I know that if I want to re-raise an exception, I simple use raise without arguments in the respective except block. But given a nested expression like
try: something()
except SomeError as e: try: plan_B() except AlsoFailsError: raise e # I'd like to raise the SomeError as if plan_B() # didn't raise the AlsoFailsErrorhow can I re-raise the SomeError without breaking the stack trace? raise alone would in this case re-raise the more recent AlsoFailsError. Or how could I refactor my code to avoid this issue?
4 Answers
As of Python 3, the traceback is stored in the exception, so a simple raise e will do the (mostly) right thing:
try: something()
except SomeError as e: try: plan_B() except AlsoFailsError: raise e # or raise e from None - see belowThe traceback produced will include an additional notice that SomeError occurred while handling AlsoFailsError (because of raise e being inside except AlsoFailsError). This is misleading because what actually happened is the other way around - we encountered AlsoFailsError, and handled it, while trying to recover from SomeError. To obtain a traceback that doesn't include AlsoFailsError, replace raise e with raise e from None.
In Python 2 you'd store the exception type, value, and traceback in local variables and use the three-argument form of raise:
try: something()
except SomeError: t, v, tb = sys.exc_info() try: plan_B() except AlsoFailsError: raise t, v, tbEven if the accepted solution is right, it's good to point to the Six library which has a Python 2+3 solution, using six.reraise.
six.reraise(exc_type, exc_value, exc_traceback=None)
Reraise an exception, possibly with a different traceback. [...]
So, you can write:
import six
try: something()
except SomeError: t, v, tb = sys.exc_info() try: plan_B() except AlsoFailsError: six.reraise(t, v, tb)As per Drew McGowen's suggestion, but taking care of a general case (where a return value s is present), here's an alternative to user4815162342's answer:
try: s = something()
except SomeError as e: def wrapped_plan_B(): try: return False, plan_B() except: return True, None failed, s = wrapped_plan_B() if failed: raisePython 3.5+ attaches the traceback information to the error anyway, so it's no longer necessary to save it separately.
>>> def f():
... try:
... raise SyntaxError
... except Exception as e:
... err = e
... try:
... raise AttributeError
... except Exception as e1:
... raise err from None
>>> f()
Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 9, in f File "<stdin>", line 3, in f
SyntaxError: None
>>> 
