Python 如何在嵌套的 try/except 块中重新引发异常？
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How to re-raise an exception in nested try/except blocks?
提问by Tobias Kienzler
I know that if I want to re-raise an exception, I simple use raisewithout arguments in the respective exceptblock. But given a nested expression like
我知道如果我想重新引发异常，我会raise在相应的except块中简单地使用而不带参数。但给定一个嵌套表达式，如
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 AlsoFailsError
how can I re-raise the SomeErrorwithout breaking the stack trace? raisealone would in this case re-raise the more recent AlsoFailsError. Or how could I refactor my code to avoid this issue?
如何在SomeError不破坏堆栈跟踪的情况下重新提高？raise在这种情况下，单独会重新加注最近的AlsoFailsError。或者我如何重构我的代码以避免这个问题？
采纳答案by user4815162342
You can store the exception type, value, and traceback in local variables and use the three-argument form of raise:
您可以将异常类型、值和回溯存储在局部变量中，并使用以下三参数形式raise：
try:
something()
except SomeError:
t, v, tb = sys.exc_info()
try:
plan_B()
except AlsoFailsError:
raise t, v, tb
In Python 3 the traceback is stored in the exception, so raise ewill do the (mostly) right thing:
在 Python 3 中，回溯存储在异常中，因此raise e会做（大部分）正确的事情：
try:
something()
except SomeError as e:
try:
plan_B()
except AlsoFailsError:
raise e # or raise e from None - see below
Note that the traceback produced by Python 3 will include a message that SomeErroroccurred while handling AlsoFailsError(because of raise ebeing inside except AlsoFailsError). This is misleading because what actually happened is the other way around - we handled AlsoFailsErrorwhile trying to recover from SomeError. To obtain a traceback that doesn't include AlsoFailsError, as was the default in Python 2, replace raise ewith raise e from None.
请注意，Python 3 生成的回溯将包括SomeError处理时发生的消息AlsoFailsError（因为raise e在 inside except AlsoFailsError）。这是一种误导，因为实际发生的事情是相反的 - 我们AlsoFailsError在尝试从SomeError. 要获得不包含 的回溯AlsoFailsError（Python 2 中的默认设置），请替换raise e为raise e from None.
回答by Tobias Kienzler
As per Drew McGowen's suggestion, but taking care of a general case (where a return value sis present), here's an alternative to user4815162342's answer:
根据Drew McGowen 的建议，但考虑到一般情况（存在返回值s），这里是user4815162342 答案的替代方案：
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:
raise
回答by Matthias Urlichs
Python 3.5+ attaches the traceback information to the error anyway, so it's no longer necessary to save it separately.
Python 3.5+ 无论如何都会将回溯信息附加到错误中，因此不再需要单独保存它。
>>> 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
>>>
回答by Laurent LAPORTE
Even if the accepted solutionis right, it's good to point to the Sixlibrary which has a Python 2+3 solution, using six.reraise.
即使接受的解决方案是正确的，最好指向具有 Python 2+3 解决方案的Six库，使用six.reraise.
six.reraise(exc_type, exc_value, exc_traceback=None)
Reraise an exception, possibly with a different traceback. [...]
六。重新加注（exc_type，exc_value，exc_traceback=None）
重新引发异常，可能具有不同的回溯。[...]
So, you can write:
所以，你可以写：
import six
try:
something()
except SomeError:
t, v, tb = sys.exc_info()
try:
plan_B()
except AlsoFailsError:
six.reraise(t, v, tb)
