[Python-Dev] Decimal & returning NotImplemented (or not) (original) (raw)

M.-A. Lemburg mal at egenix.com
Thu Mar 10 11:15:42 CET 2005

Nick Coghlan wrote:

Guido van Rossum wrote:

No, the reason is that if we did this with exceptions, it would be liable to mask errors; an exception does not necessarily originate immediately with the code you invoked, it could have been raised by something else that was invoked by that code. The special value NotImplemented must pretty much originate directly in the invoked code, since the implementation guarantees that e.g. a+b can never return NotImplemented: if a.add(b) and b.radd(a) both return NotImplemented, TypeError is raised. That makes sense - although for that reasoning, a TypeError subclass that the binary operation machinery promotes to a standard TypeError would seem to work too (with the advantage of also raising at least some sort of exception when the method is called directly)

I have to correct Guido's explanation a bit: the original reason for using a special singleton instead of an exception was indeed a significant difference in performance (raising exceptions at the Python level is expensive, not so much at the C level). I don't remember the details, but I did some measurements at the time which made the decision to use a singleton rather easy :-)

The NotImplemented singleton was part of the coercion proposal: http://www.egenix.com/files/python/CoercionProposal.html (pretty old stuff, now that I look at it again ;-)

That said, Guido's point is just valid. We would have had to add code that saves the current exception to avoid masking any pending errors - code like that always turns into a mess sooner or later.

Cheers,

Marc-Andre Lemburg eGenix.com

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