try pattern for database connection with the close method

[Peter Otten]

Ian Kelly wrote:

> On Sat, Feb 21, 2015 at 5:22 AM, Mark Lawrence <breamoreboy at yahoo.co.uk>
> wrote:
>> On 21/02/2015 02:42, Mario Figueiredo wrote:
>>>
>>> Hello all,
>>>
>>> I'm using the following pattern for db access  that requires me to
>>> close the connection as soon as it is not needed:
>>>
>>>          import sqlite3 as lite
>>>
>>>          try:
>>>              db = lite.connect('data.db')
>>>          except lite.DatabaseError:
>>>              raise OSError('database file corrupt or not found.')
>>>          else:
>>>              try:
>>>                  with db:
>>>                      db.execute(sql, parms)
>>>              except lite.IntegrityError:
>>>                  raise ValueError('invalid data')
>>>              finally:
>>>                  db.close()
>>>
>>> Since it's a bit verbose, is there a better way?
>>>
>>> Note: The user of this API has the whole database functionality
>>> abstracted away. Hence the exception channeling in the except clauses.
>>>
>>
>> Use your context manager at the outer level.
>>
>> import sqlite3 as lite
>>
>> try:
>>     with lite.connect('data.db') as db:
>>     try:
>>         db.execute(sql, parms)
>>     except lite.IntegrityError:
>>         raise ValueError('invalid data')
>> except lite.DatabaseError:
>>     raise OSError('database file corrupt or not found.')
> 
> This could result in the OSError being misleadingly raised due to some
> DatabaseError raised by the execute rather than the connect.

The OP probably wants to catch these DatabaseErrors, too. Also, the chance 
of a misleading traceback has been greatly reduced with the advent of 
chained exceptions.