Duncan Booth wrote:
> "NeBlackCat (lists)" <[EMAIL PROTECTED]> wrote:
>
>> Depending on what you read, sys.exc_info() is supposed to return
>> (None,None,None) when there is no active exception, but it seems that
>> it returns info about the last exception when there isn't one
>> currently active.
>>
>> For example:
>>
>> try:
>> a = a + 1
>> except:
>> pass
>>
>> print sys.exc_info()
>>
>> produces:
>> <class exceptions.NameError at 0x009648D0>, <exceptions.NameError
>> instance at 0x00B5E508>, <traceback object at 0x00B5E4E0>
>>
>> Where the traceback object identifies the offending a=a+1 line (of
>> course).
>>
>> Is there another way of doing this? Note that I can't rely on using
>> sys.exc_clear() in any solution, unfortunately.
>
> I think you have misunderstood the definition of when an exception is
> 'currently active'. When an exception is caught, it remains currently
> active so long as you are in the same function, or in a function which it
> calls (i.e. so long as the current scope is still active). When you return
> from that function the exception is no longer active and the previous
> exception becomes active (or None if there has not been one or you have
> used sys.exc_clear()).
>
> Try this:
> --------- t.py -------------
> import sys
>
> def f():
> try:
> a = a + 1
> except:
> pass
>
> g()
> print "f", sys.exc_info()
>
> def g():
> print "g", sys.exc_info()
>
> def h():
> f()
> print "h", sys.exc_info()
>
> h()
> ----------------------------
> The output is:
>
> g (<type 'exceptions.UnboundLocalError'>, UnboundLocalError("local variable
> 'a' referenced before assignment",), <traceback object at 0x00A8B300>)
> f (<type 'exceptions.UnboundLocalError'>, UnboundLocalError("local variable
> 'a' referenced before assignment",), <traceback object at 0x00A8B300>)
> h (None, None, None)
>
> As you can see the exception remains 'currently active' only until the
> function in which it was caught returns.
Duncan, that's a great description that should go in the docs (I suspect
you aren't just quoting from them here). Kristján V. Jónsson has also
raised some great points about this issue in a multi-threaded
multi-processing application architecture both simplified and
complicated by the presence of Stackless, though I can't lay my hands on
any summary of his findings right now.
regards
Steve
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