Replacement for sys.atexit()

[jscholes]

CircuitPython version

Adafruit CircuitPython 7.0.0-alpha.4 on 2021-07-08; Adafruit Feather nRF52840 Express with nRF52840

Code/REPL

import sys
sys.print_exception

Behavior

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'print_exception'

Description

We were using the sys.print_exception function to add tracebacks to our log entries, similar to what CPython allows. Unfortunately this (and a whole other bunch of stuff) seems to have been removed from the sys module around about commit b35fa44.

print_exception is included according to the latest MicroPython docs, although their module is called usys and not just sys. I did try to import usys instead:

>>> import usys                                                                 
Traceback (most recent call last):                                              
  File "<stdin>", line 1, in <module>                                           
ImportError: no module named 'usys'                                             

I also tried the traceback module from micropython-lib, although that just calls sys.print_exception anyway. It also uses sys.exc_info in its print_exc function, which is also not present in CircuitPython.

If these facilities have been removed intensionally, is there some other recommended way in CircuitPython to capture a traceback for reuse? Alternatively, if they are still present, where can they be located?

Additional information

No response

[anecdata]

duplicate of #4748 I believe

[jscholes]

@anecdata Thanks, and apologies for the duplicate. I am curious how merging in a MicroPython update removed so many things from the sys module, despite them still being present in the usys MicroPython module?

[tannewt]

@anecdata Thanks, and apologies for the duplicate. I am curious how merging in a MicroPython update removed so many things from the sys module, despite them still being present in the usys MicroPython module?

I removed print_exception during the merge because it was not CPython compatible. We hadn't validated sys previously. What else did I remove?

[jscholes]

@tannewt I think I was reading things wrong in terms of how much was removed from the sys module. As a minimum, print_exception and atexit seem to no longer be there, but are still present in MicroPython. Additionally (and I'm not overly clear on this one), exc_info is used by some MicroPython modules, but not documented under usys. So I'm not sure if it actually exists or not.

I removed print_exception during the merge because it was not CPython compatible.

I assume atexit was removed for the same reason, because in CPython the same functionality is fulfilled by the dedicated atexit module? I understand the desire for compatibility, but it seems problematic to remove working functionality with no replacement.

[microdev1]

exc_info is disabled by default in both CPY & MPY.

circuitpython/py/mpconfig.h

Lines 1303 to 1307 in ce73015

	// Whether to provide "sys.exc_info" function
	// Avoid enabling this, this function is Python2 heritage
	#ifndef MICROPY_PY_SYS_EXC_INFO
	#define MICROPY_PY_SYS_EXC_INFO (0)
	#endif

[tannewt]

I removed print_exception during the merge because it was not CPython compatible.

I assume atexit was removed for the same reason, because in CPython the same functionality is fulfilled by the dedicated atexit module? I understand the desire for compatibility, but it seems problematic to remove working functionality with no replacement.

Correct. It should be relatively easy to move to a dedicated and compatible atexit native module. I don't think it's problematic because all of the source code is available to anyone who wants to add it in a compatible way.

Merging it as-is would potentially lead to new code that doesn't work in CPython with Blinka. Supporting that incompatibility is a burden I'm not willing to assume to have the functionality. Supporting APIs has a bigger impact than simply the API boundary itself. API choices cascade into libraries and example code and are harder to change once they've been supported. I'd rather get it right up front than have to go back and find all the places to change.

[microdev1]

Converting this to a feature request for CPython atexit module. We already have an issue open for print_exception() (#4748).