exceptions and items in a list

[rbt]

If I have a Python list that I'm iterating over and one of the objects
in the list raises an exception and I have code like this:

try:
do something to object in list
except Exception:
pass

Does the code just skip the bad object and continue with the other
objects in the list, or does it stop?

Thanks

[Andrey Tatarinov]

Re: exceptions and items in a list

rbt wrote:[color=blue]
> If I have a Python list that I'm iterating over and one of the objects
> in the list raises an exception and I have code like this:
>
> try:
> do something to object in list
> except Exception:
> pass
>
> Does the code just skip the bad object and continue with the other
> objects in the list, or does it stop?[/color]

# skip bad object and continue with others
for object in objects:
try:
#do something to object
except Exception:
pass

# stop at first bad object
try:
for object in objects:
#do something to object
except Exception:
pass

[vincent wehren]

Re: exceptions and items in a list

rbt wrote:[color=blue]
> If I have a Python list that I'm iterating over and one of the objects
> in the list raises an exception and I have code like this:
>
> try:
> do something to object in list
> except Exception:
> pass
>
> Does the code just skip the bad object and continue with the other
> objects in the list, or does it stop?
>
> Thanks[/color]

Fire up a shell and try:
[color=blue][color=green][color=darkred]
>>> seq = ["1", "2", "a", "4", "5", 6.0]
>>> for elem in seq:[/color][/color][/color]
.... try:
.... print int(elem)
.... except ValueError:
.... pass


and see what happens...

--
Vincent Wehren

[rbt]

Re: exceptions and items in a list

Andrey Tatarinov wrote:[color=blue]
> rbt wrote:
>[color=green]
>> If I have a Python list that I'm iterating over and one of the objects
>> in the list raises an exception and I have code like this:
>>
>> try:
>> do something to object in list
>> except Exception:
>> pass
>>
>> Does the code just skip the bad object and continue with the other
>> objects in the list, or does it stop?[/color]
>
>
> # skip bad object and continue with others
> for object in objects:
> try:
> #do something to object
> except Exception:
> pass
>
> # stop at first bad object
> try:
> for object in objects:
> #do something to object
> except Exception:
> pass[/color]

Thanks Andrey. That's a great example of how to do it.

[Peter Hansen]

Re: exceptions and items in a list

rbt wrote:[color=blue]
> Andrey Tatarinov wrote:[color=green]
>> # skip bad object and continue with others
>> for object in objects:
>> try:
>> #do something to object
>> except Exception:
>> pass[/color]
>
> Thanks Andrey. That's a great example of how to do it.[/color]

Actually, it's not really a "great" example, since it catches
_all_ exceptions that might happen, and quietly ignores
them. For example, in the following code, you might not
realize that you've made a typo and none of the items in
the list are actually being checked, even though there is
obviously no problem with any of these simple items
(integers from 0 to 9).

def fornat_value(va l):
return '-- %5d --' % val

L = range(10)
for val in L:
try:
print format_value(va l)
except Exception:
pass

It's almost always better to identify the *specific*
exceptions which you are expecting and to catch those,
or not do a simple "pass". See Vincent's response,
for example, where he explicitly asks only for ValueErrors
and lets others propagate upwards, to be reported.

(I realize these were contrived examples, but examples
that don't mention this important issue risk the propagation
of buggy code...)

-Peter

[Steve Holden]

Re: exceptions and items in a list

vincent wehren wrote:
[color=blue]
> rbt wrote:
>[color=green]
>> If I have a Python list that I'm iterating over and one of the objects
>> in the list raises an exception and I have code like this:
>>
>> try:
>> do something to object in list
>> except Exception:
>> pass
>>
>> Does the code just skip the bad object and continue with the other
>> objects in the list, or does it stop?
>>
>> Thanks[/color]
>
>
> Fire up a shell and try:
>[color=green][color=darkred]
> >>> seq = ["1", "2", "a", "4", "5", 6.0]
> >>> for elem in seq:[/color][/color]
> .... try:
> .... print int(elem)
> .... except ValueError:
> .... pass
>
>
> and see what happens...
>
> --
> Vincent Wehren[/color]

I suspect the more recent versions of Python allow a much more elegant
solution. I can't remember precisely when we were allowed to use
continue in an except suite, but I know we couldn't in Python 2.1.

