New submission from Garrett Berg <googb...@gmail.com>:
# Summary
In python (2 or 3) it should always be valid to have a bare *raise* statement
in an exception block.
try:
do_something()
except SomeException:
do_cleanup()
raise # always reraises SomeException with original stack trace
You can see this in the python exception documentation:
https://docs.python.org/2.7/tutorial/errors.html#raising-exceptions
# Reproduction of Failure of invariants
Example python file where this doesn't work:
from __future__ import print_function
import sys
def doclear():
"""This should basically be a noop"""
sys.exc_clear()
try:
1/0
except ZeroDivisionError:
exc = sys.exc_info()[0]
print("first exc:", exc)
assert exc is ZeroDivisionError
try:
1/0
except ZeroDivisionError:
exc = sys.exc_info()[0]
print("second exc (before doclear):", exc)
doclear()
exc = sys.exc_info()[0]
print("second exc (after doclear):", exc)
assert sys.exc_info()[0] is ZeroDivisionError # fails
Running in python2.7 gives:
first exc: <type 'exceptions.ZeroDivisionError'>
second exc (before doclear): <type 'exceptions.ZeroDivisionError'>
second exc (after doclear): None
Traceback (most recent call last):
File "doclear.py", line 23, in <module>
assert sys.exc_info()[0] is ZeroDivisionError # fails
AssertionError
# Conclusion
There seems to be a bug in python 2.7 where it doesn't follow it's own spec.
[sys.exc_clear()](https://docs.python.org/2/library/sys.html#sys.exc_clear)
states:
> This function clears all information relating to the current or last
> exception that occurred in the current thread. After calling this function,
> exc_info() will return three None values until another exception is raised in
> the current thread or *the execution stack returns to a frame where another
> exception is being handled*.
The above code is clear example where sys.exc_clear() is changing the value of
sys.exc_info() *in a different stack frame*, which is against what is stated in
the above docs.
This bug makes it impossible to reraise with a bare raise statement when doing
cleanup that could use sys.exc_clear(). In other words, calling into functions
can *change your local state* with regards to what exception is being handled.
----------
components: Interpreter Core
messages: 308264
nosy: Garrett Berg
priority: normal
severity: normal
status: open
title: sys.exc_clear() clears exception in other stack frames
type: behavior
versions: Python 2.7
_______________________________________
Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue32317>
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