Handle specific exception type in python

Question

I have some code that handles an exception, and I want to do something specific only if it's a specific exception, and only in debug mode. So for example:

try:
    stuff()
except Exception as e:
    if _debug and e is KeyboardInterrupt:
        sys.exit()
    logging.exception("Normal handling")

As such, I don't want to just add a:

except KeyboardInterrupt:
    sys.exit()

because I'm trying to keep the difference in this debug code minimal

Solution 1

Well, really, you probably should keep the handler for KeyboardInterrupt separated. Why would you only want to handle keyboard interrupts in debug mode, but swallow them otherwise?

That said, you can use isinstance to check the type of an object:

try:
    stuff()
except Exception as e:
    if _debug and isinstance(e, KeyboardInterrupt):
        sys.exit()
    logger.exception("Normal handling")

Solution 2

This is pretty much the way it's done.

try:
    stuff()
except KeyboardInterrupt:
    if _debug:
        sys.exit()
    logging.exception("Normal handling")
except Exception as e:
    logging.exception("Normal handling")

There's minimal repetition. Not zero, however, but minimal.

If the "normal handling" is more than one line of code, you can define a function to avoid repetition of the two lines of code.

Solution 3

You should let KeyboardInterrupt bubble all the way up and trap it at the highest level.

if __name__ == '__main__':
    try:
        main()
    except KeyboardInterrupt:
        sys.exit()
    except:
        pass

def main():
    try:
        stuff()
    except Exception as e:
        logging.exception("Normal handling")
        if _debug:
            raise e

Solution 4

Either use the standard method mentioned in the other answers, or if you really want to test within the except block then you can use isinstance().

try:
    stuff()
except Exception as e:
   if _debug and isinstance(e, KeyboardInterrupt):
        sys.exit()
    logging.exception("Normal handling")

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Comments

[Velociraptors]

You beat me by 30 seconds, I think

[Falmarri]

This is on an embedded system running without a shell. So we could only possibly get a keyboard interrupt in debug mode. So we just handle all exceptions.

[mipadi]

@marcog: except Exception as e is valid in Python 2.6+.

[Paulo Scardine]

-1: 'else' should be 'except', 'else' clause is for code to be executed if there is no exception.

[Lex]

Yes, 'else' branch will be executed when there is no exception. It's just for example. But you are right I should edit code example to show 'real' program state.