. You can use gc.disable() when unpacking a large message.

A list is the default sequence type in Python. However, a tuple is lighter than a list. You can use use_list=False while unpacking when performance is important.

Major breaking changes in the history

msgpack 0.5

The package name on PyPI was changed from msgpack-python to msgpack in 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 no longer supports Python 2. 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 supports Python 2.7.

• Packer

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

• Unpacker

  • Unpacker uses raw=False by default. It assumes str values are valid UTF-8 strings and decodes them to Python str (Unicode) objects.
  • encoding option is removed. You can use raw=True to support old format (e.g. unpack into bytes, not str).
  • The default value of max_buffer_size is changed from 0 to 100 MiB to avoid DoS attacks. You need to pass max_buffer_size=0 if you have large but safe data.
  • The default value of strict_map_key is changed to True to avoid hash DoS. You need to pass strict_map_key=False if you have data that contain map keys whose type is neither bytes nor str.