>>> import shelve
>>> from contextlib import contextmanager
>>> class Database:
...     def __init__(self, path):
...         self.path = path
...
...     @contextmanager
...     def open(self):
...         s = shelve.open(self.path, writeback=True)
...         try:
...             yield s
...         finally:
...             s.close()
...
>>>
>>> d = Database('/home/zach/.tweet_poster/cache')
>>>
>>> with d.open() as db:
...     print(db['108710608601493504'])
...
Traceback (most recent call last):
  File "/usr/lib/python3.2/shelve.py", line 111, in __getitem__
    value = self.cache[key]
KeyError: '108710608601493504'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/usr/lib/python3.2/shelve.py", line 113, in __getitem__
    f = BytesIO(self.dict[key.encode(self.keyencoding)])
KeyError: b'108710608601493504'
>>>