On 24/10/2010 13:28, Steve Holden wrote:
On 10/24/2010 4:48 AM, Martin v. Loewis wrote:
Am 24.10.2010 07:01, schrieb Steve Holden:
I was somewhat surprised to discover that Python 3 no longer allows an
exception to be raised in an except clause (or rather that it reports it
as a separate exception that occurred during the handling of the first).
I think you are misinterpreting what you are seeing. The exception being
raised actually *is* an attribute error, and it actually is the
attribute error that gets reported. It's only that reporting an
exception that has a __context__ first reports the context, then reports
the actual exception.
I don't believe I *am* misinterpreting it. The fact of the matter is
that the context is irrelevant to the user, and there should be some way
to suppress it to avoid over-complicating the traceback.
This behavior is quite reasonable during testing, but I would prefer to
exclude an explicit raise directly in the except handler since that is
hardly to be construed as accidental (whereas an exception in a function
called in the handler perhaps *should* be reported).
You may now wonder whether it is possible to set __context__ to None
somehow. See PEP 3134:
Open Issue: Suppressing Context
As written, this PEP makes it impossible to suppress '__context__',
since setting exc.__context__ to None in an 'except' or 'finally'
clause will only result in it being set again when exc is raised.
I have already read that. Peter Otten has separately explained how to
suppress the behavior using sys.excepthook, which appears to be a
halfway satisfactory solution.
Suggestion: an explicit 'raise' in the exception handler excludes the
context, but if you want to include it then 'raise with'. For example:
# Exclude the context
try:
command_dict[command]()
except KeyError:
raise CommandError("Unknown command")
# Include the context
try:
command_dict[command]()
except KeyError:
raise with CommandError("Unknown command")
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