Python style: exceptions vs. sys.exit()

[Drake]

I have a general question of Python style, or perhaps just good
programming practice.

My group is developing a medium-sized library of general-purpose
Python functions, some of which do I/O. Therefore it is possible for
many of the library functions to raise IOError Exceptions. The
question is: should the library function be able to just dump to
sys.exit() with a message about the error (like "couldn't open this
file"), or should the exception propagate to the calling program which
handles the issue?

Thanks in advance for anyone who can either answer my question or
point me to where this question has already been answered.

[Larry Bates]

Re: Python style: exceptions vs. sys.exit()

Drake wrote:

I have a general question of Python style, or perhaps just good
programming practice.
>
My group is developing a medium-sized library of general-purpose
Python functions, some of which do I/O. Therefore it is possible for
many of the library functions to raise IOError Exceptions. The
question is: should the library function be able to just dump to
sys.exit() with a message about the error (like "couldn't open this
file"), or should the exception propagate to the calling program which
handles the issue?
>
Thanks in advance for anyone who can either answer my question or
point me to where this question has already been answered.
>

IMHO libraries should always just let the exception propagate up to the caller.
That allows the caller the option of taking the appropriate action.

-Larry

[Grant Edwards]

Re: Python style: exceptions vs. sys.exit()

On 2008-09-23, Drake <cjdrake@gmail. comwrote:

My group is developing a medium-sized library of general-purpose
Python functions, some of which do I/O. Therefore it is possible for
many of the library functions to raise IOError Exceptions. The
question is: should the library function be able to just dump to
sys.exit() with a message about the error (like "couldn't open this
file"),

No. A library module should never call sys.exit().

or should the exception propagate to the calling program which
handles the issue?

Yes. Let the application handle the error if it wants to. If
it's not handled, it'll end up causing the program to exit.

--
Grant Edwards grante Yow! It's a hole all the
at way to downtown Burbank!
visi.com

[Craig Allen]

Re: Python style: exceptions vs. sys.exit()

The

question is: should the library function be able to just dump to
sys.exit() with a message about the error (like "couldn't open this
file"), or should the exception propagate to the calling program which
handles the issue?
>

my view is that the exceptions are there precisely to tell the calling
program about the error and give the programmer a chance to do
something smart about it. A library, properly, doesn't even know the
context in which it is running, and sys.exit() is pretty heavy handed
for a library to call and shows assumptions beyond what a library
should assume about its running environment.

imho

[Terry Reedy]

Re: Python style: exceptions vs. sys.exit()

Drake wrote:

I have a general question of Python style, or perhaps just good
programming practice.
>
My group is developing a medium-sized library of general-purpose
Python functions, some of which do I/O. Therefore it is possible for
many of the library functions to raise IOError Exceptions. The
question is: should the library function be able to just dump to
sys.exit() with a message about the error (like "couldn't open this
file"),

No

or should the exception propagate to the calling program which
handles the issue?

Yes -- with an informative error message.

If the caller ignores the exception, the program will exit with a full
stack trace anyway.

[Christian Heimes]

Re: Python style: exceptions vs. sys.exit()

Drake wrote:

I have a general question of Python style, or perhaps just good
programming practice.
>
My group is developing a medium-sized library of general-purpose
Python functions, some of which do I/O. Therefore it is possible for
many of the library functions to raise IOError Exceptions. The
question is: should the library function be able to just dump to
sys.exit() with a message about the error (like "couldn't open this
file"), or should the exception propagate to the calling program which
handles the issue?

