python: How do I know what type of exception occurred?

Question

The other answers all point out that you should not catch generic exceptions, but no one seems to want to tell you why, which is essential to understanding when you can break the "rule". Here is an explanation. Basically, it's so that you don't hide:

• the fact that an error occurred
• the specifics of the error that occurred (error hiding antipattern)

So as long as you take care to do none of those things, it's OK to catch the generic exception. For instance, you could provide information about the exception to the user another way, like:

• Present exceptions as dialogs in a GUI
• Transfer exceptions from a worker thread or process to the controlling thread or process in a multithreading or multiprocessing application

So how to catch the generic exception? There are several ways. If you just want the exception object, do it like this:

try:
    someFunction()
except Exception as ex:
    template = "An exception of type {0} occurred. Arguments:\n{1!r}"
    message = template.format(type(ex).__name__, ex.args)
    print message

Make sure message is brought to the attention of the user in a hard-to-miss way! Printing it, as shown above, may not be enough if the message is buried in lots of other messages. Failing to get the users attention is tantamount to swallowing all exceptions, and if there's one impression you should have come away with after reading the answers on this page, it's that this is not a good thing. Ending the except block with a raise statement will remedy the problem by transparently reraising the exception that was caught.

The difference between the above and using just except: without any argument is twofold:

• A bare except: doesn't give you the exception object to inspect
• The exceptions SystemExit, KeyboardInterrupt and GeneratorExit aren't caught by the above code, which is generally what you want. See the exception hierarchy.

If you also want the same stacktrace you get if you do not catch the exception, you can get that like this (still inside the except clause):

import traceback
print traceback.format_exc()

If you use the logging module, you can print the exception to the log (along with a message) like this:

import logging
log = logging.getLogger()
log.exception("Message for you, sir!")

If you want to dig deeper and examine the stack, look at variables etc., use the post_mortem function of the pdb module inside the except block:

import pdb
pdb.post_mortem()

I've found this last method to be invaluable when hunting down bugs.

Get the name of the class that exception object belongs:

e.__class__.__name__

and using print_exc() function will also print stack trace which is essential info for any error message.

Like this:

from traceback import print_exc

class CustomException(Exception): pass

try:
    raise CustomException("hi")
except Exception as e:
    print ('type is:', e.__class__.__name__)
    print_exc()
    # print("exception happened!")

You will get output like this:

type is: CustomException
Traceback (most recent call last):
  File "exc.py", line 7, in <module>
    raise CustomException("hi")
CustomException: hi

And after print and analysis, the code can decide not to handle exception and just execute raise:

from traceback import print_exc

class CustomException(Exception): pass

def calculate():
    raise CustomException("hi")

try:
    calculate()
except CustomException as e:
    # here do some extra steps in case of CustomException
    print('custom logic doing cleanup and more')
    # then re raise same exception
    raise

Output:

custom logic doing cleanup and more

And interpreter prints exception:

Traceback (most recent call last):
  File "test.py", line 9, in <module>
    calculate()
  File "test.py", line 6, in calculate
    raise CustomException("hi")
__main__.CustomException: hi

After raise original exception continues to propagate further up the call stack. (Beware of possible pitfall) If you raise new exception it caries new (shorter) stack trace.

from traceback import print_exc

class CustomException(Exception):
    def __init__(self, ok):
        self.ok = ok

def calculate():
    raise CustomException(False)

try:
    calculate()
except CustomException as e:
    if not e.ok:
        # Always use `raise` to rethrow exception
        # following is usually mistake, but here we want to stress this point
        raise CustomException(e.ok)
    print("handling exception")

Output:

Traceback (most recent call last):
  File "test.py", line 13, in <module>
    raise CustomException(e.message)
__main__.CustomException: hi    

Notice how traceback does not include calculate() function from line 9 which is the origin of original exception e.

You usually should not catch all possible exceptions with try: ... except as this is overly broad. Just catch those that are expected to happen for whatever reason. If you really must, for example if you want to find out more about some problem while debugging, you should do

try:
    ...
except Exception as ex:
    print ex # do whatever you want for debugging.
    raise    # re-raise exception.

Most answers point to except (…) as (…): syntax (rightly so) but at the same time nobody wants to talk about an elephant in the room, where the elephant is sys.exc_info() function. From the documentation of sys module (emphasis mine):

This function returns a tuple of three values that give information about the exception that is currently being handled.
(…)
If no exception is being handled anywhere on the stack, a tuple containing three None values is returned. Otherwise, the values returned are (type, value, traceback). Their meaning is: type gets the type of the exception being handled (a subclass of BaseException); value gets the exception instance (an instance of the exception type); traceback gets a traceback object (see the Reference Manual) which encapsulates the call stack at the point where the exception originally occurred.

I think the sys.exc_info() could be treated as the most direct answer to the original question of How do I know what type of exception occurred?

Unless somefunction is a very bad coded legacy function, you shouldn't need what you're asking.

Use multiple except clause to handle in different ways different exceptions:

try:
    someFunction()
except ValueError:
    # do something
except ZeroDivision:
    # do something else

The main point is that you shouldn't catch generic exception, but only the ones that you need to. I'm sure that you don't want to shadow unexpected errors or bugs.

In Python 2, the following are useful

except Exception, exc:

    # This is how you get the type
    excType = exc.__class__.__name__

    # Here we are printing out information about the Exception
    print 'exception type', excType
    print 'exception msg', str(exc)

    # It's easy to reraise an exception with more information added to it
    msg = 'there was a problem with someFunction'
    raise Exception(msg + 'because of %s: %s' % (excType, exc))