[Python-Dev] PEP 409 - final? (original) (raw)

Ethan Furman ethan at stoneleaf.us
Wed Feb 1 19:48:33 CET 2012

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()

Stage 2 - exception caught, exception raised

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

Stage 3 - exception raised from [exception | None]

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

Sorry that life isn't easier,

Where would be the fun without the challenge?

Ethan

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