CODEWITHSUNDEEP
python
James Kanze
James Kanze

Reputation: 153929

Error exception must derive from BaseException even when it does (Python 2.7)

What's wrong with the following code (under 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 run 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

But it looks to me that TestFailed does derive from BaseException.

Upvotes: 39

Views: 127224

Answers (5)

aVral
aVral

Reputation: 133

try this:

try:
    do something ...

except AttributeError:
        raise AttributeError("Please select the right frame from dropdown !")

Upvotes: 0

Raymond Hettinger
Raymond Hettinger

Reputation: 226376

__new__ is a staticmethod that needs to return an instance.

Instead, use the __init__ method:

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

Upvotes: 34

kindall
kindall

Reputation: 184221

Others have shown you how to fix your implementation, but I feel it important to point out that the behavior you are implementing is already the standard behavior of exceptions in Python so most of your code is completely unnecessary. Just derive from Exception (the appropriate base class for runtime exceptions) and put pass as the body.

class TestFailed(Exception):
    pass

Upvotes: 26

Florian
Florian

Reputation: 2429

Use __init__() instead of __new__() to "initialize" classes. In most cases overriding __new__ is not necessary. It is called before __init__ during object creation.

See also Python's use of __new__ and __init__?

Upvotes: 5

ekhumoro
ekhumoro

Reputation: 120628

The __new__ implementation should return an instance of the class, but it's currently returning None (by default).

However, it looks like you should be using __init__ here, rather than __new__.

Upvotes: 3

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