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.
Very important notes for existing users
PyPI package name
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.
Compatibility with the old format
You can use use_bin_type=False option to pack bytes object into raw type in the old msgpack spec, instead of bin type in new msgpack spec.
You can unpack old msgpack format using raw=True option. It unpacks str (raw) type in msgpack into Python bytes.
See note below for detail.
Major breaking changes in msgpack 1.0
-
Python 2
- The extension module does not support Python 2 anymore. The pure Python implementation (
msgpack.fallback) is used for Python 2.
- The extension module does not support Python 2 anymore. The pure Python implementation (
-
Packer
use_bin_type=Trueby default. bytes are encoded in bin type in msgpack. If you are still using Python 2, you must use unicode for all string types. You can useuse_bin_type=Falseto encode into old msgpack format.encodingoption is removed. UTF-8 is used always.
-
Unpacker
raw=Falseby default. It assumes str types are valid UTF-8 string and decode them to Python str (unicode) object.encodingoption is removed. You can useraw=Trueto support old format.- Default value of
max_buffer_sizeis changed from 0 to 100 MiB. - Default value of
strict_map_keyis changed to True to avoid hashdos. You need to passstrict_map_key=Falseif you have data which contain map keys which type is not bytes or str.
Install
$ pip install msgpack
Pure Python implementation
The extension module in msgpack (msgpack._cmsgpack) does not support Python 2 and PyPy.
But msgpack provides a pure Python implementation (msgpack.fallback) for PyPy and Python 2.
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
NOTE: In examples below, I use raw=False and use_bin_type=True for users using msgpack < 1.0. These options are default from msgpack 1.0 so you can omit them.
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], use_bin_type=True)
'\x93\x01\x02\x03'
>>> msgpack.unpackb(_, raw=False)
[1, 2, 3]
unpack unpacks msgpack's array to Python's list, but can also unpack to tuple:
>>> msgpack.unpackb(b'\x93\x01\x02\x03', use_list=False, raw=False)
(1, 2, 3)
You should always specify the use_list keyword argument for backward compatibility. See performance issues relating to use_list option_ below.
Read the docstring for other 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, use_bin_type=True))
buf.seek(0)
unpacker = msgpack.Unpacker(buf, raw=False)
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, use_bin_type=True)
this_dict_again = msgpack.unpackb(packed_dict, object_hook=decode_datetime, raw=False)
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.
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, use_bin_type=True)
>>> unpacked = msgpack.unpackb(packed, ext_hook=ext_hook, raw=False)
>>> 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
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', u'eggs'], use_bin_type=False), raw=True)
[b'spam', b'eggs']
>>> msgpack.unpackb(msgpack.packb([b'spam', u'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.
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