Nowadays you can write:

Python 2.4 (#1, Dec 4 2004, 20:10:33)
[GCC 3.3.3 (cygwin special)] on cygwin
Type "help", "copyright" , "credits" or "license" for more information.[color=blue][color=green][color=darkred]
>>> for i in [1, 2, 3]:[/color][/color][/color]
... try:
... print i
... if i == 2: raise AttributeError, "Bugger!"
... except AttributeError:
... print "Caught exception"
... continue
...
1
2
Caught exception
3[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

To terminate the loop on the exception you would use "break" instead of
"continue".

regards
Steve
--
Steve Holden http://www.holdenweb.com/
Python Web Programming http://pydish.holdenweb.com/
Holden Web LLC +1 703 861 4237 +1 800 494 3119

[vincent wehren]

Re: exceptions and items in a list

Steve Holden wrote:[color=blue]
> vincent wehren wrote:
>[color=green]
>> rbt wrote:
>>[color=darkred]
>>> If I have a Python list that I'm iterating over and one of the
>>> objects in the list raises an exception and I have code like this:
>>>
>>> try:
>>> do something to object in list
>>> except Exception:
>>> pass
>>>
>>> Does the code just skip the bad object and continue with the other
>>> objects in the list, or does it stop?
>>>
>>> Thanks[/color]
>>
>>
>>
>> Fire up a shell and try:
>>[color=darkred]
>> >>> seq = ["1", "2", "a", "4", "5", 6.0]
>> >>> for elem in seq:[/color]
>> .... try:
>> .... print int(elem)
>> .... except ValueError:
>> .... pass
>>
>>
>> and see what happens...
>>
>> --
>> Vincent Wehren[/color]
>
>
> I suspect the more recent versions of Python allow a much more elegant
> solution. I can't remember precisely when we were allowed to use
> continue in an except suite, but I know we couldn't in Python 2.1.
>
> Nowadays you can write:
>
> Python 2.4 (#1, Dec 4 2004, 20:10:33)
> [GCC 3.3.3 (cygwin special)] on cygwin
> Type "help", "copyright" , "credits" or "license" for more information.[color=green][color=darkred]
> >>> for i in [1, 2, 3]:[/color][/color]
> ... try:
> ... print i
> ... if i == 2: raise AttributeError, "Bugger!"
> ... except AttributeError:
> ... print "Caught exception"
> ... continue
> ...
> 1
> 2
> Caught exception
> 3[color=green][color=darkred]
> >>>[/color][/color]
>
> To terminate the loop on the exception you would use "break" instead of
> "continue".[/color]

What do you mean by a more elegant solution to the problem? I thought
the question was if a well-handled exception would allow the iteration
to continue with the next object or that it would stop. Why would you
want to use the continue statement when in the above case that is
obviously unnecessary?:

$ python
Python 2.4 (#1, Dec 4 2004, 20:10:33)
[GCC 3.3.3 (cygwin special)] on cygwin
Type "help", "copyright" , "credits" or "license" for more information.[color=blue][color=green][color=darkred]
>>> for i in [1,2,3]:[/color][/color][/color]
.... try:
.... if i == 2: raise AttributeError, "Darn!"
.... except AttributeError:
.... print "Caught Exception"
....
1
2
Caught Exception
3[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

Or do you mean that using "continue" is more elegant than using "pass"
if there are no other statements in the except block?


Regards,
--
Vincent Wehren[color=blue]
>
> regards
> Steve[/color]