python - 错误异常必须从 BaseException 派生，即使它确实如此(Python 2.7)

Question

以下代码有什么问题(在 Python 2.7.1 下):

class TestFailed(BaseException):
    def __new__(self, m):
        self.message = m
    def __str__(self):
        return self.message

try:
    raise TestFailed('Oops')
except TestFailed as x:
    print x

当我运行它时，我得到:

Traceback (most recent call last):
  File "x.py", line 9, in <module>
    raise TestFailed('Oops')
TypeError: exceptions must be old-style classes or derived from BaseException, not NoneType

但在我看来，TestFailed 确实派生自 BaseException。

Answer 1

__new__是一个staticmethod，需要返回一个实例。

改为使用 __init__方法:

class TestFailed(Exception):
    def __init__(self, m):
        self.message = m
    def __str__(self):
        return self.message

try:
    raise TestFailed('Oops')
except TestFailed as x:
    print x