paho.mqtt + python TypeError: a bytes-like object is required, not 'str'

Question

Two paho.mqtt clients (client1 and client2) are created in Python, which connects to mosquitto brokers. Two clients subscribe to different topics. client1 runs loop_forever(), and client2 runs loop_start(). client1's on_message() callback stores the message in MySQL database. client2's on_message() callback runs a simple test.

The test contains a test method which uses mock object paho.mqtt.client.MQTTMessage. For client2, I am publishing the message from mosquitto_pub in a command line. when I receive the message to run the test, the test runs successfully. But after that I get this below error message. when I comment the test method which uses this mock object paho.mqtt.client.MQTTMessage for test, I don't get this error message.

Is this error coming from Python or paho.mqtt? or is it from client1.loop_forever() method?

File "/usr/local/lib/python3.6/site-packages/paho_mqtt-1.3.1-py3.6.egg/paho/mqtt/client.py", line 1481, in loop_forever
rc = self.loop(timeout, max_packets)
File "/usr/local/lib/python3.6/site-packages/paho_mqtt-1.3.1-py3.6.egg/paho/mqtt/client.py", line 1003, in loop
rc = self.loop_read(max_packets)
File "/usr/local/lib/python3.6/site-packages/paho_mqtt-1.3.1-py3.6.egg/paho/mqtt/client.py", line 1284, in loop_read
rc = self._packet_read()
File "/usr/local/lib/python3.6/site-packages/paho_mqtt-1.3.1-py3.6.egg/paho/mqtt/client.py", line 1849, in _packet_read
rc = self._packet_handle()
File "/usr/local/lib/python3.6/site-packages/paho_mqtt-1.3.1-py3.6.egg/paho/mqtt/client.py", line 2305, in _packet_handle
return self._handle_publish()
File "/usr/local/lib/python3.6/site-packages/paho_mqtt-1.3.1-py3.6.egg/paho/mqtt/client.py", line 2500, in _handle_publish
self._handle_on_message(message)
File "/usr/local/lib/python3.6/site-packages/paho_mqtt-1.3.1-py3.6.egg/paho/mqtt/client.py", line 2640, in _handle_on_message
for callback in self._on_message_filtered.iter_match(message.topic):
File "/usr/local/lib/python3.6/site-packages/paho_mqtt-1.3.1-py3.6.egg/paho/mqtt/matcher.py", line 60, in iter_match
lst = topic.split('/')
TypeError: a bytes-like object is required, not 'str'

Answer 1

As mentioned in the paho documentation, your topic must be of type bytes:

class MQTTMessage(object):
""" This is a class that describes an incoming or outgoing message. It is
passed to the on_message callback as the message parameter.

Members:

topic : String/bytes. topic that the message was published on.
payload : String/bytes the message payload.
qos : Integer. The message Quality of Service 0, 1 or 2.
retain : Boolean. If true, the message is a retained message and not fresh.
mid : Integer. The message id.

On Python 3, topic must be bytes.
"""

__slots__ = 'timestamp', 'state', 'dup', 'mid', '_topic', 'payload', 'qos', 'retain', 'info'

def __init__(self, mid=0, topic=b""):
    self.timestamp = 0
    self.state = mqtt_ms_invalid
    self.dup = False
    self.mid = mid
    self._topic = topic
    self.payload = b""
    self.qos = 0
    self.retain = False
    self.info = MQTTMessageInfo(mid)

def __eq__(self, other):
    """Override the default Equals behavior"""
    if isinstance(other, self.__class__):
        return self.mid == other.mid
    return False

def __ne__(self, other):
    """Define a non-equality test"""
    return not self.__eq__(other)

@property
def topic(self):
    if sys.version_info[0] >= 3:
        return self._topic.decode('utf-8')
    else:
        return self._topic

@topic.setter
def topic(self, value):
    self._topic = value

-> On Python 3, topic must be bytes.

So ensure that your topic is a bytes object, then it should work

BTW, here a short example for string and byte converting:

mystring = 'hello_string'
b1 = bytes(mystring, 'utf-8')
b2 = str(b1, 'utf-8')
print(b1)
print(b2)

output is:

b'hello_string'
hello_string