[Python-ideas] notify exception during raise (once magic properties are set)

[Chris Angelico]

On Sat, Apr 11, 2015 at 5:54 AM, Travis Everett
<travis.a.everett at gmail.com> wrote:
> I have a specialized use case for exceptions which need to analyze their
> __cause__ property before they're fully initialized. The property isn't set
> until after init, and (forgive my ignorance of Python's source!) appears to
> be done at the C level in a way that it can't be observed with a property
> setter or __setattr__. I currently create a method to do this post-init work
> as a workaround, and push the burden of making sure it gets called on the
> code using the exceptions--but this makes the exceptions idiomatic and
> error-prone.

On principle I'd be in favour of making the setting of __cause__ go
through the normal mechanisms, such that @property etc would work; but
just out of curiosity, what would happen if your code raised an
exception during the raising of another exception? Because that's when
__cause__ will get set - the new exception object is fully
constructed, and now it goes into exception handling to start bubbling
up. Could be a bit weird! But if that's handlable (maybe it just
chains yet another exception in), then that's the route I'd be
inclined towards.

There are quite a few things that happen somewhat weirdly in Python,
largely (I think) because of specific CPython optimizations or
implementation details. Anything that makes the language more flexible
may well have a hefty performance cost, but if it doesn't, then it
would be a good thing IMO.

ChrisA