Python – Best Practices for Python Exceptions


What are the best practices for creating exceptions? I just saw this, and I don't know if I should be horrified, or like it. I read several times in books that exceptions should never ever hold a string, because strings themselves can throw exceptions. Any real truth to this?

Basically from my understanding from the scripts is that this was done so all the inhouse Python libraries will have a common error message format (something that is desperately needed) so I can understand why putting the error message string is a good idea. (Almost every method throws exceptions due to the utter need for nothing invalid getting through).

The code in question is the following:

Base Exception, Error
class Error(Exception):
    def __init__(self, message):
        self.message = message

    def __str__(self):
        return "[ERROR] %s\n" % str(self.message)

    def log(self):
        ret = "%s" % str(self.message)
        if(hasattr(self, "reason")):
            return "".join([ret, "\n==> %s" % str(self.reason)])
        return ret

class PCSException(Error):
    def __init__(self, message, reason = None):
        self.message = message
        self.reason = reason
    def __str__(self):
        ret = "[PCS_ERROR] %s\n" % str(self.message)
        if(self.reason != None):
            ret += "[REASON] %s\n" % str(self.reason)
        return ret

This is just the tip of the iceberg, but can someone give me some insight in what makes this a terrible idea? Or if there is a much better exception coding process/style.

Best Solution

Robust exception handling (in Python) - a "best practices for Python exceptions" blog post I wrote a while ago. You may find it useful.

Some key points from the blog:

Never use exceptions for flow-control

Exceptions exist for exceptional situations: events that are not a part of normal execution.

Consider 'find' on a string returning -1 if the pattern isn't found, but indexing beyond the end of a string raises an exception. Not finding the string is normal execution.

Handle exceptions at the level that knows how to handle them


The best place is that piece of code that can handle the exception. For some exceptions, like programming errors (e.g. IndexError, TypeError, NameError etc.) exceptions are best left to the programmer / user, because "handling" them will just hide real bugs.

Always ask "is this the right place to handle this exception?" and be careful with catching all exceptions.

Document the exceptions thrown by your code


thinking about which exceptions your code may throw will help you write better, safer and more encapsulated code