On Thu, 2007-06-21 at 10:41 -0600, Anthony Tuininga wrote:
> I have been recently playing with context managers and the new "with"
> statement added in Python 2.5. I would like to suggest the addition of
> the following methods to connections:
>
> def __enter__(self):
> return self
>
> def __exit__(self, excType, excValue, excTraceback):
> if excType is None and excValue is None and excTraceback is None:
> self.commit()
> else:
> self.rollback()
>
> This allows the following code:
>
> from __future__ import with_statement
>
> connection = .....
> with connection:
> cursor = connection.cursor()
> cursor.execute("update SomeTable set SomeColumn = "SomeValue")
> cursor.execute("delete from SomeOtherTable where SomeOtherColumn = 5)
>
>
> rather than
>
> try:
> cursor = connection.cursor()
> cursor.execute("update SomeTable set SomeColumn = "SomeValue")
> cursor.execute("delete from SomeOtherTable where SomeOtherColumn = 5)
> connection.commit()
> except:
> connection.rollback()
>
> I've implemented this in cx_Oracle and it appears to work quite
> nicely. Thoughts?
It's a good idea, but there is a conflicting use case "in the wild" for
having a connection behave as a context manager. InformixDB allows you
to use connections and cursors as context managers that close
themselves, analogous to what file objects do:
with api.connect(dbname) as conn:
with conn.cursor() as cur:
cur.execute(stmt)
To disambiguate these use cases and to improve code-readability, I think
a context manager that encapsulates a transaction should have a name
that makes this clear. Something like this:
with conn.transaction():
do_stuff()
I think we'd also have to discuss the semantics of how this with-managed
transaction coexists with the transaction that is already active on the
connection. Should that transaction be committed, rolled back, or
continued? If the engine underneath supports nested transactions, should
the with-statement's transaction enter a nested transaction?
Maybe the transaction() function should grow a parameter for specifying
this.
Regards,
--
Carsten Haese
http://informixdb.sourceforge.net
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