What is "finally:" for?

[Max Ivanov]

Hi all!
When and where I should use try-except-finally statement? What is the
difference between:
--------
try:
A...
except:
B....
finally:
C...
--------

and
-------
try:
A...
except:
B....
C...

[Andreas Kaiser]

On 16 Sep., 13:25, "Max Ivanov" <[e-mail]:

Hi all!
When and where I should use try-except-finally statement? What is the
difference between:
--------
try:
* A...
except:
* B....
finally:
* C...
--------

It does A if no exception or B if an exception occurs. THEN it does
always C.

>
and
-------
try:
* A...
except:
* B....
C...

If an exception occurs, C is never reaching.

Andreas

[Andreas Kaiser]

On 16 Sep., 13:25, "Max Ivanov" <[e-mail]:

Hi all!
When and where I should use try-except-finally statement? What is the
difference between:
--------
try:
* A...
except:
* B....
finally:
* C...
--------

and
-------
try:
* A...
except:
* B....
C...

Look at: http://docs.python.org/ref/try.html and http://bytes.com/forum/thread24648.html

The python doc says:
"Changed in version 2.5: In previous versions of Python,
try...except...finally did not work. try...except had to be nested in
try...finally."

Andreas

[Paul McGuire]

On Sep 16, 6:25*am, "Max Ivanov" <[e-mail]:

Hi all!
When and where I should use try-except-finally statement? What is the
difference between:
--------
try:
* A...
except:
* B....
finally:
* C...
--------

and
-------
try:
* A...
except:
* B....
C...

If B raises or rethrows another exception, finally: ensures that C
still gets called. In the second form that you give, C would not get
called. Here is one way to use try-except-finally:

try:
open database
do database stuff
except DatabaseException, de:
log exception
throw
finally:
close database

Or

try:
transaction = new database transaction
do database stuff
do more database stuff
commit transaction
transaction = None
except DatabaseException, de:
log exception
throw
finally:
if transaction:
rollback transaction
If you are allocating a resource, lock, file, database, etc., then
exception-safe coding style is to get the resource in a try: and
release the resource in finally:

-- Paul

[Ant]

On Sep 16, 12:30*pm, Andreas Kaiser <[e-mail]:

On 16 Sep., 13:25, "Max Ivanov" <[e-mail]:Hi all!

When and where I should use try-except-finally statement? What is the
difference between:
--------
try:
* A...
except:
* B....
finally:
* C...
--------

It does A if no exception or B if an exception occurs. THEN it does
always C.

Rather it runs the code in A. If an exception is thrown in A, B is
then run. C is run regardless.

-------
try:
* A...
except:
* B....
C...

If an exception occurs, C is never reaching.

Whether or not C runs will depend on what code is present in B (e.g.
it could return, or throw a different exception).

Typically you will use a finally clause if there is some cleanup that
should occur regardless of whether an exception is thrown. For
example:

def getNumbers(filename):
numbers = []

fh = open("test.txt") # Should contain lines of numbers
try:
for line in fh:
if line.strip():
numbers.append(int(line.strip()))
except ValueError, e:
return None
finally:
fh.close()

return numbers
This ensures that open files are closed for example.

--
Ant.

[Scott David Daniels]

Paul McGuire wrote:
....

try:
open database
do database stuff
except DatabaseException, de:
log exception
throw
finally:
close database

Or

try:
transaction = new database transaction
do database stuff
do more database stuff
commit transaction
transaction = None
except DatabaseException, de:
log exception
throw
finally:
if transaction:
rollback transaction

Generally it is a good idea to try the advice you are about to offer.

"raise" is the Python way to do the Lisp/Scheme "throw" which will
likely produce another error, but not the one you want.

--Scott David Daniels
Sc***********@Acm.Org