Package: paho-mqtt
Thefull documentation is available here.This document describes the source code for theEclipse PahoMQTT Python client library, which implements versions 5.0, 3.1.1, and 3.1 of the MQTT protocol.This code provides a client class which enables applications to connect to anMQTTbroker to publish messages, and to subscribe to topics and receive published messages. It also provides some helper functions to make publishing one off messages to an MQTT server very straightforward.It supports Python 3.7+.The MQTT protocol is a machine-to-machine (M2M)/”Internet of Things” connectivity protocol. Designed as an extremely lightweight publish/subscribe messaging transport, it is useful for connections with remote locations where a small code footprint is required and/or network bandwidth is at a premium.Paho is anEclipse Foundationproject.ContentsInstallationKnown limitationsUsage and APIGetting StartedClientNetwork loopCallbacksLoggerExternal event loop supportGlobal helper functionsPublishSingleMultipleSubscribeSimpleUsing CallbackReporting bugsMore informationInstallationThe latest stable version is available in the Python Package Index (PyPi) and can be installed usingpip install paho-mqttOr withvirtualenv:virtualenv paho-mqtt
source paho-mqtt/bin/activate
pip install paho-mqttTo obtain the full code, including examples and tests, you can clone the git repository:git clone https://github.com/eclipse/paho.mqtt.pythonOnce you have the code, it can be installed from your repository as well:cd paho.mqtt.python
pip install -e .To perform all tests (including MQTT v5 tests), you also need to clone paho.mqtt.testing in paho.mqtt.python folder:git clone https://github.com/eclipse/paho.mqtt.testing.git
cd paho.mqtt.testing
git checkout a4dc694010217b291ee78ee13a6d1db812f9babdKnown limitationsThe following are the known unimplemented MQTT features.Whenclean_sessionis False, the session is only stored in memory and not persisted. This means that
when the client is restarted (not just reconnected, the object is recreated usually because the
program was restarted) the session is lost. This results in a possible message loss.The following part of the client session is lost:QoS 2 messages which have been received from the server, but have not been completely acknowledged.Since the client will blindly acknowledge any PUBCOMP (last message of a QoS 2 transaction), it
won’t hang but will lose this QoS 2 message.QoS 1 and QoS 2 messages which have been sent to the server, but have not been completely acknowledged.This means that messages passed topublish()may be lost. This could be mitigated by taking care
that all messages passed topublish()have a correspondingon_publish()call or usewait_for_publish.It also means that the broker may have the QoS2 message in the session. Since the client starts
with an empty session it don’t know it and will reuse the mid. This is not yet fixed.Also, whenclean_sessionis True, this library will republish QoS > 0 message across network
reconnection. This means that QoS > 0 message won’t be lost. But the standard says that
we should discard any message for which the publish packet was sent. Our choice means that
we are not compliant with the standard and it’s possible for QoS 2 to be received twice.You should setclean_session = Falseif you need the QoS 2 guarantee of only one delivery.Usage and APIDetailed API documentationis available onlineor could be built fromdocs/and samples are available in theexamplesdirectory.The package provides two modules, a fullClientand fewhelpersfor simple publishing or subscribing.Getting StartedHere is a very simple example that subscribes to the broker $SYS topic tree and prints out the resulting messages:importpaho.mqtt.clientasmqtt# The callback for when the client receives a CONNACK response from the server.defon_connect(client,userdata,flags,reason_code,properties):print(f"Connected with result code{reason_code}")# Subscribing in on_connect() means that if we lose the connection and# reconnect then subscriptions will be renewed.client.subscribe("$SYS/#")# The callback for when a PUBLISH message is received from the server.defon_message(client,userdata,msg):print(msg.topic+" "+str(msg.payload))mqttc=mqtt.Client(mqtt.CallbackAPIVersion.VERSION2)mqttc.on_connect=on_connectmqttc.on_message=on_messagemqttc.connect("mqtt.eclipseprojects.io",1883,60)# Blocking call that processes network traffic, dispatches callbacks and# handles reconnecting.# Other loop*() functions are available that give a threaded interface and a# manual interface.mqttc.loop_forever()ClientYou can use the client class as an instance, within a class or by subclassing. The general usage flow is as follows:Create a client instanceConnect to a broker using one of theconnect*()functionsCall one of theloop*()functions to maintain network traffic flow with the brokerUsesubscribe()to subscribe to a topic and receive messagesUsepublish()to publish messages to the brokerUsedisconnect()to disconnect from the brokerCallbacks will be called to allow the application to process events as necessary. These callbacks are described below.Network loopThese functions are the driving force behind the client. If they are not
called, incoming network data will not be processed and outgoing network data
will not be sent. There are four options for managing the
network loop. Three are described here, the fourth in “External event loop
support” below. Do not mix the different loop functions.loop_start() / loop_stop()mqttc.loop_start()whileTrue:temperature=sensor.blocking_read()mqttc.publish("paho/temperature",temperature)mqttc.loop_stop()These functions implement a threaded interface to the network loop. Callingloop_start()once, before or afterconnect*(), runs a thread in the
background to callloop()automatically. This frees up the main thread for
other work that may be blocking. This call also handles reconnecting to the
broker. Callloop_stop()to stop the background thread.
