Barry Warsaw wrote:
> I agree about bare except, but there is a very valid use case for an
> except clause that catches every possible exception. We need to make
> sure we don't overlook this use case. As an example, say I'm building a
> transaction-aware system, I'm going to want to write code like this:
>
> txn = new_transaction()
> try:
> txn.begin()
> rtn = do_work()
> except AllPossibleExceptions:
> txn.abort()
> raise
> else:
> txn.commit()
> return rtn
>
> I'm fine with not spelling that except statement as "except:" but I
> don't want there to be any exceptions that can sneak past that middle
> suite, including non-errors like SystemExit or KeyboardInterrupt.
>
> I can't remember ever writing a bare except with a suite that didn't
> contain (end in?) a bare raise. Maybe we can allow bare except, but
> constrain things so that the only way out of its suite is via a bare
> raise.
I also use this idiom quite frequently, but I wonder if a finally clause
would be a better way to write it:
txn = new_transaction()
try:
txn.begin()
rtn = do_work()
finally:
if exception_occurred():
txn.abort()
else:
txn.commit()
return rtn
Since this doesn't use try/except/else, it's not affected by changes to
the meaning of except clauses. However, it forces more indentation and
expects a new builtin, and the name "exception_occurred" is probably too
long for a builtin.
Now for a weird idea.
txn = new_transaction()
try:
txn.begin()
rtn = do_work()
finally except:
txn.abort()
finally else:
txn.commit()
return rtn
This is what I would call qualified finally clauses. The interpreter
chooses exactly one of the finally clauses. If a "finally except"
clause is chosen, the exception is re-raised before execution continues.
Most code that currently uses bare raise inside bare except could just
prefix the "except" and "else" keywords with "finally".
Shane
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