I am trying to catch a SystemExit exception in the following fashion:
try:
raise SystemExit
except Exception as exception:
print "success"
But, it doesn't work.
It does work however when I change my code like that:
try:
raise SystemExit
except:
print "success"
As far as I am aware, except Exception as exception should catch any exception. This is how it is described here as well. Why isn't that working for me here?
As documented, SystemExit does not inherit from Exception. You would have to use except BaseException.
However, this is for a reason:
The exception inherits from BaseException instead of StandardError or Exception so that it is not accidentally caught by code that catches Exception.
It is unusual to want to handle "real" exceptions in the same way you want to handle SystemExit. You might be better off catching SystemExit explicitly with except SystemExit.
Related
I have a try block case in my code and I want to ignore one particular exception and all the rest should be raised.
For example:
try:
blah
except <exception> as e:
raise Exception(e)
In this kind of case, I want all the exceptions to be raised except for one case, say if the exception is "query not found" I have to ignore it.
How do I ignore that single exception?
I can use multiple except blocks but how to define a exception?
You can give something like this:
try:
print(x)
except NameError:
print("Variable x is not defined")
except:
print("Something else went wrong")
In this case, you want to catch NameError and specify a message. For all others, you want to specify another message.
Let's say you want to ignore NameError, then you can just give continue or pass.
Alternatively, you can also raise an exception.
Example will be:
x = -1
if x < 0:
raise Exception("Sorry, no numbers below zero")
So you can use a combination of all this to get you what you want.
If you want more details on exception, see the below links:
https://docs.python.org/3/tutorial/errors.html
https://www.w3schools.com/python/python_try_except.asp
https://realpython.com/python-exceptions/
And on stack overflow (as Gino highlighted), see
Handling all but one exception
As an alternative to #Joe Ferndz's answer, in case you don't want the exception to be raised but still want the block to exit, you can use suppress from the contextlib module:
from contextlib import suppress
with suppress(ValueError):
print('hello world')
raise ValueError
print('this will not be printed')
In this case, the block still exits on raise ValueError, but an exception is not raised.
I am trying to handle exceptions in Python 3.6. I want to handle every possible exception and print the exception. When i do
try:
raise RuntimeError("Test")
except:
e = sys.exc_info()[0]
print(e)
it just prints
class '_mysql_exceptions.OperationalError'
How do i get the message of the Exception? In this case i would like the output to be "Test".
You can catch and print the Exception as follows:
try:
raise RuntimeError("Test")
except Exception as e:
print(e)
# Test
I'm not quite sure why you're trying to catch every Exception though, it would seem more prudent to let Python handle and raise these for you in general. Normally you would only catch specific Exceptions.
This behavior is not specific to Python 3.6.
I have a list of functions that may fail and, if one fails, I don't want the script to stop, but to continue with next function.
I am executing it with something like this :
list_of_functions = [f_a, f_b, f_c]
for current_function in list_of_functions:
try:
current_function()
except Exception:
print(traceback.format_exc())
It's working fine, but it is not PEP8 compliant:
When catching exceptions, mention specific exceptions whenever
possible instead of using a bare except: clause.
For example, use:
try:
import platform_specific_module
except ImportError:
platform_specific_module = None
A bare except: clause will catch SystemExit and KeyboardInterrupt
exceptions, making it harder to interrupt a program with Control-C,
and can disguise other problems. If you want to catch all exceptions
that signal program errors, use except Exception: (bare except is
equivalent to except BaseException: ).
A good rule of thumb is to limit use of bare 'except' clauses to two
cases:
If the exception handler will be printing out or logging the traceback; at least the user will be aware that an error has occurred.
If the code needs to do some cleanup work, but then lets the exception propagate upwards with raise . try...finally can be a better
way to handle this case.
How can I do this the good way?
The PEP8 guide you quote suggests that it is okay to use a bare exception in your case provided you are logging the errors. I would think that you should cover as many exceptions as you can/know how to deal with and then log the rest and pass, e.g.
import logging
list_of_functions = [f_a,f_b,f_c]
for current_function in list_of_functions:
try:
current_function()
except KnownException:
raise
except Exception as e:
logging.exception(e)
Use this to cheat PEP8:
try:
"""code"""
except (Exception,):
pass
I think in some rare cases catching general exception is just justified and there is a way to trick PEP8 inspection:
list_of_functions = [f_a,f_b,f_c]
for current_function in list_of_functions:
try:
current_function()
except (ValueError, Exception):
print(traceback.format_exc())
You can replace ValueError by any other. It works for me (at least in PyCharm).
You can just put a comment like except Exception as error: # pylint: disable=broad-except that's worked for me actually. I hope it could be work for you.
