Catching all exceptions in Python

Question

In Python, what's the best way to catch "all" exceptions?

except: # do stuff with sys.exc_info()[1]

except BaseException as exc:

except Exception as exc:

The catch may be executing in a thread.

My aim is to log any exception that might be thrown by normal code without masking any special Python exceptions, such as those that indicate process termination etc.

Getting a handle to the exception (such as by the clauses above that contain exc) is also desirable.

score 67 · Answer 1 · edited Aug 24 '11 at 06:24

except Exception: vs except BaseException::

The difference between catching Exception and BaseException is that according to the exception hierarchy exception like SystemExit, KeyboardInterrupt and GeneratorExit will not be caught when using except Exception because they inherit directly from BaseException.

except: vs except BaseException::

The difference between this two is mainly in python 2 (AFAIK), and it's only when using an old style class as an Exception to be raised, in this case only expression-less except clause will be able to catch the exception eg.

class NewStyleException(Exception): pass

try:
   raise NewStyleException
except BaseException:
   print "Caught"

class OldStyleException: pass

try:
   raise OldStyleException
except BaseException:
   print "BaseException caught when raising OldStyleException"
except:
   print "Caught"

I made `NewStyleException` inherit from `Exception`, as is officially recommended (http://docs.python.org/tutorial/errors.html#user-defined-exceptions). — Eric O. Lebigot, Aug 24 '11 at 06:24

score 32 · Accepted Answer · answered Aug 23 '11 at 12:28

32

If you need to catch all exceptions and do the same stuff for all, I'll suggest you this :

try:
   #stuff
except:
   # do some stuff

If you don't want to mask "special" python exceptions, use the Exception base class

try:
   #stuff
except Exception:
   # do some stuff

for some exceptions related management, catch them explicitly :

try:
   #stuff
except FirstExceptionBaseClassYouWantToCatch as exc:
   # do some stuff
except SecondExceptionBaseClassYouWantToCatch as exc:
   # do some other stuff based
except (ThirdExceptionBaseClassYouWantToCatch, FourthExceptionBaseClassYouWantToCatch) as exc:
   # do some other stuff based

The exception hierarchy from the python docs should be a usefull reading.

answered Aug 23 '11 at 12:28

Cédric Julien

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Thanks for the link to the exception hierarchy. :) – Eric O. Lebigot Aug 24 '11 at 06:16
How can I then get the caught exception instance if the except class is not defined? – mvorisek Jul 31 '18 at 07:39
@Mvorisek use `except Exception as exc`, Eception is the base class of all exceptions, so it will catch everything. – Cédric Julien Jul 31 '18 at 07:50

Peter · Answer 3 · 2011-08-25T14:10:42.437

-1

In order to avoid masking basic exceptions, you need to "re-raise" any exceptions which aren't the ones which you explicitly want to handle, e.g. (adapted from 8. Errors and Exceptions):

import sys

try:
    # do something that could throw an exception:
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:
    # maybe log the exception (e.g. in debug mode)
    # re-raise the exception:
    raise

edited Aug 25 '11 at 14:10

answered Aug 23 '11 at 12:48

Peter

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The final `except: raise` does not do anything: it can be skipped altogether; non-IOError and non-ValueError exceptions are simply not caught and are automatically raised by the interpreter. – Eric O. Lebigot Aug 24 '11 at 06:20
3

@EOL You're right. I have edited my answer to indicate why the final except: might be useful. – Peter Aug 25 '11 at 14:11

Catching all exceptions in Python

3 Answers3