[Python-Dev] Another PEP 343 contextmanager glitch

[Phillip J. Eby]

At 03:05 PM 3/24/2006 -0800, Guido van Rossum wrote:
>Oops, that's definitely a bug. Please go ahead and fix in both places.

Will do.


>(I'm still trying to fathom the other PEP 343 issue you brought up.)

Here's the paragraph I'm proposing to add to the PEP to explain that issue:

     NOTE: to facilitate chaining of contexts, __exit__() methods should
     *not* reraise the error that is passed in, because it is always the
     responsibility of the *caller* to reraise.  If an __exit__() method
     exits with *any* exception, the caller of __exit__() should
     interpret this as meaning that the __exit__() method itself
     failed to perform the desired cleanup or other operations.

Currently, the PEP is ambiguous on this point.  When "with" calls 
__exit__() this point is irrelevant because either way, the same exception 
will be raised/propagated.

But if you are writing Python code that explicitly uses the context 
protocol (e.g. the 'nested' context manager), it should be clear as to 
whether a given __exit__() invocation failed or succeeded.  Thus, 
__exit__() should only raise an exception if it *failed*.

(This then implies that the @contextmanager decorator must catch any 
re-raising done by the wrapped generator.)