Python 3 allows raising exceptions from other exceptions e.g.:
try:
raise CustomException()
except CustomException as e:
try:
raise TypeError() from e
except TypeError as e:
print(type(e.__cause__))
The CustomException instance is stored in the exception object's __cause__ attribute.
The code above should print CustomException.
Is there a way to catch the original exception instead of the newly raised one?
try:
raise CustomException()
except CustomException as e:
try:
raise TypeError() from e
except CustomException as e:
print(type(e)) # should reach here
Overriding __subclasscheck__ does not work since I don't have access to the instance and I it is impossible to specify that CustomException is a subclass of all classes or of the cause class.
Is there a way to trick Python into thinking that the exception we're catching is of the type of __cause__?
If you have control over the exception that is raised you can perhaps make it a subclass of the raised exception:
try:
raise TypeError()
except TypeError as e:
try:
class CustomException(TypeError.__class__):
pass
raise CustomException() from e
except TypeError as e:
print(type(e)) # Reaches here
That said, the mechanism of catch-and-reraise is meant to hide what the original exception was so that later code doesn't depend on implementation details.
Simple solution: catch all exceptions and filter required:
try:
raise ZeroDivisionError()
except ZeroDivisionError as e:
try:
raise TypeError() from e
except Exception as ex:
if type(ex.__cause__) is ZeroDivisionError:
print('ZeroDivisionError')
else:
raise # re-raise exception if it has different __cause__
Related
I know I can store an exception name in a variable with this syntax:
try:
code
except TypeError as e:
logger.error(e)
except NameError as e:
logger.error(e)
How do I do the same for the generic except: message? I assume that this (which is the general idea) won't work:
try:
code
except as e:
logger.error(e)
You can you use type(e).__name__ to capture the name of any error you encounter, and access the message as you a normal variable, with e.message. All the built in errors (indexError, TypeError, etc.) are children of the class Exception, so they will be picked up. to save it as a variable named 'err':
try:
code
except Exception as e:
err = type(e).__name__
message = e.message
This will save the error type of any exception of the base python type Exception that you run into, using the built in __name__ variable
BaseException is the broadest type you can catch:
try:
# some code
except BaseException as e:
logger.error(e)
You are able to catch Exception:
import logging
try:
code
except TypeError as e:
logger.error(e)
except NameError as e:
logger.error(e)
except Exception as e:
logging.error(e)
I ran into this with Python 2, where old-style classes aren't caught by except Exception or except BaseException. I solved it by using sys.exc_info to access the current exception:
import sys
try:
code
except:
e = sys.exc_info()[1]
logging.error(e)
This question already has answers here:
Python: Catching specific exception
(3 answers)
Closed 5 years ago.
I'm using a python library in which at one point an exception is defined as follows:
raise Exception("Key empty")
I now want to be able to catch that specific exception, but I'm not sure how to do that.
I tried the following
try:
raise Exception('Key empty')
except Exception('Key empty'):
print 'caught the specific exception'
except Exception:
print 'caught the general exception'
but that just prints out caught the general exception.
Does anybody know how I can catch that specific Key empty exception? All tips are welcome!
Define your exception:
class KeyEmptyException(Exception):
def __init__(self, message='Key Empty'):
# Call the base class constructor with the parameters it needs
super(KeyEmptyException, self).__init__(message)
Use it:
try:
raise KeyEmptyException()
except KeyEmptyException as e:
print e
Update: based on the discussion in comment OP posted:
But the lib is not under my control. It's open source, so I can edit it, but I would preferably try to catch it without editing the library. Is that not possible?
say library raises an exception as
# this try is just for demonstration
try:
try:
# call your library code that can raise `Key empty` Exception
raise Exception('Key empty')
except Exception as e:
# if exception occurs, we will check if its
# `Key empty` and raise our own exception
if str(e) == 'Key empty':
raise KeyEmptyException()
else:
# else raise the same exception
raise e
except Exception as e:
# we will finally check what exception we are getting
print('Caught Exception', e)
you need to subclass Exception:
class EmptyKeyError(Exception):
pass
try:
raise EmptyKeyError('Key empty')
except EmptyKeyError as exc:
print(exc)
except Exception:
print('caught the general exception')
I am curious if there is a way in python to continue on within try/catch block, after you catch an exception, look at its properties, and if not relevant, then continue down the stack.
try:
# Code
except AppleError as apple_ex:
# look at 'apple_ex.error_code' error body, and if not relevant,
# continue on to next down the catch block...
# In other words, proceed to except BananaError and so on down.
except BananaError as banana_ex:
# ...
except Exception as ex:
# ...
That is not how exceptions are handled in Python. When you raise an exception in a try block, if you handle catching it in the except, it will fall inside that block, but will not continue to the next except at that same level. Observe this functional example:
try:
raise AttributeError()
except AttributeError:
raise TypeError()
except TypeError:
print("it got caught") # will not catch the TypeError raised above
So, in your try, we raise an AttributeError, we catch it, and then raise a TypeError inside catching the AttributeError.
The except TypeError will not catch that TypeError.
Based on how you are explaining your problem, you need to rethink how you are handling your exceptions and see if you can determine the handling of errors somewhere else, and raise the error there.
