Example: crc.py, AttributeError: 'list' object has no attribute 'channel'

[Sicksym]

There is a problem with compiling crc.py.

  for counter in range(16):
        checksum = compute_xbr_checksum(msg, counter)
        msg.data[7] = counter + (checksum << 4)
        messages.append(
            can.Message(arbitration_id=msg.arbitration_id, data=msg.data[:])
        )

In this for loop the messages are appending itself -> Error.

Is there any chance by changing the message for each transmit in order to append the CRC correctly?

[karlding]

You're seeing this exception right?

Traceback (most recent call last):
  File "examples/crc.py", line 82, in <module>
    crc_send(BUS)
  File "examples/crc.py", line 32, in crc_send
    task = bus.send_periodic(messages, 1)
  File "/home/karl/Documents/Code/python-can/can/bus.py", line 205, in send_periodic
    task = self._send_periodic_internal(msg, period, duration)
  File "/home/karl/Documents/Code/python-can/can/interfaces/socketcan/socketcan.py", line 610, in _send_periodic_internal
    bcm_socket = self._get_bcm_socket(msg.channel or self.channel)
AttributeError: 'list' object has no attribute 'channel'

This is because #610 is missing from the 3.3.2 release (which I presume is what you're using), which adds support for arrays of Message in interfaces that support this natively (SocketCAN via BCM). The example is written for what's currently in develop, using an array of Messages. We didn't have this example in 3.3.2, which seems to be the confusion. Prior to this, a Task mapped 1:1 to a periodic message. Essentially you're trying to use newer features on an older version.

We can potentially request for this feature to be ported over to the 3.3.3 release in #715, as the original plan for these releases was maintenance only (ie. no new features) until the new library major version was released. Not sure if that's still the plan, @hardbyte?

[Sicksym]

Hey @karlding,

yes this is the exception, thank you for your answer. I am using 3.3.2.

Is there any chance to modify the periodic message from send_periodic() every time before it gets send in 3.3.2? Is there any TX request flag which i could check?

Thank you very much in advance!

[karlding]

Modifying the periodic Message being sent every tick is not supported at the moment. This is because currently, the semantics that send_periodic aims to provide are what the SocketCAN BCM provides. In addition, this isn't really a feature supported by most interfaces (or their drivers), so it would have to be done in userspace.

You can implement this yourself using a separate thread, essentially doing something similar to what ThreadBasedCyclicSendTask does.

However, this has come up a few times in the past, and there's a PR currently being worked on (829) that is trying to address parts of this by turning the data field a generator (although I make no guarantees about whether the change will ultimately be accepted).

[karlding]

Closing this out as there doesn't seem to be anything actionable here and the initial query is resolved. Please reopen if necessary.