What I was trying to say was: I think of ValueError as applying to a single ("bad") value whereas ArithmeticError can arise from trying to combine multiple ("good") values. It wouldn't be appropriate for ArithmeticError to be a subclass of ValueError, because there is no single erroneous value that you can point to. On 25/05/2016 09:08, Rob Cliffe wrote:
Or more generally, any operation on multiple values, where each value is valid individually, but the result of the operation is not:
p = Decimal('1E999999999')
q = Decimal('10')
try:
... p*q
... except ArithmeticError:
... print 'ArithmeticError'
...
ArithmeticError
On 25/05/2016 06:48, Guido van Rossum wrote:
Well, ZeroDivisionError doesn't derive from ValueError, does it? So it may not be a good idea, but it's certainly a tradition.
On Tuesday, May 24, 2016, Greg Ewing <greg.ewing@canterbury.ac.nz <mailto:greg.ewing@canterbury.ac.nz>> wrote:
Guido van Rossum wrote:
It's very common for modules to define their own root exception class.
And it's a nuisance when their exceptions only belong to their own private hierarchy and don't participate in any the standard classifications.
I don't think Decimal is directly to blame here, because deriving its exceptions from ArithmeticError seems like a reasonable thing to do. What *doesn't* seem reasonable to me is that ArithmeticError doesn't derive from ValueError. So far nobody has explained why that's a good idea.
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