Show error after xBox controller disconnected the TechnicHub

[BertLindeman]

At the moment using an xBox controller on a TechnicHub disconnects the hub.
No way to see any exception then.

The error / exception can be written to the user storage on the hub.
After reconnect that info can be shown in the REPL.

(click the small triangle(s) below to see more)

REPL explanation

The REPL (Read-Eval-Print Loop) is an interactive tool that lets you type Python commands, run them, and see the results immediately.

Start the REPL in code.pybricks.com by pressing the Blue ">>>" button after the hub is connected.

program to show how to write exception data to user storage: "technichub_save_exception.py"

"""
technichub_save_exception.py
This program shows how to save error message(s) in the hub's storage
so you can show it later in the REPL.
You can see the error after re-connecting the hub was disconnected.
"""


# Initial modules imports
import usys
import uio
from pybricks.hubs import ThisHub
from pybricks import version

# Initialize the hub and get version details
hub = ThisHub()
print(f'Hub name = "{hub.system.name()}" {version}')

# Determine the size limit for user data storage based on hub type
USER_DATA_SIZE = 512 if version[0] == "primehub" else 128

# TEST shorter user storage size. Can be commented-out
USER_DATA_SIZE = 128


# TEST function that will raise an exception for demonstration
def myproblem(aa):
    return aa / 0  # Intentional division by zero to generate an exception


# Clean the user storage to prevent interpreting old data as new
hub.system.storage(0, write=b'\x00' * USER_DATA_SIZE)

# Try block to catch and store the exception traceback of your program
try:
    # ====== START Here should come your program ======
    myproblem(12345)  # Will trigger an exception
    # ======  END  Here should come your program ======
except Exception as e:
    my_file = uio.StringIO()
    usys.print_exception(e, my_file)  # Capture the exception details
    traceback_info = my_file.getvalue()

    # Remove the traceback header if present to save space.
    TRACEBACK_PREFIX = 'Traceback (most recent call last):\n'
    if traceback_info.startswith(TRACEBACK_PREFIX):
        traceback_info = traceback_info[len(TRACEBACK_PREFIX):].lstrip()

    # Ensure we write no more than the size of the user storage size.
    hub.system.storage(0, write=bytes(traceback_info[-USER_DATA_SIZE:], 'utf-8'))


# test print of the exception data from user storage
print("\nException read from storage:\n=============================================\n")
# Paste in REPL and do an extra enter if the REPL prompt is ... :
for loc in range(0, 128): print(str(hub.system.storage(loc, read=1), 'utf-8'), end="")
print("\n=============================================\n")

Demo xbox program: "test_xbox_on_technichub.py"

"""
test_xbox_on_technichub.py
Demo program to show an example writing exception data to user storage
"""

from pybricks.hubs import TechnicHub
from pybricks.iodevices import XboxController
from pybricks.parameters import Button, Color
from pybricks.tools import wait
from pybricks import version


# Initial modules imports
import usys
import uio

hub = TechnicHub()

# setup variables needed by "save exception"

# Determine the size limit for user data storage based on hub type
USER_DATA_SIZE = 512 if version[0] == "primehub" else 128

# Clean the user storage to prevent interpreting old data as new
hub.system.storage(0, write=b'\x00' * USER_DATA_SIZE)

# Define your program
def my_xbox_program():
    print("""
        After connection to the xBox controller is made
        - press A - hub light gets GREEN
        - press B - hub light gets RED
        - press Y - hub light gets YELLOW
        - press X - hub light gets CYAN
        Any other button creates an error and the hub stops

        Reconnect and start the REPL (Blue button ">>>" on the top)
        Paste the command on the output window (the bottom one) and do [Enter] twice
    """)
    xbox = XboxController()
    while True:
        if xbox.buttons.pressed():
            if Button.Y in xbox.buttons.pressed():
                hub.light.on(Color.YELLOW)
            elif Button.B in xbox.buttons.pressed():
                hub.light.on(Color.RED)
            elif Button.X in xbox.buttons.pressed():
                hub.light.on(Color.CYAN)
            elif Button.A in xbox.buttons.pressed():
                hub.light.on(Color.GREEN)
            else:
                hub.light.animate([Color.MAGENTA, Color.VIOLET], 100)
                wait(1000)
                # divide by zero
                print(100 / 0)
                break
        wait(100)

# execute the program and write exeption to user storage
try:
    my_xbox_program()
except Exception as e:
    my_file = uio.StringIO()
    usys.print_exception(e, my_file)  # Capture the exception details
    traceback_info = my_file.getvalue()

    # Remove the traceback header if present to save space.
    TRACEBACK_PREFIX = 'Traceback (most recent call last):\n'
    if traceback_info.startswith(TRACEBACK_PREFIX):
        traceback_info = traceback_info[len(TRACEBACK_PREFIX):].lstrip()

    # Ensure we write no more than the size of the user storage size.
    hub.system.storage(0, write=bytes(traceback_info[-USER_DATA_SIZE:], 'utf-8'))


# test print of the exception data from user storage
print("\nException read from storage:\n=============================================\n")
# After re-connect:
# paste the next line in REPL
# and do an extra enter if the REPL prompt is ...
for loc in range(0, 128): print(str(hub.system.storage(loc, read=1), 'utf-8'), end="")
print("\n=============================================\n")

# expect to see something like this:
"""
>>> for loc in range(0, 128): print(str(hub.system.storage(loc, read=1), 'utf-8'), end="")
...
ub.py", line 63, in <module>
  File "test_xbox_on_technichub.py", line 56, in my_xbox_program
ZeroDivisionError: divide by zero
>>>
"""

While testing the demo program I needed to use the exception data in storage a few times.
I can say it works.

Tested on stable version in code.pybricks.com and beta.
Hope it is not too complicated for beginners and I am curious about the comments of others.

Knowing me I will [EDIT] this post a few times to repair my typo's. 😃

Bert