[Python-Dev] 'With' context documentation draft (was Re: Terminology for PEP 343 (original) (raw)

Michael Hudson mwh at python.net
Fri Jul 8 13:39:58 CEST 2005

Walter Dörwald <walter at livinglogic.de> writes:

Am 07.07.2005 um 20:00 schrieb Guido van Rossum:

+1 on @contextmanager +1. [enter, exit]

These names should be changed to beginwith and endwith.

-1. The PEP has had an extensive review period and several alternatives were discussed and rejected. These names are clear, they do match, and as Fred says the * namespace has lots of room. Also, the PEP was accepted with these names. Now, if it was accepted with a major flaw or some semantics left unspecified, What is still unspecified (or at least not explicitely mentioned) in the PEP is the lifetime of VAR in: with EXPR as VAR: BLOCK Does VAR overwrite or shadow any previous values of VAR?

Yup! Or at least in my half-an-implementation, that's what happens.

(PEP 343 is a lot harder to implement then you might think, btw -- the stack discipline around finally: statements is a bit mind-bending).

I.e. will VAR still exist after the end of the block with its value the return value of enter() or will it revert to the previous value (if any)?

Eh, no. Where would you store the old value?

I'd prefer the later although interpreting the translation of

with EXPR as VAR: BLOCK into abc = EXPR exc = (None, None, None) VAR = abc.enter() try: try: BLOCK except: exc = sys.excinfo() raise finally: abc.exit(*exc)

literally would indicate the former.

Please continue to interpret it like that :)

Cheers, mwh

--

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