On Thu, 06 Nov 2008 01:02:34 -0800, brasse wrote:
> Hello!
>
> I have been running in to some problems when using contextlib.nested().
> My problem arises when using code similar to this:
>
> from __future__ import with_statement
>
> from contextlib import nested
>
> class Foo(object):
>
> def __init__(self, tag, fail=False):
> print 'ctor', tag
> self.tag = tag
> if fail:
> raise Exception()
>
> def __enter__(self):
> print '__enter__', self.tag
> return self
>
> def __exit__(self, *args):
> print '__exit__', self.tag
>
> with nested(Foo('a'), Foo('b', True)) as (a, b):
> print a.tag
> print b.tag
>
> Here the construction of b fails which in turn means that the
> contextmanager fails to be created leaving me a constructed object (a)
> that needs to be deconstructed in some way. I realize that nested() is
> in a tight spot here to do anything about it since it doesn't exist.
> This behavior makes it hard for me to use the with statement (using
> nested()) the way I want.
>
> Has anyone else been running in to this? Any tips on how to handle
> multiple resources?
Your problem does not seem to be connected to context managers. The error
occurs before calling `contextlib.nested` at all::
>>> foo = [Foo('a')]
ctor a
>>> with nested(*foo) as a: print a
...
__enter__ a
[<__main__.Foo object at 0x7fbc29408b90>]
__exit__ a
>>> foo = [Foo('a'), Foo('b', True)]
ctor a
ctor b
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 7, in __init__
raise Exception()
Exception
If you need to deconstruct object `a` from your example, your staging is
probably broken. Allocate the resource in `__init__` but only go live
just in `__enter__`. If you do not enter the context, then, you won't
need to deconstruct it as well.
HTH,
--
Robert "Stargaming" Lehmann
--
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