8.5. 异常链

raise 语句允许可选的 from 子句，它通过设置所引发异常的 __cause__ 属性来启用异常链。 例如:

raise RuntimeError from OSError

这在你要转换异常时很有用。 例如:

>>> def func():
...    raise IOError
...
>>> try:
...     func()
... except IOError 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
OSError
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
RuntimeError

跟在 from 之后的表达式必须是一个异常或为 None。 当在一个异常处理程序或 finally 子句中又引发了一个异常时异常链将自动生成。 异常链可使用 from None 形式来禁用:

>>> try:
...     open('database.sqlite')
... except IOError:
...     raise RuntimeError from None
...
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
RuntimeError