"raise (type, value, traceback)" and "raise type, value, traceback"

[Duncan Booth]

Ian Kelly <ian.g.kelly at gmail.com> wrote:

> t = (type, value, traceback)
> raise t
> 
> That it accepts the tuple and raises a value-less expression of type
> `type` surprises me.  The docs don't say anything about this, and I
> would have expected a TypeError, but it appears to be extracting the
> first element of the tuple and using that as the value of the first
> expression.
> 
Not only that but you can nest tuples and it will drill down as far as 
necessary through the tuples until it finds something that isn't a 
tuple.

It doesn't appear to be documented, but I think it may be intended to 
provide some kind of symmetry with 'try...except': when you catch an 
exception the expression used to catch the exception must be 
'compatible' with the exception, i.e. the exception itself, one of its 
base classes, or a tuple which contains an item 'compatible' with the 
exception.

So my guess would be that since it allows:

MyExceptionSpec = (RuntimeError, ValueError)

try:
    ...
except MyExceptionSpec:
    ...

the intent is to allow you to also throw 'MyExceptionSpec' from within 
the 'try'.

I would also guess that it is a hangover from the days when exceptions 
were strings and you couldn't use inheritance to group exceptions.

-- 
Duncan Booth http://kupuguy.blogspot.com