The loop is also stopped if you calldisconnect().loop_forever()mqttc.loop_forever(retry_first_connection=False)This is a blocking form of the network loop and will not return until the
client callsdisconnect(). It automatically handles reconnecting.Except for the first connection attempt when usingconnect_async, useretry_first_connection=Trueto make it retry the first connection.Warning: This might lead to situations where the client keeps connecting to an
non existing host without failing.loop()run=Truewhilerun:rc=mqttc.loop(timeout=1.0)ifrc!=0:# need to handle error, possible reconnecting or stopping the applicationCall regularly to process network events. This call waits inselect()until
the network socket is available for reading or writing, if appropriate, then
handles the incoming/outgoing data. This function blocks for up totimeoutseconds.timeoutmust not exceed thekeepalivevalue for the client or
your client will be regularly disconnected by the broker.Using this kind of loop, require you to handle reconnection strategie.CallbacksThe interface to interact with paho-mqtt include various callback that are called by
the library when some events occur.The callbacks are functions defined in your code, to implement the require action on those events. This could
be simply printing received message or much more complex behaviour.Callbacks API is versioned, and the selected version is theCallbackAPIVersionyou provided toClientconstructor. Currently two version are supported:CallbackAPIVersion.VERSION1: it’s the historical version used in paho-mqtt before version 2.0.
It’s the API used before the introduction ofCallbackAPIVersion.
This version is deprecated and will be removed in paho-mqtt version 3.0.CallbackAPIVersion.VERSION2: This version is more consistent between protocol MQTT 3.x and MQTT 5.x. It’s also
much more usable with MQTT 5.x since reason code and properties are always provided when available.
It’s recommended for all user to upgrade to this version. It’s highly recommended for MQTT 5.x user.The following callbacks exists:on_connect(): called when the CONNACK from the broker is received. The call could be for a refused connection,
check the reason_code to see if the connection is successful or rejected.on_connect_fail(): called byloop_forever()andloop_start()when the TCP connection failed to establish.
This callback is not called when usingconnect()orreconnect()directly. It’s only called following
an automatic (re)connection made byloop_start()andloop_forever()on_disconnect(): called when the connection is closed.on_message(): called when a MQTT message is received from the broker.on_publish(): called when an MQTT message was sent to the broker. Depending on QoS level the callback is called
at different moment:For QoS == 0, it’s called as soon as the message is sent over the network. This could be before the correspondingpublish()return.For QoS == 1, it’s called when the corresponding PUBACK is received from the brokerFor QoS == 2, it’s called when the corresponding PUBCOMP is received from the brokeron_subscribe(): called when the SUBACK is received from the brokeron_unsubscribe(): called when the UNSUBACK is received from the brokeron_log(): called when the library log a messageon_socket_open,on_socket_close,on_socket_register_write,on_socket_unregister_write: callbacks used for external loop support. See below for details.For the signature of each callback, see theonline documentation.Subscriber exampleimportpaho.mqtt.clientasmqttdefon_subscribe(client,userdata,mid,reason_code_list,properties):# Since we subscribed only for a single channel, reason_code_list contains# a single entryifreason_code_list[0].is_failure:print(f"Broker rejected you subscription:{reason_code_list[0]}")else:print(f"Broker granted the following QoS:{reason_code_list[0].value}")defon_unsubscribe(client,userdata,mid,reason_code_list,properties):# Be careful, the reason_code_list is only present in MQTTv5.