else in try/except

[Ethan Furman]

The code in 'else' in a 'try/except/else[/finally]' block seems
pointless to me, as I am not seeing any difference between having the
code in the 'else' suite vs having the code in the 'try' suite.

Can anybody shed some light on this for me?

~Ethan~

[MRAB]

The difference is that if an exception occurs in the else block it
won't be caught by the exception handlers of the try statement.

[Arnaud Delobelle]

On 14 November 2011 21:53, Ethan Furman <et...@stoneleaf.us> wrote:
> The code in 'else' in a 'try/except/else[/finally]' block seems pointless to
> me, as I am not seeing any difference between having the code in the 'else'
> suite vs having the code in the 'try' suite.
>
> Can anybody shed some light on this for me?

Exceptions in the else clause will not be caught. Look at the
difference between the two try clauses below:

>>> def f(): raise TypeError
...
>>> try:
... print("try")
... f()
... except TypeError:
... print("except")
...
try
except
>>> try:
... print("try")
... except TypeError:
... print("except")
... else:
... f()
...
try
Traceback (most recent call last):
File "<stdin>", line 6, in <module>
File "<stdin>", line 1, in f
TypeError
>>>

HTH

--
Arnaud

[Chris Kaynor]

On Mon, Nov 14, 2011 at 2:59 PM, MRAB <pyt...@mrabarnett.plus.com> wrote:

> On 14/11/2011 21:53, Ethan Furman wrote:
>>
>> The code in 'else' in a 'try/except/else[/finally]' block seems
>> pointless to me, as I am not seeing any difference between having the
>> code in the 'else' suite vs having the code in the 'try' suite.
>>
>> Can anybody shed some light on this for me?
>>

> The difference is that if an exception occurs in the else block it
> won't be caught by the exception handlers of the try statement.

Consider the examples:

try:
a
raise RuntimeError()
except RuntimeError:
pass

vs

try:
a
except RuntimeError:
pass
else:
raise RuntimeError()

The first example will not raise an exception, while the second will
result in a RuntimeError.

Effectively, the first block will either:
1) If the "a" block raises a RuntimeError, continue,
2) If the "a" block raised any other error, propergate it,
3) If the "a" block does not raise an exception, continue.
while the second block will either:
1) If the "a" block raises a RuntimeError, continue,
2) If the "a" block raised any other error, propergate it,
3) If the "a" block does not raise an exception, raise a RuntimeError.

Note the difference in the 3rd case.

> --
> http://mail.python.org/mailman/listinfo/python-list
>

[Ethan Furman]

Thanks, all!

~Ethan~

[Barry W Brown]

I thought that the point of the else clause is that it is reached only
if there is no exception in the try clause.

[Barry W Brown]

[Grant Edwards]

On 2011-11-15, Barry W Brown <brow...@gmail.com> wrote:

> I thought that the point of the else clause is that it is reached
> only if there is no exception in the try clause.

Not really. If that's all you wanted, then you just put the code at
the end of the try block.

--
Grant Edwards grant.b.edwards Yow! ... I see TOILET
at SEATS ...
gmail.com

[Robert Kern]

On 11/15/11 2:31 PM, Grant Edwards wrote:
> On 2011-11-15, Barry W Brown<brow...@gmail.com> wrote:
>
>> I thought that the point of the else clause is that it is reached
>> only if there is no exception in the try clause.
>
> Not really. If that's all you wanted, then you just put the code at
> the end of the try block.

No, he's right. You should only put code in the try: block where you want
exceptions to be caught. Everything else should be outside of the block. You
really do want to minimize the amount of code inside the try: block.

try:
# minimal code that might raise exceptions that you want to catch
except ThisError:
# handle ThisError exceptions, and probably continue execution
except ThatError:
# handle ThatError exceptions, and probably continue execution
else:
# Code that only runs if ThisError or ThatError were not
# raised in the try: block. This code may raise ThisError or ThatError
# exceptions that should not be caught by the above except: blocks.

# Other code that runs regardless if a caught-and-continued exception
# was raised or not

--
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco