Python missing __exit__ method

Question

Some background: I work in a large bank and I'm trying to re-use some Python modules, which I cannot change, only import. I also don't have the option of installing any new utilities/functions etc (running Python 2.6 on Linux).

I've got this at present:

In my module:

from common.databaseHelper import BacktestingDatabaseHelper

class mfReportProcess(testingResource):
    def __init__(self):
        self.db = BacktestingDatabaseHelper.fromConfig('db_name')

One of the methods called within the 'testingResource' class has this:

 with self.db as handler:

which falls over with this:

with self.db as handler:
AttributeError: 'BacktestingDatabaseHelper' object has no attribute '__exit__'

and, indeed, there is no __exit__ method in the 'BacktestingDatabaseHelper' class, a class which I cannot change.

However, this code I'm trying to re-use works perfectly well for other apps - does anyone know why I get this error and no-one else? Is there some way of defining __exit__ locally?

Many thanks in advance.

EDITED to add:

I've tried to add my own class to setup DB access but can't get it to work - added this to my module:

class myDB(BacktestingDatabaseHelper): 
    def __enter__(self): 
        self.db = fromConfig('db_name') 
    def __exit__(self): 
        self.db.close() 

and added:

self.db = myDB 

into my init attribute for my main class but I get this error:

with self.db as handler:
TypeError: unbound method __enter__() must be called with myDB instance as first argument (got nothing instead)

Any suggestions as to how to do this properly?

Answer 1

Using the with protocol assumes that the object used in with implements the context manager protocol.

Basically this means that the class definition should have __enter__() and __exit__() methods defined. If you use an object without these, python will throw an AttributeError complaining about the missing __exit__ attribute.