User Guide

The mqtt_codec package is a “weapons grade” stateless package for encoding and decoding MQTT 3.1.1 packets as defined in the specification at

The package is hyperbolically “weapons grade” because its primary goal is thoroughness of implementation and reliability. Speed is a distant secondary priority; having said this, nobody has ever lodged a complaint about the library being too slow.


A basic encode/decode cycle looks like this:

>>> from io import BytesIO
>>> from binascii import b2a_hex
>>> import mqtt_codec.packet
>>> import
>>> # Encode a Connect packet
>>> will = mqtt_codec.packet.MqttWill(qos=0, topic='hello', message='message', retain=True)
>>> connect = mqtt_codec.packet.MqttConnect(client_id='client_id', clean_session=False, keep_alive=0, will=will)
>>> with BytesIO() as f:
...   num_bytes_written = connect.encode(f)
...   buf = f.getvalue()
>>> assert len(buf) == num_bytes_written
>>> print('0x{} ({} bytes)'.format(b2a_hex(buf), len(buf)))
0x102500044d515454042400000009636c69656e745f6964000568656c6c6f00076d657373616765 (39 bytes)
>>> # Decode the connect packet and assert equality.
>>> with as f:
...   num_bytes_read, decoded_connect = connect.decode(f)
>>> assert len(buf) == num_bytes_written
>>> assert connect == decoded_connect
>>> print('  Encoded {}'.format(connect))
  Encoded MqttConnect(client_id='client_id', clean_session=False, keep_alive=0, username=***, password=***, will=MqttWill(topic=hello, payload=0x6d657373616765, retain=True, qos=0))
>>> print('= Decoded {}'.format(decoded_connect))
= Decoded MqttConnect(client_id=u'client_id', clean_session=False, keep_alive=0, username=***, password=***, will=MqttWill(topic=hello, payload=0x6d657373616765, retain=True, qos=0))


The mqtt-codec project has been tested on Linux against these environments:

  • Python 2.7
  • Python 3.4
  • Python 3.5
  • Python 3.6
  • Python 3.7

The codec likely works on Python 3.0 - 3.3 as well but these tests are not part of the standard docker container test suite.

Package Dependencies

When running Python versions less than 3.4 the enum34 package is required. Besides there are no other required packages.

Processor and Memory Usage

The maximum size of an MQTT packet is mqtt_codec.packet.MqttFixedHeader.MAX_REMAINING_LEN (=268435455 bytes). Encoding or decoding an mqtt message may consume up to this many bytes. Smaller messages require less memory to encode or decode.

While constructing an MQTT packet it is necessary to temporarily encode it so that the byte size of contained UTF-8 strings can be determined and the final packet size calculated. This means that constructing an MQTT packet can temporarily consume up to mqtt_codec.packet.MqttFixedHeader.MAX_REMAINING_LEN (=268435455 bytes) of memory and a proportionate amount of processor time. In practice most packets tend to be much smaller than this and the processor time seems small enough for most applications.

Testing and Quality

The mqtt-codec package is tested against most use and abuse cases. It has proven itself in distributed IoT environments with thousands of nodes and expected to perform as well or better than most quality industrial scale systems. There is a high bar to marking a release as stable and it usually takes more than a month of field data collection on a prospective release before this happens.

The codec has not proven itself in hostile and malicious environments and has not seen thorough 3rd-party review from a security specialist. If you are interested in assisting then please contact the author, Keegan Callin.

Semantic Versioning

The mqtt-codec package is versioned according to Semantic Versioning 2.0.0 guidelines. A summary of SemVer is included here for your convenience:

Given a version number MAJOR.MINOR.PATCH, increment the:

  1. MAJOR version when you make incompatible API changes,
  2. MINOR version when you add functionality in a backwards-compatible manner, and
  3. PATCH version when you make backwards-compatible bug fixes.

Additional labels for pre-release and build metadata are available as extensions to the MAJOR.MINOR.PATCH format.

—Semantic Versioning Summary, <>, retrieved 2018-10-01.

Bugs and Enhancements

As the maintainer of this library I, Keegan Callin, welcome your polite, constructive comments and criticisms of this library at the github issue tracker.