Side note:

sys.exit() is just another way to write raise SystemExit. The function
is defined as:

static PyObject *
sys_exit(PyObje ct *self, PyObject *args)
{
PyObject *exit_code = 0;
if (!PyArg_UnpackT uple(args, "exit", 0, 1, &exit_code))
return NULL;
/* Raise SystemExit so callers may catch it or clean up. */
PyErr_SetObject (PyExc_SystemEx it, exit_code);
return NULL;
}

Christian

[Steven D'Aprano]

Re: Python style: exceptions vs. sys.exit()

On Tue, 23 Sep 2008 13:25:26 -0700, Drake wrote:

I have a general question of Python style, or perhaps just good
programming practice.
>
My group is developing a medium-sized library of general-purpose Python
functions, some of which do I/O. Therefore it is possible for many of
the library functions to raise IOError Exceptions. The question is:
should the library function be able to just dump to sys.exit() with a
message about the error (like "couldn't open this file"), or should the
exception propagate to the calling program which handles the issue?
>
Thanks in advance for anyone who can either answer my question or point
me to where this question has already been answered.

Presumably somebody has suggested that calling sys.exit() was a good
option. I'm curious to what possible reason they could give for such a
poor choice.

Hint: if a library function can't open a file, the application should be
given the opportunity to open a different file. Or do something
completely different instead. Whatever. That's not up to the library to
decide, not even if the library is in such a broken state that it can't
continue. Why not? Because the application might deal with that by
unloading the library and continuing regardless.



--
Steven

[Asun Friere]

Re: Python style: exceptions vs. sys.exit()

On Sep 24, 8:10 am, Christian Heimes <li...@cheimes. dewrote:

Side note:
>
sys.exit() is just another way to write raise SystemExit. The function
is defined as:
>

As can be seen if you were ever silly enough to call sys.exit() in
IDLE. ;)

[Bruno Desthuilliers]

Re: Python style: exceptions vs. sys.exit()

Drake a écrit :

I have a general question of Python style, or perhaps just good
programming practice.
>
My group is developing a medium-sized library of general-purpose
Python functions, some of which do I/O. Therefore it is possible for
many of the library functions to raise IOError Exceptions. The
question is: should the library function be able to just dump to
sys.exit() with a message about the error (like "couldn't open this
file"),

Arrghll ! NO, DONT !

or should the exception propagate to the calling program which
handles the issue?

Yes, by all means and for God's sake.

Thanks in advance for anyone who can either answer my question or
point me to where this question has already been answered.

There have been numerous threads about this here.

[Carl Banks]

Re: Python style: exceptions vs. sys.exit()

On Sep 23, 4:25 pm, Drake <cjdr...@gmail. comwrote:

The
question is: should the library function be able to just dump to
sys.exit() with a message about the error (like "couldn't open this
file"),

I'm kind of curious what your library is for. Is it something where
exiting the app be the only appropriate action for an IO error?

Even if it is, I will echo other people's advice: a library should
never call exit, at least not by default. For your imagined use it
might make sense to exit upon any failure, but other people using the
library might not want to do that.

For that matter, a library should never print error or status
messages. Messages should either be sent to the caller somehow, or
handled using the logging facility.


Carl Banks

[Grant Edwards]

Re: Python style: exceptions vs. sys.exit()

On 2008-09-24, Steven D'Aprano <steven@REMOVE. THIS.cybersourc e.com.auwrote:

On Tue, 23 Sep 2008 13:25:26 -0700, Drake wrote:
>

>I have a general question of Python style, or perhaps just good
>programming practice.
>>
>My group is developing a medium-sized library of general-purpose Python
>functions, some of which do I/O. Therefore it is possible for many of
>the library functions to raise IOError Exceptions. The question is:
>should the library function be able to just dump to sys.exit() with a
>message about the error (like "couldn't open this file"), or should the
>exception propagate to the calling program which handles the issue?
>>
>Thanks in advance for anyone who can either answer my question or point
>me to where this question has already been answered.

>
>
Presumably somebody has suggested that calling sys.exit() was a good
option. I'm curious to what possible reason they could give for such a
poor choice.

Same here. It's like an automotive engine controls designer
asking if a failed O2 sensor should turn on the check engine
light or blow up the car.

--
Grant Edwards grante Yow! Mr and Mrs PED, can I
at borrow 26.7% of the RAYON
visi.com TEXTILE production of the
INDONESIAN archipelago?