From issue PY-9715 on yourtrack.jetbrains.com:
From pep-0348:
BaseException
The superclass that all exceptions must inherit from. It's name was
chosen to reflect that it is at the base of the exception hierarchy
while being an exception itself. "Raisable" was considered as a name,
it was passed on because its name did not properly reflect the fact
that it is an exception itself.
Direct inheritance of BaseException is not expected, and will be
discouraged for the general case. Most user-defined exceptions should
inherit from Exception instead. This allows catching Exception to
continue to work in the common case of catching all exceptions that
should be caught. Direct inheritance of BaseException should only be
done in cases where an entirely new category of exception is desired.
But, for cases where all exceptions should be caught blindly, except
BaseException will work.
You can avoid the error if you then re-raise the Exception. This way you are able to do damage control and not endanger loosing track of its occurance.
Do you perhaps mean that each function can raise different exceptions? When you name the exception type in the except clause it can be any name that refers to an exception, not just the class name.
eg.
def raise_value_error():
raise ValueError
def raise_type_error():
raise TypeError
def raise_index_error():
doesnt_exist
func_and_exceptions = [(raise_value_error, ValueError), (raise_type_error, TypeError),
(raise_index_error, IndexError)]
for function, possible_exception in func_and_exceptions:
try:
function()
except possible_exception as e:
print("caught", repr(e), "when calling", function.__name__)
prints:
caught ValueError() when calling raise_value_error
caught TypeError() when calling raise_type_error
Traceback (most recent call last):
File "run.py", line 14, in <module>
function()
File "run.py", line 8, in raise_index_error
doesnt_exist
NameError: name 'doesnt_exist' is not defined
Of course that leaves you with not knowing what to do when each exception occurs. But since you just want to ignore it and carry on then that's not a problem.
First, generate the pylintrc using the below command
pylint --generate-rcfile > .pylintrc
For reference:
https://learn.microsoft.com/en-us/visualstudio/python/linting-python-code?view=vs-2022
Search for disable (uncomment if needed) in the generate pylintrc file and add the below exception.
broad-except
Rerun the pylint command and see the magic
I want to pause a python script when Crtl+C is pressed but to handle the rest of exceptions with different code.
If I got:
except KeyboardInterrupt:
print '\nPausing... (Hit ENTER to continue, type quit to exit.)'
try:
response = raw_input()
if response == 'quit':
break
print 'Resuming...'
except KeyboardInterrupt:
print 'Resuming...'
continue
except Exception, e:
print traceback.print_exc()
continue
sleep(170)
Won't second except go also for KeyboardInterrupt, isn't a keyboard interrupt supposed to be an exception?
Won't second except go also for KeyboardInterrupt, isn't a keyboard interrupt supposed to be an exception?
Well, this is interesting... Apparently it is not a subclass of the Exception class, but is child to a superclass called BaseException.
Python's doc Built-in Exceptions explains why they implemented it this way:
The exception inherits from BaseException so as to not be accidentally caught by code that catches Exception and thus prevent the interpreter from exiting.
And it can be checked like this:
>>> issubclass(KeyboardInterrupt, Exception)
False
>>> issubclass(KeyboardInterrupt, BaseException)
True
>>> issubclass(Exception, BaseException)
True
Nevertheless, even if you changed your last except block to catch a BaseException instead of a Exception, it would still not enter it (because your inner except KeyboardInterrupt avoids the exception to be thrown to the outer indentation or "parent" levels).
In case you also removed this inner except KeyboardInterrupt block, the exception would be thrown to the outer indentation, which I assume it does not exist (given your indentation level) and the execution would terminate...
zip_file_name = "not_exist.py"
try:
with zipfile.ZipFile(zip_file_name) as f_handle:
print("open it successfully")
except (zipfile.BadZipfile, zipfile.LargeZipFile), e:
print(e)
Is this the correct way to handle exception throw by a with statement?
Yes, this is how you would want to handle exceptions from inside a with statement. ContextManagers (that implement the behavior behind the with statement) can handle exceptions but should only do so to properly clean up resources used by the object.
Here's a snippet from the relevant documentation:
If BLOCK raises an exception, the context manager’s exit() method
is called with three arguments, the exception details (type, value,
traceback, the same values returned by sys.exc_info(), which can also
be None if no exception occurred). The method’s return value controls
whether an exception is re-raised: any false value re-raises the
exception, and True will result in suppressing it. You’ll only rarely
want to suppress the exception, because if you do the author of the
code containing the ‘with‘ statement will never realize anything went
wrong.
Yes, that's fine.
Another alternative is:
try:
f_handle = zipfile.ZipFile(zip_file_name)
# no more code here
except (zipfile.BadZipfile, zipfile.LargeZipFile), e:
print(e)
else:
with f_handle:
print("open it successfully")
This prevents you from accidentally catching an exception from the body of the with statement in your except handler.