On Sun, May 22, 2011 at 8:48 PM, Shunichi Wakabayashi
<shunichi_wakabaya...@yahoo.co.jp> wrote:
> One idea is using contextlib.nested(),
>
> from contextlib import nested
>
> with nested(*[open('list_%d.txt' % i, 'w') for i in range(LIST_LEN)]) as
> fobjlist:
> for i in range(1000):
> fobjlist[random.randrange(LIST_LEN)].write(str(i)+"\n")
>
> with nested(*[open('dict_%s.txt' % k, 'w') for k in DICT_KEYS]) as fobjlist:
> fobjdict = dict(zip(DICT_KEYS, fobjlist)) #convert list to dict
> for i in range(1000):
> fobjdict[random.choice(DICT_KEYS)].write(str(i)+"\n")
>
> On Python2.x, this is OK. but 3.x warns that nested() is deprecated.
Not merely deprecated. It has already been removed in 3.2.
> Another idea is to make container classes having __exit__() myself.
>
> class MyList(list):
> def __enter__(self):
> return [ v.__enter__() for v in self ]
> def __exit__(self, exc_type, exc_value, traceback):
> ret = False
> for v in self:
> if v.__exit__(exc_type, exc_value, traceback):
> ret = True
> exc_type = exc_value = traceback = None
> return ret
This has a number of subtle bugs in it:
1) Each context manager's __exit__ method is not loaded before the
corresponding __enter__ method is invoked.
2) If the second context manager's __enter__ method raises an
exception, the first context manager's __exit__ method is never
called, breaking the with statement guarantee.
3) The __exit__ methods are called in the same order that the
__enter__ methods were called. Since they form a stack, they should
be called in the reverse order.
These highlight the complexity of handling context managers correctly,
which I think suggests that a custom implementation is probably a bad
idea.
> So, do you have another, more smart and pythonic way?
Copy the implementation of contextlib.nested to your own custom module
and use that. The last revision prior to its removal is here:
http://hg.python.org/cpython/file/45506be44514/Lib/contextlib.py
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