Exceptions should not be singletons in Python 3

[kouli]

Due to exception chaining introduced in Python 3 (probably), exceptions should not be "singletons": i.e. each exception raised should be a separate individual instance of Exception. Please, do not use those singletons as e.g. in pysnmp.proto.errind (get rid of the requestTimedOut instance and always instantiate exceptions using RequestTimedOut()). See output (exception's traceback) of the following example to see why it is bad - singleton obtains an accumulated traceback from all places it was raised at:

class TimeoutError(Exception): pass
timeoutError = TimeoutError()

def work():
    raise timeoutError

def main(m):
    for i in range(10):
        try:
            work()
        except TimeoutError:
            if i >= m: # ignore first `m` timeouts
                raise

main(15)
main(3) # raises a TimeoutError with 18 "cycles" in its traceback

[etingof]

That's a great point, thank you!

[lextm]

I don't think this is a valid report.

Commonly the code looks like

            raise error.StatusInformation(
                errorIndication=errind.decryptionError
            )

Since none of the instances from errind is called directly by raise, the trackback information is not accumulated.