[Python-Dev] PEP for allowing 'raise NewException from None' (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Sat Jan 28 07:31:41 CET 2012

On Sat, Jan 28, 2012 at 10:33 AM, Ethan Furman <ethan at stoneleaf.us> wrote:

Because at this point it is possible to do:

 raise ValueError from NameError outside a try block.  I don't see it as incredibly useful, but I don't know that it's worth making it illegal. So the question is:  - should 'raise ... from ...' be legal outside a try block?  - should 'raise ... from None' be legal outside a try block?

Given that it would be quite a bit of work to make it illegal, my preference is to leave it alone.

I believe that means there's only one open question. Should "raise ex from None" be syntactic sugar for:

  1. clearing the current thread's exception state (as I believe Ethan's patch currently does), thus meaning that context and cause both end up being None
  2. setting cause to None (so that context still gets set normally, as it is now when cause is set to a specific exception), and having cause default to a new sentinel object that indicates "use context"

I've already stated my own preference in favour of 2 - that approach means developers that think about it can explicitly change exception types such that the context isn't displayed by default, but application and framework developers remain free to insert their own exception handlers that always report the full exception stack.

Cheers, Nick.

-- Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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