# In MQTTv3 it will always be emptyiflen(reason_code_list)==0ornotreason_code_list[0].is_failure:print("unsubscribe succeeded (if SUBACK is received in MQTTv3 it success)")else:print(f"Broker replied with failure:{reason_code_list[0]}")client.disconnect()defon_message(client,userdata,message):# userdata is the structure we choose to provide, here it's a list()userdata.append(message.payload)# We only want to process 10 messagesiflen(userdata)>=10:client.unsubscribe("$SYS/#")defon_connect(client,userdata,flags,reason_code,properties):ifreason_code.is_failure:print(f"Failed to connect:{reason_code}. loop_forever() will retry connection")else:# we should always subscribe from on_connect callback to be sure# our subscribed is persisted across reconnections.client.subscribe("$SYS/#")mqttc=mqtt.Client(mqtt.CallbackAPIVersion.VERSION2)mqttc.on_connect=on_connectmqttc.on_message=on_messagemqttc.on_subscribe=on_subscribemqttc.on_unsubscribe=on_unsubscribemqttc.user_data_set([])mqttc.connect("mqtt.eclipseprojects.io")mqttc.loop_forever()print(f"Received the following message:{mqttc.user_data_get()}")publisher exampleimporttimeimportpaho.mqtt.clientasmqttdefon_publish(client,userdata,mid,reason_code,properties):# reason_code and properties will only be present in MQTTv5. It's always unset in MQTTv3try:userdata.remove(mid)exceptKeyError:print("on_publish() is called with a mid not present in unacked_publish")print("This is due to an unavoidable race-condition:")print("* publish() return the mid of the message sent.")print("* mid from publish() is added to unacked_publish by the main thread")print("* on_publish() is called by the loop_start thread")print("While unlikely (because on_publish() will be called after a network round-trip),")print(" this is a race-condition that COULD happen")print("")print("The best solution to avoid race-condition is using the msg_info from publish()")print("We could also try using a list of acknowledged mid rather than removing from pending list,")print("but remember that mid could be re-used !")unacked_publish=set()mqttc=mqtt.Client(mqtt.CallbackAPIVersion.VERSION2)mqttc.on_publish=on_publishmqttc.user_data_set(unacked_publish)mqttc.connect("mqtt.eclipseprojects.io")mqttc.loop_start()# Our application produce some messagesmsg_info=mqttc.publish("paho/test/topic","my message",qos=1)unacked_publish.add(msg_info.mid)msg_info2=mqttc.publish("paho/test/topic","my message2",qos=1)unacked_publish.add(msg_info2.mid)# Wait for all message to be publishedwhilelen(unacked_publish):time.sleep(0.1)# Due to race-condition described above, the following way to wait for all publish is safermsg_info.wait_for_publish()msg_info2.wait_for_publish()mqttc.disconnect()mqttc.loop_stop()LoggerThe Client emit some log message that could be useful during troubleshooting. The easiest way to
enable logs is the callenable_logger(). It’s possible to provide a custom logger or let the
default logger being used.Example:importloggingimportpaho.mqtt.clientasmqttlogging.basicConfig(level=logging.DEBUG)mqttc=mqtt.Client(mqtt.CallbackAPIVersion.VERSION2)mqttc.enable_logger()mqttc.connect("mqtt.eclipseprojects.io",1883,60)mqttc.loop_start()# Do additional action needed, publish, subscribe, ...[...]It’s also possible to define a on_log callback that will receive a copy of all log messages. Example:importpaho.mqtt.clientasmqttdefon_log(client,userdata,paho_log_level,messages):ifpaho_log_level==mqtt.LogLevel.MQTT_LOG_ERR:print(message)mqttc=mqtt.Client(mqtt.CallbackAPIVersion.VERSION2)mqttc.on_log=on_logmqttc.connect("mqtt.eclipseprojects.io",1883,60)mqttc.loop_start()# Do additional action needed, publish, subscribe, ...[...]The correspondence with Paho logging levels and standard ones is the following:PahologgingMQTT_LOG_ERRlogging.ERRORMQTT_LOG_WARNINGlogging.WARNINGMQTT_LOG_NOTICElogging.INFO(no direct equivalent)MQTT_LOG_INFOlogging.INFOMQTT_LOG_DEBUGlogging.DEBUGExternal event loop supportTo support other network loop like asyncio (seeexamples), the library expose some
method and callback to support those use-case.The following loop method exists:loop_read: should be called when the socket is ready for reading.loop_write: should be called when the socket is ready for writing AND the library want to write data.loop_misc: should be called every few seconds to handle message retrying and pings.In pseudo code, it give the following:whilerun:ifneed_read:mqttc.loop_read()ifneed_write:mqttc.loop_write()mqttc.loop_misc()ifnotneed_readandnotneed_write:# But don't wait more than few seconds, loop_misc() need to be called regularlywait_for_change_in_need_read_or_write()updated_need_read_and_write()The tricky part is implementing the update of need_read / need_write and wait for condition change. To support
this, the following method exists:socket(): which return the socket object when the TCP connection is open.
