python的异常链

[不喝水的杰]

raise 语句支持可选的 from 子句，该子句用于启用链式异常。 例如：

# 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

异常链会在 except 或 finally 子句内部引发异常时自动生成。 这可以通过使用 from None 这样的写法来禁用:

>>>

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