Crash when extending native class

[ali1234]

CircuitPython version

Adafruit CircuitPython 8.0.0-beta.4 on 2022-10-30; Raspberry Pi Pico with rp2040

(It also crashes 7.)

Code/REPL

import board
import pwmio

class PWMOutExtend(pwmio.PWMOut):
    def __init__(self, pin, frequency=1000, slew_rate=1000):
        super().__init__(pin, frequency=frequency)
        self.foo_bar_baz = 0
        self.duty_cycle = 0

led = PWMOutExtend(board.GP25, frequency=1000)

print('before')
led.foo_bar_baz = 0
print('after')

Behavior

You see "before" on the REPL and then the device freezes. If you put it in code.py you now have to enter safe mode to access CIRCUITPY.

Description

Every line of the example is necessary except for the prints. In particular all three assignments are necessary.

I can't reproduce it when extending a class I defined myself.

Additional information

No response

[tannewt]

Generally native classes aren't subclassable because our C code doesn't always check to ensure it was given the native struct representation. Ideally we'd have a way to do this for everything.

In the meantime, I'd suggest wrapping PWMOut instead of subclassing.

[dhalbert]

It would be nice if we could forbid or warn when a native class that can't handle subclassing is subclassed. Classes that do handle this properly could be marked as such. This is probably a change to make upstream.

[TonyLHansen]

Such an error message would be appreciated. It definitely tripped me up. When it happened to me (#8778), I was only able to recover fully after I reflashed the board back to the boot loader.