How can I write a try
/except
block that catches all exceptions?
10 Answers
Apart from a bare except:
clause (which as others have said you shouldn't use), you can simply catch Exception
:
import traceback
import logging
try:
whatever()
except Exception as e:
logging.error(traceback.format_exc())
# Logs the error appropriately.
You would normally only ever consider doing this at the outermost level of your code if for example you wanted to handle any otherwise uncaught exceptions before terminating.
The advantage of except Exception
over the bare except
is that there are a few exceptions that it wont catch, most obviously KeyboardInterrupt
and SystemExit
: if you caught and swallowed those then you could make it hard for anyone to exit your script.
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9For anyone wondering, totally contrary to my expectation this will still catch non-exception subclassing things like ints, at least in python 2.x. Commented Oct 1, 2014 at 22:17
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13@JosephGarvin, that's incorrect, i.e. this will not catch "non-exceptions" that don't subclass
Exception
. Note that it's impossible to raise anint
as an exception, and attempting to do so raises aTypeError
exception, which is what would be caught by the enclosingexcept Exception
clause in such a case. On the other hand, an old-style class can be raised and qualifies as a "non-exception" that doesn't subclassException
- this will be caught by a bareexcept
clause but not by anexcept Exception
clause.– YoelCommented Nov 14, 2016 at 14:14 -
4@JosephGarvin check this blog entry: chris-lamb.co.uk/posts/no-one-expects-string-literal-exception I'm with @Yoel on this one, your testing just masked the
TypeError
– DuncanCommented Nov 14, 2016 at 15:12 -
2@CharlieParker nothing wrong with catching them if that's what you want but you mostly don't. Calling
sys.exit()
usually means you expect the app to terminate but if you catch SystemExit it won't. Likewise if you hit control-C on a running script (Ctrl-break on windows) you expect the program to stop, not to catch the error and keep going. But you can catch either/both of these if you want to do cleanup before existing.– DuncanCommented May 4, 2020 at 9:13 -
3@CharlieParker you could try
except BaseException as e: notify_user(e); raise
that would catch all exceptions and do whatever notification you need, but I don't know HPC so you might want to ask as a new SO question.– DuncanCommented Sep 30, 2020 at 8:33
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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35Possible workaround: effbot.org/zone/stupid-exceptions-keyboardinterrupt.htm– MikelCommented Feb 14, 2011 at 9:52
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9You really should print to stderr.– user492203Commented Jan 14, 2015 at 20:34
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83I very very strongly disagree with the statement, "shouldn't." You should do it sparingly. There are times when you're dealing with third party libraries (sometimes dynamically loaded!!) that have gone totally crazy with exceptions and tracking them all down can be a very painful task, and if you miss just one, you have a very very huge painful bug in your system. That being said, it's good to track down as many as you can and handle them appropriately and then have a backup catch all for the ones you miss.– BlazeCommented Oct 19, 2015 at 8:04
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58What I find also weird is that in a duck typing language where you don't declare instance variables, it's suddenly very concerned about not typing all of your exceptions. Hmm!– BlazeCommented Oct 19, 2015 at 8:06
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10
To catch all possible exceptions, catch BaseException
. It's on top of the Exception class hierarchy:
Python 3: https://docs.python.org/3.12/library/exceptions.html#exception-hierarchy
Python 2.7: https://docs.python.org/2.7/library/exceptions.html#exception-hierarchy
try:
something()
except BaseException as error:
print('An exception occurred: {}'.format(error))
But as other people mentioned, you would usually not need this, only for very specific cases.
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8Is wishing to save progress of a long-running job after pressing Ctrl-C that unusual? Commented Aug 28, 2018 at 15:56
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I have my jobs running from a HPC manager. I want to capture when the (slurm, qsub, condor) manager does an exit (to email myself about it with my custom email). Will
except:
catch that? But it doesn't give me a handle fore
though :( Commented Sep 29, 2020 at 15:23 -
2
In Python, all exceptions must be instances of a class that derives from BaseException
, but if you can omit it for a general case - omit it, problem is, linters wine about it.– jave.webCommented Nov 3, 2020 at 16:23 -
2@BallpointBen That's what signal handlers are for. It's the default signal handler for
SIGINT
that raisesKeyboardInterrupt
in the first place. Sure, you can catchKeyboardInterrupt
, but it's just one of many signals that could terminated your program prematurely. Those don't generate any kind of exception, so you may as well handle them all uniformly.– chepnerCommented Jul 23, 2021 at 20:18 -
3CAUTION: using
BaseException
like this is rarely what you want. Do you seriously want to catch keyboard interupts and sys.exit? Probably NOT! All user-defined exceptions should inherit fromException
. take a look at the exception class heirarchy. dotnettutorials.net/wp-content/uploads/2020/07/… Commented Feb 28, 2022 at 15:45
You can do this to handle general exceptions
try:
a = 2/0
except Exception as e:
print e.__doc__
print e.message
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13This may not catch all exceptions, as the base class for all exceptions is BaseException and I have encountered production code that is not in the Exception class family. See docs.python.org/3/library/… for details about this.– DDayCommented Jun 2, 2016 at 16:54
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7
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13Technically, it should catch all non-system-exiting exceptions. From the docs @DDay linked: "exception BaseException: The base class for all built-in exceptions. It is not meant to be directly inherited by user-defined classes (for that, use Exception)." Unless you're working with code that ignores this, or you need to catch system-exiting exceptions, the above should be ok to use. Commented Feb 13, 2018 at 0:19
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3
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2Because in Python 3 print is a function and not a statement. Thus you need to call it with (). e.g print(e.message) Commented Dec 24, 2020 at 13:53
Very simple example, similar to the one found here:
http://docs.python.org/tutorial/errors.html#defining-clean-up-actions
If you're attempting to catch ALL exceptions, then put all your code within the "try:" statement, in place of 'print "Performing an action which may throw an exception."'.
try:
print "Performing an action which may throw an exception."
except Exception, error:
print "An exception was thrown!"
print str(error)
else:
print "Everything looks great!"
finally:
print "Finally is called directly after executing the try statement whether an exception is thrown or not."
