[Python-Dev] PEP 409 - final?

[Ethan Furman]

Guido van Rossum wrote:
> Hm... Reading this draft, I like the idea of using "raise X from
> None", but I still have one quibble. It seems the from clause sets
> __cause__, and __cause__ can indicate three things: (1) print
> __cause__ (explicitly set), (2) print __context__ (default), (3) print
> neither (raise X from None). For (1), __cause__ must of course be a
> traceback object. 

Actually, for (1) __cause__ is an exception object, not a traceback.


> The PEP currently proposes to use two special
> values: False for (2), None for (3). To me, this has a pretty strong
> code smell, and I don't want this pattern to be enshrined in a PEP as
> an example for all to follow. (And I also don't like "do as I say,
> don't do as I do." :-)

My apologies for my ignorance, but is the code smell because both False 
and None evaluate to bool(False)?  I suppose we could use True for (2) 
to indicate that __context__ should be printed, leaving None for (3)... 
but having __context__ at None and __cause__ at True could certainly be 
confusing (the default case when no chaining is in effect).


> Can we think of a different special value to distinguish between (2)
> and (3)? Ideally one that doesn't change the nice "from None" idiom,
> which I actually like as a way to spell this.

How about this:


Exception Life Cycle
====================


Stage 1 - brand new exception
-----------------------------

raise ValueError()

* __context__ is None
* __cause__ is None


Stage 2 - exception caught, exception raised
--------------------------------------------

try:
    raise ValueError()
except Exception:
    raise CustomError()

* __context__ is previous exception
* __cause__ is True


Stage 3 - exception raised from [exception | None]
--------------------------------------------------

try:
    raise ValueError()
except Exception:
    raise CustomError() from [OtherException | None]

* __context__ is previous exception
* __cause__ is [OtherException | None]


> 
> Sorry that life isn't easier,

Where would be the fun without the challenge?

~Ethan~