python - 异常链

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异常链
如果一个未处理的异常发生在 except 部分内，它将会有被处理的异常附加到它上面，并包括在错误信息中:

>>>
try:
    open("database.sqlite")
except OSError:
    raise RuntimeError("unable to handle error")

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
FileNotFoundError: [Errno 2] No such file or directory: 'database.sqlite'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
RuntimeError: unable to handle error
为了表明一个异常是另一个异常的直接后果， raise 语句允许一个可选的 from 子句:

# exc must be exception instance or None.
raise RuntimeError from exc
转换异常时，这种方式很有用。例如：

>>>
def func():
    raise ConnectionError

try:
    func()
except ConnectionError as exc:
    raise RuntimeError('Failed to open database') from exc

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "<stdin>", line 2, in func
ConnectionError

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

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
RuntimeError: Failed to open database
它还允许使用 from None 表达禁用自动异常链:

>>>
try:
    open('database.sqlite')
except OSError:
    raise RuntimeError from None

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
RuntimeError
异常链机制详见 内置异常。