Python: Catching Specific Exception

Python: Catching specific exception

in except ValueError,e, e is an instance of the exception, not a string. So when you test if e is not equal to a particular string, that test is always False. Try:

if str(e) != "..."

instead.

Example:

def catch(msg):
    try:
        raise ValueError(msg)
    except ValueError as e:  # as e syntax added in ~python2.5
        if str(e) != "foo":
            raise
        else:
            print("caught!")

catch("foo")
catch("bar")

Typically, you don't really want to rely on the error message if you can help it -- It's a little too fragile. If you have control over the callable macdat, instead of raising a ValueError in macdat, you could raise a custom exception which inherits from ValueError:

class MyValueError(ValueError): pass

Then you can only catch MyValueError and let other ValueErrors continue on their way to be caught by something else (or not). Simple except ValueError will still catch this type of exception as well so it should behave the same in other code which might also be catching ValueErrors from this function.

Catching specific error messages in try / except

You were close. The trick is to use ValueError as e and compare your strings against str(e). It's also good practice to use if / elif rather than repeated if statements.

Here's a working example:

import calendar, datetime

try: 
    print(datetime.date(int(birthYear), int(birthMonth), int(birthDay)))
except ValueError as e:
    if str(e) == 'month must be in 1..12': 
        print('Month ' + str(birthMonth) + ' is out of range. The month must be a number in 1...12')
    elif str(e) == 'year {0} is out of range'.format(birthYear): 
        print('Year ' + str(birthMonth) + ' is out of range. The year must be a number in ' + str(datetime.MINYEAR) + '...' + str(datetime.MAXYEAR))
    elif str(e) == 'day is out of range for month': 
        print('Day ' + str(birthDay) + ' is out of range. The day must be a number in 1...' + str(calendar.monthrange(birthYear, birthMonth)))

How to import and catch module-specific exceptions in Python?

This is how I usually do it:

>>> import multiprocessing
... q = multiprocessing.Queue()
... q.get_nowait()
... 
... 
---------------------------------------------------------------------------
Empty                                     Traceback (most recent call last)
<...snip...>
>>> import sys
>>> err = sys.last_value
>>> err.__module__
'queue'
>>> from queue import Empty
>>> isinstance(err, Empty)
True

There is no foolproof way that works for all modules in the generic case, because they don't usually know (or care) about all their dependencies exception hierarchy. An exception in 3rd party code would just bubble up the stack, and there is generally no point to catch it unless one can actually do something to handle it and continue. Good projects will usually document the exception hierarchy clearly in their API guide.

Pythonic exception handling: only catching specific errno

I just stumbled across the probably most elegant solution: creating the ignored context manager:

import errno
from contextlib import contextmanager

@contextmanager
def ignorednr(exception, *errornrs):
    try:
        yield
    except exception as e:
        if e.errno not in errornrs:
            raise
        pass

with ignorednr(OSError, errno.EEXIST):
     os.mkdir(dir)

This way I just have the ugly job of creating the context manager once, from then on the syntax is quite nice and readable.

The solution is taken from https://www.youtube.com/watch?v=OSGv2VnC0go.

Handling both specific and general Python exceptions?

You could reraise the exception, and handle the generic case in the outer handler of a nested setup:

try:
    try:
        output_var = some_magical_function()
    except IntegrityError as zde:
        integrity_error_handling()
        raise
except ALLExceptions as ae: # all exceptions INCLUDING the IntregityError
    shared_exception_handling_function(ae) # could be error reporting

The unqualified raise statement re-raises the current exception, so the IntegrityError exception is thrown again to be handled by the AllExceptions handler.

The other path you could take is to test for the exception type:

try:
    output_var = some_magical_function()
except ALLExceptions as ae: # all exceptions INCLUDING the IntregityError
    if isinstance(ae, IntegrityError):
        integrity_error_handling()
    shared_exception_handling_function(ae) # could be error reporting

How do I determine what type of exception occurred?

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.

How can I write a `try`/`except` block that catches all exceptions?

You can but you probably shouldn't:

try:
    do_something()
except:
    print("Caught it!")

However, this will also catch exceptions like KeyboardInterrupt and you usually don't want that, do you? Unless you re-raise the exception right away - see the following example from the docs:

try:
    f = open('myfile.txt')
    s = f.readline()
    i = int(s.strip())
except IOError as (errno, strerror):
    print("I/O error({0}): {1}".format(errno, strerror))
except ValueError:
    print("Could not convert data to an integer.")
except:
    print("Unexpected error:", sys.exc_info()[0])
    raise

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