TypeError:exceptions must be old-style classes or derived from BaseException, not str

Question

Following is my code:

test = 'abc'
if True:
    raise test + 'def'

And when i run this, it gives me the TypeError

TypeError: exceptions must be old-style classes or derived from BaseException, not str

So what kind of type should the test be?

Asked By: 2342G456DI8

||

Source

Answer 1

It should be an exception.

You want to do something like:

raise RuntimeError(test + 'def')

In Python 2.5 and below, your code would work, as then it was allowed to raise strings as exceptions. This was a very bad decision, and so removed in 2.6.

Answered By: Abe Karplus

Answer 2

You can’t raise a str. Only Exceptions can be raised.

So, you’re better off constructing an exception with that string and raising that. For example, you could do:

test = 'abc'
if True:
    raise Exception(test + 'def')

OR

test = 'abc'
if True:
    raise ValueError(test + 'def')

Hope that helps

Answered By: inspectorG4dget

Answer 3

The sole argument to raise indicates the exception to be raised. This must be either an exception instance or an exception class (a class that derives from Exception).

Try this:

test = 'abc'
if True:
    raise Exception(test + 'def')

Answered By: user1393258

TypeError:exceptions must be old-style classes or derived from BaseException, not str

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