Nested try...except

bruno.desthuilliers at gmail.com bruno.desthuilliers at gmail.com
Thu Apr 3 01:22:16 CEST 2008


On 2 avr, 15:22, Magnus.Morab... at gmail.com wrote:
> On 2 Apr, 15:15, Magnus.Morab... at gmail.com wrote:
>
>
>
> > On 2 Apr, 15:12, cokofree... at gmail.com wrote:
>
> > > On Apr 2, 3:06 pm, Magnus.Morab... at gmail.com wrote:
>
> > > > Hi,
>
> > > > I found the following code on the net -
>
> > > > http://mail-archives.apache.org/mod_mbox/httpd-python-cvs/200509.mbox/%3C20050924104732.5116.qm...@minotaur.apache.org%3E
>
> > > > def count(self):
> > > > -            db = sqlite.connect(self.filename,
> > > > isolation_level=ISOLATION_LEVEL)
> > > > -            try:
> > > > -                try:
> > > > -                    cur = db.cursor()
> > > > -                    cur.execute("select count(*) from sessions")
> > > > -                    return cur.fetchone()[0]
> > > > -                finally:
> > > > -                    cur.close()
> > > > -            finally:
> > > > -                db.close()
>
> > > > I don't understand though why the second try is not after the line cur
> > > > = db.cursor(). Can anyone explain for me why?
>
> > > > /Barry.
>
> > > Better question is why is there a try with no except...
>
> > > Better yet, WHY is there two TRY statements when there could quite
> > > happily be only one...
>
> > > Towards what you are asking, I GUESS...because the author hoped to
> > > handle the cases where cur failed to get assigned...but then
> > > his .close method of it would likely not work anyway...I mean...does
> > > this even work...YUCK
>
> > I shouldn't have written "Nested try...except" as the title, instead I
> > mean "Nested try...finally". Sorry about that...
>
> > Anyway, how would you do this? That is, use a finally to close the
> > network connection and the cursor?
>
> > Thanks for your help,
>
> > Barry
>
> Here's what I would do. Is it OK?
>
>     def ExecuteWithNoFetching(self, queryString):
>
>         sqlServerConnection = adodbapi.connect (";".join (connectors))

slightly OT : where this 'connectors' comes from ?

>         try:
>             cursor = sqlServerConnection.cursor()
>             try:
>                 cursor.execute(queryString)
>                 raise Exception("Exception")
>                 sqlServerConnection.commit()
>             finally:
>                 cursor.close()
>         finally:
>             sqlServerConnection.close()

Else it looks ok, even if a bit paranoïd. Also, opening a db
connection for each and every single query might not be the best
solution... And finally (no pun intented), this won't let you chain
calls within a single transaction.

Another, a bit more flexible solution could be something like:

def run_in_transaction(*funcs):
    sqlServerConnection = adodbapi.connect (";".join (connectors))
    try:
        cursor = sqlServerConnection.cursor()
        try:
            for func in funcs:
                func(cursor)
        except Exception, e:
            sqlServerConnection.rollback()
            raise
        else:
            sqlServerConnection.commit()
        finally:
             cursor.close()
    finally:
        sqlServerConnection.close()

def do_something(cursor):
    cursor.execute("some update query")

def do_something_else(cursor):
    cursor.execute("some select query")
    for row in cursor.fetchall():
       if some_condition(row):
          cursor.execute("some update query")

run_in_transaction(do_something, do_something_else)

And also, there's this new 'with' statement...

My 2 cents...



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