Easiest way to add a function to existing class

I'm using the python's built-in shelve module to manage some simple dictionaries. The problem I'm having is I want to use with shelve.open(filename) as f:, but when I try it claims DbfilenameShelf has no attribute __exit__.

So, I'm guessing the easiest way to do this is to wrap it in another class and add an __exit__ function to that wrapper. I tried this:

class Wrapper(shelve.DbfilenameShelf):
    def __exit__(self):
        self.close()
    def __init__(self, filename, writeback=False):
        shelve.DbfilenameShelf.__init__(self, filename, flag='c', protocol=None, writeback=False)

But when I tried to instantiate the wrapper like so: wrapped = Wrapper(filename) it tells me I'm giving it an invalid argument.

Error as requested:

Traceback (most recent call last):
File "<input>", line 1, in <module>
File "<input>", line 5, in __init__
File "C:\Python27\Lib\shelve.py", line 223, in __init__
Shelf.__init__(self, anydbm.open(filename, flag), protocol, writeback)
File "C:\Python27\Lib\anydbm.py", line 85, in open
return mod.open(file, flag, mode)
File "C:\Python27\Lib\dbhash.py", line 18, in open
return bsddb.hashopen(file, flag, mode)
File "C:\Python27\Lib\bsddb\__init__.py", line 364, in hashopen
d.open(file, db.DB_HASH, flags, mode)
DBInvalidArgError: (22, 'Invalid argume开发者_如何学Cnt')    

Don't subclass it. Python comes with a tool for automatically calling close(), contextlib.closing:

from contextlib import closing
with closing(shelve.open(filename)) as f:
    # your 'with' block here

will automatically call the close() method of the object returned by shelve.open(filename) at the end of the with block.

You're subclassing the wrong thing and missing an __enter__ method. You probably want this:

class contextShelf(shelve.shelve):
  def __enter__(self):
    return self

  def __exit__(self, exc_type, exc_value, exc_trace):
    self.close()

Because you're adding methods, but not changing the __init__ signature or adding any extra steps there's no reason you need to redefine __init__. The base class' __init__ will be called automatically.