Replacement for sys.print_exception()

[anecdata]

Removed with the MicroPython 1.12 merge:
#4693
But something like it is super useful for debugging, especially for intermittent exceptions.

There was a brief discussion on Discord:
https://discord.com/channels/327254708534116352/327298996332658690/839326516596178944

[tannewt]

I think the CPython equivalent is: https://docs.python.org/3/library/traceback.html#traceback.print_exception

[Neradoc]

C python relies on sys.exec_info() and a traceback class missing from Circuitpython I believe, which is why I was worried a C python compatible version for traceback.print_exception would take more space and be more complicated to use. But I might just worry for nothing.

I personally would prefer we implement format_exception()/format_exc() or similar since getting the traceback as a list of strings is more generic and allows for easier piping into logging, uploading to a server, etc.

[tannewt]

@Neradoc would you mind posting an example of handling an exception in CPython that we could make work in CircuitPython? (I've never done it in CPython.) Thanks!

[Neradoc]

I haven't used it in C Python, my concern is how to implement code like that (6.2.0).

import adafruit_logging as logging
import io
import sys

logger = logging.getLogger(__name__)

try:
    do_some_stuff()
except NameError as ex:
    # just print exception without raising (debugging)
    sys.print_exception(ex)
    # log exception with a logger (stdout or file)
    with io.StringIO() as sio:
        sys.print_exception(ex, sio)
        logger.error(sio.getvalue())
    # log exception to a file
    with open("/log.txt", "a") as fp:
        sys.print_exception(ex, fp)

Traceback (most recent call last):
  File "test_circuitpython.py", line 8, in <module>
NameError: name 'do_some_stuff' is not defined
3139.43: ERROR - Traceback (most recent call last):
  File "test_circuitpython.py", line 8, in <module>
NameError: name 'do_some_stuff' is not defined

To replicate that feature, I think it's easier to give access to the string, and with a function that doesn't take 3 arguments. We could also have print_exc().

import logging
import traceback
import os

logger = logging.getLogger(__name__)

try:
    do_some_stuff()
except NameError:
    trace = traceback.format_exc()
    # just print exception without raising
    print(trace)
    # log exception with a logger (stdout or file)
    logger.error(trace)
    # log exception to a file
    with open("log.txt", "a") as fp:
        fp.write(trace)

Traceback (most recent call last):
  File "/path/to/the/test_cpython.py", line 8, in <module>
    do_some_stuff()
NameError: name 'do_some_stuff' is not defined

Traceback (most recent call last):
  File "/path/to/the/test_cpython.py", line 8, in <module>
    do_some_stuff()
NameError: name 'do_some_stuff' is not defined

Alternatives.
Note the first argument is etype and the doc says:

Changed in version 3.5: The etype argument is ignored and inferred from the type of value.

    trace = traceback.format_exception(None, ex, ex.__traceback__)
    print("".join(trace))
    traceback.print_exception(None, ex, ex.__traceback__)
    traceback.print_exception(*sys.exc_info())
    traceback.print_exc()
    with open("log.txt", "a") as fp:
        traceback.print_exception(None, ex, ex.__traceback__, file=fp)

Note: Circuitpython does not have Exception.__traceback__ exposed.

Practical use cases:
Note how these use Exception.__traceback__ and sys.exc_info(), both absent from Circuitpython.

circuitpython/tools/extract_pyi.py

Lines 224 to 226 in 689ec86

	except SyntaxError as e:
	traceback.print_exception(type(e), e, e.__traceback__)
	return (ok, total)

https://github.com/adafruit/adabot/blob/41b24322c0f686a50238ef78339690a7a591702c/adabot/circuitpython_libraries.py#L557-L566

    except:
        if output_filename is not None:
            exc_type, exc_val, exc_tb = sys.exc_info()
            output_handler("Exception Occurred!", quiet=True)
            output_handler(("-"*60), quiet=True)
            output_handler("Traceback (most recent call last):", quiet=True)
            tb = traceback.format_tb(exc_tb)
            for line in tb:
                output_handler(line, quiet=True)
            output_handler(exc_val, quiet=True)

[deshipu]

I would just like to note, that sometimes you don't want a complete traceback, but just the last one or two lines of it, so that you can scroll the error in a similar way as the microbit does it.

[microdev1]

I implemented a traceback module for this here.
Currently it just moves sys.print_exception() to traceback.print_exception().

[ThomasAtBBTF]

This is very useful!
An old issue #3301 would be fixed with this!
Thank you for working on this.

[ThomasAtBBTF]

I implemented a traceback module for this here.
Currently it just moves sys.print_exception() to traceback.print_exception().

It would also be super helpful to be able to obtain the traceback-string instead of just printing it.

[microdev1]

It would also be super helpful to be able to obtain the traceback-string instead of just printing it.

Yup! the plan is to have an api equivalent to cpython traceback module but the availibilty of the module functions will vary depending on the board due to flash restrictions.

[tannewt]

Note that function availability shouldn't vary with board. variation should be done at the module level. For string support we could support supplying a file-like object that code just be a string buffer.