Exception chaining

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官方文档

在 raise 后接 from 可以启用异常链。

# exc must be exception instance or None.
raise RuntimeError from exc

可以用来转义一个异常的信息，例如

try:
	raise ConnectionError
except ConnectionError as exc:
	raise RuntimeError('Failed to open database') from exc
>>>
Traceback (most recent call last):
  File "/home/yuchuan/code/selfuse/test.py", line 20, in <module>
    raise ConnectionError
ConnectionError

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/yuchuan/code/selfuse/test.py", line 22, in <module>
    raise RuntimeError('Failed to open database') from exc
RuntimeError: Failed to open database

表明 RuntimeError 是由 ConnectionError 直接导致的，并且提供了更具体的信息。

若不使用 raise ... from ...

try:
	raise ConnectionError
except ConnectionError as exc:
	raise RuntimeError('Failed to open database')
>>>
Traceback (most recent call last):
  File "/home/yuchuan/code/selfuse/test.py", line 20, in <module>
    raise ConnectionError
ConnectionError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/yuchuan/code/selfuse/test.py", line 22, in <module>
    raise RuntimeError('Failed to open database')
RuntimeError: Failed to open database

则两个异常会被视为无联系的异常。

可用于对异常进行进一步解释，或者表明引发异常的异常链。使用 from 与否在程序运行结果上并无区别，但在报错输出的解释与程序设计的逻辑上有着明显的区别。应尽量使用 from 使得异常链完整，便于追踪问题所在。

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Errors and exceptions
[[Exception Chaining]]
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