In the above example, you'd see output in this order:
1) Performing an action which may throw an exception.
2) Finally is called directly after executing the try statement whether an exception is thrown or not.
3) "An exception was thrown!" or "Everything looks great!" depending on whether an exception was thrown.
Hope this helps!
-
I have my jobs running from a HPC manager. I want to capture when the (slurm, qsub, condor) manager does an exit (to email myself about it with my custom email). Will
except:
catch that? But it doesn't give me a handle fore
though :( Commented Sep 29, 2020 at 15:23 -
2
-
2@Tony try:
except Exception as error:
-- If you're running Python3. Commented Aug 3, 2021 at 18:53
There are multiple ways to do this in particular with Python 3.0 and above
Approach 1
This is simple approach but not recommended because you would not know exactly which line of code is actually throwing the exception:
def bad_method():
try:
sqrt = 0**-1
except Exception as e:
print(e)
bad_method()
Approach 2
This approach is recommended because it provides more detail about each exception. It includes:
- Line number for your code
- File name
- The actual error in more verbose way
The only drawback is tracback needs to be imported.
import traceback
def bad_method():
try:
sqrt = 0**-1
except Exception:
print(traceback.print_exc())
bad_method()
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I have my jobs running from a HPC manager. I want to capture when the (slurm, qsub, condor) manager does an exit (to email myself about it with my custom email). Will
except:
catch that? But it doesn't give me a handle fore
though :( Commented Sep 29, 2020 at 15:23 -
I've just found out this little trick for testing if exception names in Python 2.7 . Sometimes i have handled specific exceptions in the code, so i needed a test to see if that name is within a list of handled exceptions.
try:
raise IndexError #as test error
except Exception as e:
excepName = type(e).__name__ # returns the name of the exception
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I have my jobs running from a HPC manager. I want to capture when the (slurm, qsub, condor) manager does an exit (to email myself about it with my custom email). Will
except:
catch that? But it doesn't give me a handle fore
though :( Commented Sep 29, 2020 at 15:23 -
To be quite honest, I am not familiar with HPC... If it is integrated with/for python it should have corresponding exceptions implemented. If not you can try 3rd part library that has it (dunno which) or making a task listener that would search for flag set by HPC. If all that fails you can try and code some yourself, 'exception' class is inheritable and dive into processes/drivers. Other than that, due to lack of more information and No Opinions policy of SO, I would suggest asking a new question witch HPC in title and tags - if not asked before. Cheer :)– DaniloCommented Sep 30, 2020 at 1:55
I am adding the bonus method that can catch the exception with full traceback which can help you to understand the error more.
Python 3
import traceback
try:
# your code goes here
except Exception as e:
print(e)
traceback.print_exc()
-
1
try:
whatever()
except:
# this will catch any exception or error
It is worth mentioning this is not proper Python coding. This will catch also many errors you might not want to catch.
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1just use except don't cach all exceptions as mentionned in some other answers. You have to use BaseException for this purpose but as you said, nobody should catch all exceptions like this. I guess it's ok for a start if the goal is to add more granular except during development but I don't think it will be... Commented Oct 31, 2019 at 9:20
First of all, there are exceptions that you want them to break your code (as when this error happens your code will not function anyways!) and exceptions you want to capture silently/smoothly. Try differentiating them. You may not want to capture all exceptions there are!
Second, instead of capturing everything, you could take the time and go through the logs of your process. Let's say you are getting a different/third-party exception, for example from a cloud service provider like GCP. In the logs, you could find the exception you are getting. Then, you could do something like this:
from google.api_core.exceptions import ServiceUnavailable, RetryError
for i in range(10):
try:
print("do something")
except ValueError:
print("I know this might happen for now at times! skipping this and continuing with my loop"
except ServiceUnavailable:
print("our connection to a service (e.g. logging) of gcp has failed")
print("initializing the cloud logger again and try continuing ...")
except RetryError:
print("gcp connection retry failed. breaking the loop. try again later!)
break
For the rest (errors that might or might not happen), I am leaving room for my code to crash if I get an unexpected exception! This way I could understand what is going on and improve my code by capturing edge cases.
If you want this to never crash for some reason, for example if it is a code embedded in a remote hardware that you cannot easily access, you can add a generic exception catcher at the end:
except Exception as e:
print(f"something went wrong! - {e}")
You can also take a look at Python 3 exception hierarchy here. The difference between Exception
and BaseException
is that, Exception
will not catch SystemExit
, KeyboardInterrupt
, or GeneratorExit
sys.stderr
and possibly logged. That is a perfectly valid and common exception.try: whatever() except Exception as e: exp_capture()
?except: pass
a bad programming practice?