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On Apr 12, 2012, at 1:55 PM, Manlio Perillo wrote:
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> Hi.
>
> I looked at SQLAlchemy source code to see examples about `with`
> statement support and found something that look suspicious to me.
>
> class Engine:
> ....
>
> def begin(self, close_with_result=False)
> conn = self.contextual_connect(
> close_with_result=close_with_result)
> trans = conn.begin()
> return Engine._trans_ctx(conn, trans, close_with_result)
>
> What happens in case conn.begin() fails?
the call to begin() overall would raise an exception.
>
> I was assuming that conn.begin always executed a BEGIN SQL command, but
> this is not the case.
> However some dialects (like informix and MySQL oursql) *do* execute SQL
> commands, and these can fail.
and I guess the concern is the Connection is left to the garbage collector to
be closed. In rfc4e7ddc0777 I changed it to:
conn = self.contextual_connect(close_with_result=close_with_result)
try:
trans = conn.begin()
except:
conn.close()
raise
return Engine._trans_ctx(conn, trans, close_with_result)
note that in other cases like engine.execute(), the "close_with_result" flag is
unconditionally true meaning an exception there closes out the connection in
line 1830 in _handle_dbapi_exception().
> I have another question, about coding style.
> Current SQLAlchemy code create the connection and transaction objects in
> the Engine.begin method, and then return an instance of
> Engine._trans_ctx class.
>
> What about, instead, creating the connection and transaction objects
> inside the __enter__ method of the Engine._trans_ctx class?
>
> Doing this way, one can prevent incorrect usage of the context manager;
> that is doing so:
>
> ctx = db.begin()
>
> should not create a connection that will never be explicitly closed.
oh that's a nice idea, if you create a pull request from
https://bitbucket.org/sqlalchemy/sqlalchemy I can pull that in, thanks !
you'll need to adjust my new test
test.engine.test_execute:ConvenienceExecuteTest.test_transaction_engine_ctx_begin_fails
to use testing.run_as_contextmanager in order to hit the exception in that
case.
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