[Python-Dev] with-statement heads-up (original) (raw)

Guido van Rossum guido at python.org
Tue Feb 28 19:07:36 CET 2006

On 2/28/06, Mike Bland <mbland at acm.org> wrote:

On 2/28/06, Guido van Rossum <guido at python.org> wrote: > I just realized that there's a bug in the with-statement as currently > checked in. exit is supposed to re-raise the exception if there > was one; if it returns normally, the finally clause is NOT to re-raise > it. The fix is relatively simple (I believe) but requires updating > lots of unit tests. It'll be a while.

Hmm. My understanding was that exit was not to reraise it, but was simply given the opportunity to record the exception-in-progress.

Yes, that's what the PEP said. :-(

Unfortunately the way the PEP is specified, the intended equivalence between writing a try/except in a @contextmanager-decorated generator and writing things out explicitly is lost. The plan was that this:

@contextmanager def foo(): try: yield except Exception: pass

with foo(): 1/0

would be equivalent to this:

try: 1/0 except Exception: pass

IOW

with GENERATOR(): BLOCK

becomes a macro call, and GENERATOR() becomes a macro definition; its body is the macro expansion with "yield" replaced by BLOCK. But in order to get those semantics, it must be possible for exit() to signal that the exception passed into it should not be re-raised.

The current expansion uses roughly this:

finally: ctx.exit(*exc)

and here the finally clause will re-raise the exception (if there was one).

I ran into this when writing unit tests for @contextmanager.

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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