[Tim Rowe]

Re: Python style: exceptions vs. sys.exit()

2008/9/24 Bruno Desthuilliers <bruno.42.desth uilliers@websit eburo.invalid>:

Drake a écrit :

>many of the library functions to raise IOError Exceptions. The
>question is: should the library function be able to just dump to
>sys.exit() with a message about the error (like "couldn't open this
>file"),

>
Arrghll ! NO, DONT !

Can I put in a vote *for* the questioner's library dumping to
sys.exit() on any abnormal condition? It would reduce employment
competition for the rest of us.

Why, yes, I am wearing my BOFH hat. How could you tell?

--
Tim Rowe

[Ross Ridge]

Re: Python style: exceptions vs. sys.exit()

Steven D'Aprano <steven@REMOVE. THIS.cybersourc e.com.auwrote:

Presumably somebody has suggested that calling sys.exit() was a good
option. I'm curious to what possible reason they could give for such a
poor choice.

Grant Edwards <invalid@invali dwrote:

>Same here. It's like an automotive engine controls designer
>asking if a failed O2 sensor should turn on the check engine
>light or blow up the car.

No, it's more like asking if the failed sensor should turn on a strange
and mysterious light on the dashboard and then blow up the car if the
driver doesn't immediately stop and check the engine. The owners manual
would only vaguely hint at the fact that this could happen.

Ross Ridge

--
l/ // Ross Ridge -- The Great HTMU
[oo][oo] rridge@csclub.u waterloo.ca
-()-/()/ http://www.csclub.uwaterloo.ca/~rridge/
db //

[Grant Edwards]

Re: Python style: exceptions vs. sys.exit()

On 2008-09-24, Ross Ridge <rridge@csclub. uwaterloo.cawro te:

Steven D'Aprano <steven@REMOVE. THIS.cybersourc e.com.auwrote:

>Presumably somebody has suggested that calling sys.exit() was a good
>option. I'm curious to what possible reason they could give for such a
>poor choice.

>
Grant Edwards <invalid@invali dwrote:

>>Same here. It's like an automotive engine controls designer
>>asking if a failed O2 sensor should turn on the check engine
>>light or blow up the car.

>
No, it's more like asking if the failed sensor should turn on
a strange and mysterious light on the dashboard

You're right. I had forgotten that sys.exit() is actually
raising the system exit exception, and that the application
calling the library could handle that exception.

and then blow up the car if the driver doesn't immediately
stop and check the engine. The owners manual would only
vaguely hint at the fact that this could happen.

--
Grant Edwards grante Yow! Why is it that when
at you DIE, you can't take
visi.com your HOME ENTERTAINMENT
CENTER with you??

[Ross Ridge]

Re: Python style: exceptions vs. sys.exit()

Steven D'Aprano <steven@REMOVE. THIS.cybersourc e.com.auwrote:

Presumably somebody has suggested that calling sys.exit() was a good
option. I'm curious to what possible reason they could give for such a
poor choice.

Grant Edwards <invalid@invali dwrote:

>Same here. It's like an automotive engine controls designer
>asking if a failed O2 sensor should turn on the check engine
>light or blow up the car.

Ross Ridge <rridge@csclub. uwaterloo.cawro te:

No, it's more like asking if the failed sensor should turn on
a strange and mysterious light on the dashboard

Grant Edwards <invalid@invali dwrote:

>You're right. I had forgotten that sys.exit() is actually
>raising the system exit exception, and that the application
>calling the library could handle that exception.

Well, my point was that exceptions in Python are a bit like a car's
check engine light. Few drivers know what this mysterious light means,
and aren't prepared to do anything about it when it goes on.

Ross Ridge

--
l/ // Ross Ridge -- The Great HTMU
[oo][oo] rridge@csclub.u waterloo.ca
-()-/()/ http://www.csclub.uwaterloo.ca/~rridge/
db //