[Python-Dev] Possible context managers in stdlib (original) (raw)
Nick Coghlan ncoghlan at gmail.com
Wed Jul 13 00:15:15 CEST 2005
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Terry Reedy wrote:
"Nick Coghlan" <ncoghlan at gmail.com> wrote in message news:42D3A766.90705 at gmail.com...
The main outcome of the PEP 343 terminology discussion was some proposed documentation I put on the Sourceforge patch tracker ([1]). Is this a proposal for the Language Reference manual?
No - it's for an entry in the Library reference under 'built-in types', as a sibling to the current documentation of the iteration protocol.
The 'with' statement itself would have to be documented along with the rest of the grammar.
A simpler way to achieve this in Python is to use the 'with' statement along with the appropriate context manager. Somewhere about here we need the syntax itself.
I'm not sure. We don't reproduce the 'for' loop syntax in the documentation of iterators, so should we reproduce the 'with' statement syntax in the documentation of context managers?
Again, modelling on the existing documentation of the iteration protocol, I would expect the statement syntax to be introduced in the tutorial (e.g. as part of Section 8.6, where try/finally is introduced).
Context managers define an... the contained suite starts. If the 'as' clause of the 'with'
Else this does not mean much. ... The simplest use of context management is to strictly control the handling of key resources (such as files, generators, database connections, synchronisation locks). I have a little trouble seeing generators (as opposed to iterables) as resources needing management.
PEP 342 adds this, in order to allow 'yield' inside tyr/finally blocks.
For example, the following context manager allows prompt closure of any resource with a 'close' method (e.g. a generator or file): And I was not aware that they had close methods. You mean a iterable (not just a file) with both an associated generator and a close? Or are generators gaining close methods (which make no sense to me). Or are you using 'generator' in a different sense?
Sorry - these docs assume PEP 342 has been implemented, so generator's have close() methods. I was trying to steer clear of files, since we don't know yet whether there is going to be an "opening" or "closing" implementation in the standard library, or whether files will become context managers. The latter is my preference, but Guido didn't seem too keen on the idea last time it was brought up.
class closing(object): def init(self, resource): self.resource = resource def enter(self): return self.resource def exit(self, *excinfo): self.resource.close()
with closing(mygenerator()) as g: # mygenerator() is assigned to g via call to enter() for item in g: print item # g is closed as the with statement ends To me, this should be with closing(myiterable())... with 'for' calling g.iter to get the iterator that is possibly a generator. Otherwise, I don't understand it. The rest is pretty clear. Terry J. Reedy
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