For example:
def some_func():
try:
thing()
except SomeException:
# analyze the exception here and raise the error you *should* raise
if apple_error_thing:
raise AppleError
elif banana_error_thing:
raise BananaError
else:
raise UnknownException
def your_func():
try:
some_func()
except AppleError as e:
print('Apple')
except BananaError as e:
print('Banana')
except UnknownException as e:
print('Unknown')
An AppleError is still an AppleError and not a BananaError, even if error_code is not relevant, so it makes no sense to fall through to BananaError.
You could instead define specific errors for your different error codes:
GRANNY_SMITH_ERROR = 1
MACINTOSH_ERROR = 2
class AppleError(Exception):
def __init__(self, error_code, *args):
super(AppleError, self).__init__(*args)
self.error_code = error_code
class GrannySmithError(AppleError):
def __init__(self, *args):
super(GrannySmithError, self).__init__(GRANNY_SMITH_ERROR, *args)
class MacintoshError(AppleError):
def __init__(self, *args):
super(MacintoshError, self).__init__(MACINTOSH_ERROR, *args)
Then you can try to match the specific error:
try: raise MacintoshError()
except MacintoshError as exc: print("mac")
except GrannySmithError as exc: print("granny smith")
If you do not care to distinguish between different types of apple errors, you can still trap all apple errors:
try: raise MacintoshError()
except AppleError as exc: print("generic apple")
You can combine these, for example, only doing special processing for GrannySmith, not for Macintosh:
try: raise MacintoshError()
except GrannySmithError as exc: print("granny smith")
except AppleError as exc: print("generic apple")
The important thing is to list the errors from most specific to least specific. If you test for AppleError before GrannySmithError, then it will never enter the GrannySmith block.
No, that isn't possible. After the exception is handled by the inner except it doesn't have the ability to get handled by the outer except:
From the docs on the try statement:
When the end of this block is reached, execution continues normally after the entire try statement. (This means that if two nested handlers exist for the same exception, and the exception occurs in the try clause of the inner handler, the outer handler will not handle the exception.)
In short your only solution might be to have another handler at an outer level and re-raise the exception in the inner handler, that is:
try:
try:
raise ZeroDivisionError
except ZeroDivisionError as e:
print("caught")
raise ZeroDivisionError
except ZeroDivisionError as f:
print("caught")
Now the nested except raises an exception which is consequently caught by a similar handler.
I try to construct a except clause that catches everything except [sic] a particular type of exception:
try:
try:
asdjaslk
except not NameError as ne: #I want this block to catch everything except NameError
print("!NameError- {0}: {1}".format(ne.__class__, ne))
except Exception as e: #NameError is the only one that should get here
print("Exception- {0}: {1}".format(e.__class__, e))
The language accepts the not in the except clause, but does nothing with it:
>>> Exception- <type 'exceptions.NameError'>: name 'asdjaslk' is not defined
Is it possible to do this or should I reraise them?
You'll have to re-raise. An except statement can only whitelist, not blacklist.
try:
asdjaslk
except Exception as ne:
if isinstance(ne, NameError):
raise
The not NameError expression returns False, so you are essentially trying to catch:
except False:
but no exception will ever match a boolean instance.
The syntax allows for any valid Python expression, and the thrown exception is matched against the outcome of that expression. except SomeException if debug else SomeOtherException: is perfectly valid, for example.
You can can try this:
try:
# your code raising exceptions
except NameError:
# catch the exception you don't want to catch
# but immediately raise it again:
print("re-raising NameError- {0}: {1}".format(ne.__class__, ne))
raise
except Exception as e:
# handle all other exceptions here
print("catching Exception- {0}: {1}".format(e.__class__, e))
pass
How to handle all but one exception?
try:
something
except <any Exception except for a NoChildException>:
# handling
Something like this, except without destroying the original traceback:
try:
something
except NoChildException:
raise NoChildException
except Exception:
# handling
The answer is to simply do a bare raise:
try:
...
except NoChildException:
# optionally, do some stuff here and then ...
raise
except Exception:
# handling
This will re-raise the last thrown exception, with original stack trace intact (even if it's been handled!).
New to Python ... but is not this a viable answer?
I use it and apparently works.... and is linear.
try:
something
except NoChildException:
assert True
except Exception:
# handling
E.g., I use this to get rid of (in certain situation useless) return exception FileExistsError from os.mkdir.
That is my code is:
try:
os.mkdir(dbFileDir, mode=0o700)
except FileExistsError:
assert True
and I simply accept as an abort to execution the fact that the dir is not somehow accessible.
I'd offer this as an improvement on the accepted answer.
try:
dosomestuff()
except MySpecialException:
ttype, value, traceback = sys.exc_info()
raise ttype, value, traceback
except Exception as e:
mse = convert_to_myspecialexception_with_local_context(e, context)
raise mse
This approach improves on the accepted answer by maintaining the original stacktrace when MySpecialException is caught, so when your top-level exception handler logs the exception you'll get a traceback that points to where the original exception was thrown.
You can do type checking on exceptions! Simply write
try:
...
except Exception as e:
if type(e) == NoChildException:
raise
It still includes the original stack trace.
I found a context in which catching all errors but one is not a bad thing, namely unit testing.
If I have a method:
def my_method():
try:
something()
except IOError, e:
handle_it()
Then it could plausibly have a unit test that looks like:
def test_my_method():
try:
my_module.my_method()
except IOError, e:
print "shouldn't see this error message"
assert False
except Exception, e:
print "some other error message"
assert False
assert True
Because you have now detected that my_method just threw an unexpected exception.