Three things. One, this isn't backwards-compatible as you are not passing
any details down into Exception.__init__() to make sure that
BaseException.args gets populated.
Two, you can simplify your code by using str.format_map() instead of the
string module.
Three, I don't see enough overhead from version 2 to your version 3 since
you're only saving a line of code so say there's that much boilerplate. But
the best way to prove me wrong is to release this on PyPI and see if people
use it. But without community use proving people want this we won't be up
for adding a new built-in exception as you're asking every book on Python
to be rewritten to cover this which is no small thing.
On Thu, Aug 8, 2019 at 8:56 AM Ryan Fox <r...@rcfox.ca> wrote:
> Exception definitions in Python are not great. There are two main ways
> they're used in the code that I've read:
>
> 1) By far the most common:
>
> >>> class MyException(Exception):
> ... pass
> >>> raise MyException(f'Bad thing happened during {action} in {context}')
>
> 2) Much rarer:
>
> >>> class MyException(Exception):
> ... def __init__(self, action, context):
> ... super().__init__(f'Bad thing happened during {action} in
> {context}')
> >>> raise MyException(current_action, current_context)
>
> Version 1 is quick and easy, but messages skew and become inconsistent as
> they're copied from place to place. The Python standard library isn't
> immune to this either.
>
> Version 2 looks simple enough to do, but all of the repetitious
> boilerplate adds up when several exception types need to be defined. (And
> it's even worse when you want all of your code to include typing
> annotations.) Most people don't bother.
>
> My proposal is a new exception class as the preferred base for
> user-defined exceptions:
>
> >>> class MyException(ExceptionTemplate):
> ... message = 'Bad thing happened during {action} in {context}'
> >>> raise MyException(action=current_action, context=current_context)
>
> I have a reference implementation, implemented in an almost trivial amount
> of pure Python code:
> https://github.com/rcfox/exception-template/blob/master/exception_template/exception_template.py
>
> So why am I bothering you with this? I would like to see this become the
> preferred method of defining new exceptions as recommended by the Python
> documentation. This would also require adding ExceptionTemplate as a
> built-in exception type.
>
> I will grant that ExceptionTemplate seems a little bit magical, but all of
> the required arguments are explicitly laid out to the user in the 'message'
> field, and format strings should be a familiar concept to most users.
>
> I've also included some tests that show that you can still treat these
> exceptions as regular classes:
> https://github.com/rcfox/exception-template/blob/master/tests/test.py#L53
>
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