Error exception must derive from BaseException even when it does

Question

I don't know where I went wrong in the below code (Python 2.7.1):

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

When I execute it, I get:

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

According to me, it looks like TestFailed is derived from BaseException

Answer 1

There's a static method which will return an instance. That's __new__ method. Instead of that, you can use __init__ method

Code:

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

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