Exception __cause__ is ignored

[dneuhaeuser-zalando]

When an exception is raised as part of handling another exception, it gets a __cause__ attribute set, so that one can determine the cause.

Example:

try:
    raise ValueError("some value")
except ValueError:
    raise TypeError("some type")

If you execute this, it creates an exception with traceback that includes the cause:

$ python bar.py 
Traceback (most recent call last):
  File "bar.py", line 2, in <module>
    raise ValueError("some value")
ValueError: some value

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "bar.py", line 4, in <module>
    raise TypeError("some type")
TypeError: some type

I would expect the traceback in spans to also include the cause simply because that matches interpreter behavior when printing tracebacks and also matches how logging works. Unfortunately it doesn't.

Tracebacks are formatted in

lightstep-tracer-python/lightstep/recorder.py

Line 174 in b8a47e2

log.key_values[STACK] = util._format_exc_tb(log.key_values[STACK])

with a helper function

lightstep-tracer-python/lightstep/util.py

Lines 108 to 112 in b8a47e2

	def _format_exc_tb(exc_tb):
	if type(exc_tb) is types.TracebackType:
	return ''.join(traceback.format_tb(exc_tb))
	return exc_tb

that uses traceback.format_tb. The latter doesn't take the cause into consideration.

Amending the example used earlier to print the traceback using the same formatting used in spans shows that the cause is missing

import sys
import traceback


try:
    try:
        raise ValueError("some value")
    except ValueError:
        raise TypeError("some type")
except Exception as exc:
    exc_value, exc_type, exc_tb = sys.exc_info()
    print("".join(traceback.format_tb(exc_tb)))

Output:

  File "foo.py", line 9, in <module>
    raise TypeError("some type")

It would be great, if this could be changed to include information about the cause.