How to return a value and raise an Exception

Question

I have two objectives with this try/except statement.

1. It needs to return a value of 1 if no problems occurred, or 0 if any problems occurred.
2. It needs to raise an exception and end the script.

I have the return value working. I also have the SystemExit() working. But together, they aren't working.

My Python Script (that's relevant):

except IOError:
    value_to_return = 0
    return value_to_return
    raise SystemExit("FOOBAR")

With this, it ignores the raise SystemExit("FOOBAR") line completely. How do I go about getting a returned value and still raise SystemExit("FOOBAR")? This may be elementary to some, but I'm actually having quite a bit of difficulty with it.

Solution 1

Returning and raising are mutually exclusive.

Raising SystemExit will end the script. A few cleanup routines get to run, and if the caller really, really wants to, they can catch the SystemExit and cancel it, but mostly, you can think of it as stopping execution right there. The caller will never get a chance to see a return value or do anything meaningful with it.

Returning means you want the script to continue. Continuing might mean having the caller raise SystemExit, or it might mean ignoring the error, or it might mean something else. Whatever it means is up to you, as you're the one writing the code.

Finally, are you sure you should be handling this error at all? Catching an exception only to turn it into a system shutdown may not be the most useful behavior. It's not a user-friendly way to deal with problems, and it hides all the useful debugging information you'd get from a stack trace.

Solution 2

You can raise an error with a 'returning_value' argument to be used after the calling.

Another pythonic answer to your problem could be to make use of the error arguments in the raise and then, in your call manage the error to get the value, convert it from string and get your 'return-ish'.

def your_f():
    try:
      some_io_thingy_ok()
      return 1
    except IOError:
        raise SystemExit("FOOBAR", 0)

try:
    my_returning_value = your_f()
except SystemExit as err:
    my_returning_value = err.args[1]


print(my_returning_value)

From Python 3 docs :

When an exception occurs, it may have an associated value, also known as the exception’s argument. The presence and type of the argument depend on the exception type.

The except clause may specify a variable after the exception name. The variable is bound to an exception instance with the arguments stored in instance.args. For convenience, the exception instance defines str() so the arguments can be printed directly without having to reference .args. One may also instantiate an exception first before raising it and add any attributes to it as desired.

Solution 3

To exit a script and return an exit status, use sys.exit():

import sys
sys.exit(value_to_return)

Comments

[user2357112]

@ingyhere: No, that tutorial section just sucks. IIRC someone copied a poorly-written description from a Github comment without examining it closely enough. finally will suspend the exception's propagation, and then the return will cancel the exception and return "value" instead.

[user2357112]

The language reference has a more accurate description, particularly "If the finally clause executes a return or break statement, the saved exception is discarded". The language reference tends to be better than the tutorial (but the language reference isn't perfect either; for example, it often fails to mention how various operations can be customized by magic methods).

[ingyhere]

OK, @user2357112supportsMonica. I ran a sample program, and you're indeed correct. The Python.org link I presented above unfortunately states verbatim, "If the exception is not handled by an except clause, the exception is re-raised after the finally clause has been executed." It also discusses an example where finally is executed and then the exception. That's not necessarily contradictory to what you wrote, though. In other SO posts there is contradiction, unfortunately. Earlier comment deleted for clarity.

[UCYT5040]

Couldn't you also use __builtins__.exit(value_to_return) as well?

[Dan Getz]

@UltimateCreeper yes, it seems so. sys.exit is apparently preferred in programs, see stackoverflow.com/questions/6501121/…