Am 02.11.2012 01:15, schrieb Vernon Cole:
> I've been scratching my head over this question, too. Someone
> (Christoph?) please craft for us a little sample program, as small as
> possible, showing how this ought to look in practice.
The implementation would be like this:
class ConnectionWithContextManager(Connection):
def __enter__(self):
return self
def __exit__(self, et, ev, tb):
if et is None and ev is None and tb is None:
self.commit()
else:
self.rollback()
Cursor objects should also have an __enter__ method that returns self,
but their __exit__ method should just execute self.close(). You can also
call contextlib.closing on a normal cursor to get that.
The usage would be like this:
con = dbapi2.connect(...)
with con: # 1st transaction
with con.cursor() as cur:
cur.execute("insert into debit(amount) values (-100)")
cur.execute("insert into credit(amount) values (100)")
with con: # 2nd transaction
with con.cursor() as cur:
cur.execute("insert into debit(amount) values (-200)")
cur.execute("insert into credit(amount) values (200)")
con.close()
SQLite hasn't implemented cursors as context managers, but it has added
execute() as a shortcut method on the connection, so here you can do:
con = sqlite3.connect(...)
with con: # 1st transaction
con.execute("insert into debit(amount) values (-100)")
con.execute("insert into credit(amount) values (100)")
with con: # 2nd transaction
con.execute("insert into debit(amount) values (-200)")
con.execute("insert into credit(amount) values (200)")
con.close()
See also
http://docs.python.org/2/library/sqlite3.html#using-the-connection-as-a-context-manager
What I find a bit ugly about this approach is that now the context
managers of connections and cursors behave differently, and that code
written using these context managers is not self-explanatory. If
connections and cursors would just have closing context managers, and
connections had an extra "transaction" member that is a context manager
for handling the transaction, the code would be more readable and
explicit which is better than implicit:
with dbapi2.connect(...) as con:
with con.transaction:
with con.cursor() as cur:
cur.execute("insert into debit(amount) values (-100)")
cur.execute("insert into credit(amount) values (100)")
with con.transaction:
with con.cursor() as cur:
cur.execute("insert into debit(amount) values (-200)")
cur.execute("insert into credit(amount) values (200)")
Or, when using the shortcut methods:
with dbapi2.connect(...) as con:
with con.transaction:
con.execute("insert into debit(amount) values (-100)")
con.execute("insert into credit(amount) values (100)")
with con.transaction:
con.execute("insert into debit(amount) values (-200)")
con.execute("insert into credit(amount) values (200)")
-- Christoph
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