Custom Exceptions in Python

Question

I am trying to understand how to use custom exceptions in Python using the try-except construction.

Here is a simple example of a custom exception:

# create custom error class
class CustomError1(Exception):
    pass

I attempted to use it like so:

# using try-except
def fnFoo1(tuple1, tuple2):
    try:
        assert(len(tuple1) == len(tuple2))
    except AssertionError:
        raise(CustomError1)
    return(1)
fnFoo1((1,2), (1, 2, 3))

This raises the CustomeError1 but also raises the AssertionError, which I would like to subsume in CustomError1.

The following does what I want, but does not seem to be the way that exceptions should be handled.

# use the custom error function without try-catch
def fnFoo2(tuple1, tuple2):
    if len(tuple1) != len(tuple2): raise(CustomError1)
    print('All done!')
fnFoo2((1,2), (1, 2, 3))

What is the way to write custom exceptions that hide other exceptions?

Have a look at this: ankurankan.github.io/blog/2014/06/03/custom-assert-method-in-python/ — Ankur Ankan
– Ankur Ankan, Commented Jun 5, 2014 at 7:06
What version of Python 3 are you using? I seem to recall there were some changes in exception behavior in later 3.x versions. — BrenBarn
– BrenBarn, Commented Jun 5, 2014 at 7:09

BrenBarn · Accepted Answer · 2014-06-05 07:14:47Z

4

According to PEP 409 you should be able to suppress the original exception context by using raise CustomError1 from None. This is also documented here:

using raise new_exc from None effectively replaces the old exception with the new one for display purposes

This functionality was added in Python 3.3. In versions from 3.0 to 3.2, it was not possible to hide the original exception (ouch).

answered Jun 5, 2014 at 7:14

BrenBarn

253k39 gold badges421 silver badges392 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

tchakravarty Over a year ago

Super, thanks. I thought I had missed something because it seemed like such a simple thing to try and achieve.

Collectives™ on Stack Overflow

Custom Exceptions in Python

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related