This call is particularly useful forselectbased loops. Seeexamples/loop_select.py.want_write(): return true if there is data  waiting to be written. This is close to theneed_writewof above pseudo-code, but you should also check whether the socket is ready for writing.callbackson_socket_*:on_socket_open: called when the socket is opened.on_socket_close: called when the socket is about to be closed.on_socket_register_write: called when there is data the client want to write on the socketon_socket_unregister_write: called when there is no more data to write on the socket.Callbacks are particularly useful for event loops where you register or unregister a socket
for reading+writing. Seeexamples/loop_asyncio.pyfor an example.The callbacks are always called in this order:on_socket_openZero or more times:on_socket_register_writeon_socket_unregister_writeon_socket_closeGlobal helper functionsThe client module also offers some global helper functions.topic_matches_sub(sub, topic)can be used to check whether atopicmatches asubscription.For example:the topicfoo/barwould match the subscriptionfoo/#or+/barthe topicnon/matchingwould not match the subscriptionnon/+/+PublishThis module provides some helper functions to allow straightforward publishing
of messages in a one-shot manner. In other words, they are useful for the
situation where you have a single/multiple messages you want to publish to a
broker, then disconnect with nothing else required.The two functions provided aresingle()andmultiple().Both functions include support for MQTT v5.0, but do not currently let you
set any properties on connection or when sending messages.SinglePublish a single message to a broker, then disconnect cleanly.Example:importpaho.mqtt.publishaspublishpublish.single("paho/test/topic","payload",hostname="mqtt.eclipseprojects.io")MultiplePublish multiple messages to a broker, then disconnect cleanly.Example:frompaho.mqtt.enumsimportMQTTProtocolVersionimportpaho.mqtt.publishaspublishmsgs=[{'topic':"paho/test/topic",'payload':"multiple 1"},("paho/test/topic","multiple 2",0,False)]publish.multiple(msgs,hostname="mqtt.eclipseprojects.io",protocol=MQTTProtocolVersion.MQTTv5)SubscribeThis module provides some helper functions to allow straightforward subscribing
and processing of messages.The two functions provided aresimple()andcallback().Both functions include support for MQTT v5.0, but do not currently let you
set any properties on connection or when subscribing.SimpleSubscribe to a set of topics and return the messages received. This is a
blocking function.Example:importpaho.mqtt.subscribeassubscribemsg=subscribe.simple("paho/test/topic",hostname="mqtt.eclipseprojects.io")print("%s%s"%(msg.topic,msg.payload))Using CallbackSubscribe to a set of topics and process the messages received using a user
provided callback.Example:importpaho.mqtt.subscribeassubscribedefon_message_print(client,userdata,message):print("%s%s"%(message.topic,message.payload))userdata["message_count"]+=1ifuserdata["message_count"]>=5:# it's possible to stop the program by disconnectingclient.disconnect()subscribe.callback(on_message_print,"paho/test/topic",hostname="mqtt.eclipseprojects.io",userdata={"message_count":0})Reporting bugsPlease report bugs in the issues tracker athttps://github.com/eclipse/paho.mqtt.python/issues.More informationDiscussion of the Paho clients takes place on theEclipse paho-dev mailing list.General questions about the MQTT protocol itself (not this library) are discussed in theMQTT Google Group.There is much more information available via theMQTT community site.