# MessagePack for Python

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## What is this?

[MessagePack](https://msgpack.org/) 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.

```pycon
>>> 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).

```py
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`.

```py
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.

```pycon
>>> 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.

```pycon
>>> 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.

```pycon
>>> 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.