multiple serial ports mounted from usb_cdc breaks REPL auto connect scripts (6.2.0 beta 3)

[maholli]

Awesome new usb_cdc capability creates multiple serial ports when connecting to PC/Mac/Linux machine. The REPL port number is not consistently highest/lowest number which causes issues for scripts written to auto-connect to CP boards. @dhalbert suggested this issue be created:

Some of the scripts my students and I use:

Windows Batch:

@echo off
python -c "import serial.tools.list_ports as ls; p=ls.comports(); [print(i[0]) for i in p]" > Output
SET /p SERPORT=<Output
START putty.exe -serial %SERPORT% 115200
DEL Output

AutoHotKey

#c::
COMPort := Object()
COMPortName := Object()
Num:=0
Loop, HKLM, HARDWARE\DEVICEMAP\SERIALCOMM\
{
    RegRead, OutputVar
    If not InStr("%OutputVar%", "COM3")
      Run "putty.exe" "-serial" "%OutputVar%" "115200"
      Exit
    COMPort%Num%:=OutputVar
    COM_result:=COMPort%Num%
    Num+=1
}
Return

Python one-liner

python -c "import serial.tools.list_ports as ls; p=ls.comports(); [print(i[0]) for i in p]

[dhalbert]

Thank you for being a guinea pig for this. It turns out that there is enough information provided by pyserial to figure out which port is right. You need to use the .interface attribute on the result of comports():

import serial.tools.list_ports
for port in serial.tools.list_ports.comports():
    if port.interface == "CircuitPython CDC control":
        print(port.device)

The key is the port.interface, which is the interface name. For the REPL port, it is CircuitPython CDC control; for the secondary port, it is CircuitPython CDC2 control. Or it could be CircuitPython CDC data and CircuitPython CDC2 data. The key difference is CDC vs CDC2.

However, note that this is broken in the current pyserial on MacOS: it returns the same interface name twice. Fortunately, @Neradoc has fixed the problem and submitted a PR to pyrserial. Since the code is mostly standalone, it could be pulled out of pyserial for now. Or we could fork pyserial temporarily. See pyserial/pyserial#566.

We should write a self-contained script that returns the appropriate device for use in scripts like yours. I am also planning to add this capability to the Mu editor.

I'm thinking I will make a pip-installable module that will return the CircuitPython serial ports, with functionality to find all the REPL ports and all the secondary ports. It will depend on pyserial but also include a workaround for the MacOS problem. I just have to think about the name and the (simple) API.

[dhalbert]

Fixed by #4384.