(I still don't really have net access back after moving house - just
chiming in briefly via my mobile)
Anyway, I think there is one very good reason for NOT defining a multi-
with statement in terms of an existing tuple: it gains us nothing
except speed over contextlib.nested. The whole point of the new
syntactic support is to execute each expression inside the context of
the preceding managers. That requirement precludes the idea of using
an intermediate tuple, since every expression would have to be
evaluated before the tuple could be created.
I'm still not 100% convinced the saving in indentation levels due to
this change would be worth the increase in complexity and ambiguity
though.
--
Nick Coghlan, Brisbane, Australia
On 03/05/2009, at 6:12 AM, Georg Brandl <g.bra...@gmx.net> wrote:
Fredrik Johansson schrieb:
On Sat, May 2, 2009 at 9:01 PM, Georg Brandl <g.bra...@gmx.net>
wrote:
Hi,
this is just a short notice that Mattias Brändström and I have f
inished a
patch to implement the previously discussed and mostly warmly
welcomed
extension to with's syntax, allowing
with A() as a, B() as b:
to be written instead of
with A() as a:
with B() as b:
I was hoping for the other syntax in order to be able to create a
nested context in advance as a simple tuple:
with A, B:
pass
context = A, B
with context:
pass
(I.e. a tuple, or perhaps any iterable, would be a valid context
manager.)
I see; you want to construct your context manager programmatically
and pass
it to "with" without knowing what is in there.
While this would be possible, we have to be aware that with this we
would
effectively change the context manager protocol, rather like the
iterator
protocol's __getitem__ alternate realization. This muddies the
definition
of a context manager.
(The interesting thing is that you could already implement *that*
version
without any new syntactic support, by giving tuples an __enter__/
__exit__
method pair.)
With the syntax in the patch, I will still have to implement a custom
nesting context manager to do this, which sort of defeats the
purpose.
Not really. Having an unknown number of stacked context managers is
not
the purpose -- for that, I'd still say a custom nesting context
manager
is better, because it is also more explicit when created not at the
"with"
site. (You could even write it as a tuple subclass, if you like the
tuple
interface.)
Georg
--
Thus spake the Lord: Thou shalt indent with four spaces. No more, no
less.
Four shall be the number of spaces thou shalt indent, and the number
of thy
indenting shall be four. Eight shalt thou not indent, nor either
indent thou
two, excepting that thou then proceed to four. Tabs are right out.
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