Awesome
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.
- Packer uses
-
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 useraw=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 passmax_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 passstrict_map_key=False
if you have data which contain map keys which type is not bytes or str.
- Unpacker uses