Writing multiple files with with-context

[Shunichi Wakabayashi]

To write onto multiple files on the same time (a number of files are variable),
I'd like to code as follows, for example, IF I can do,

LIST_LEN = 4
with [ 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")

or using a dict object,

DICT_KEYS = ('foo', 'bar', 'baz')
with { k: open('dict_%s.txt' % k, 'w') for k in DICT_KEYS } as fobjdict:
  for i in range(1000):
    fobjdict[random.choice(DICT_KEYS)].write(str(i)+"\n")

However, list and dict don't has __exit__ method and so they cannot run.
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.
Moreover, on using dict, it is required to convert list to dict.

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

class MyDict(dict):
  def __enter__(self):
    return { k: v.__enter__() for k, v in self.items() }
  def __exit__(self, exc_type, exc_value, traceback):
    ret = False
    for v in self.values():
      if v.__exit__(exc_type, exc_value, traceback):
        ret = True
        exc_type = exc_value = traceback = None
    return ret

with MyList( 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 MyDict( (k, open('dict_%s.txt' % k, 'w')) for k in DICT_KEYS ) as fobjdict:
  for i in range(1000):
    fobjdict[random.choice(DICT_KEYS)].write(str(i)+"\n")

I think this is smarter a little than others,
but it cannot guaranteed to call __exit__() of members in containers
if members are changed during with-context.

So, do you have another, more smart and pythonic way?

Thanks