I know that I can do:
try:
# do something that may fail
except:
# do this if ANYTHING goes wrong
I can also do this:
try:
# do something that may fail
except IDontLikeYouException:
# say please
except YouAreTooShortException:
# stand on a ladder
But if I want to do the same thing inside two different exceptions, the best I can think of right now is to do this:
try:
# do something that may fail
except IDontLikeYouException:
# say please
except YouAreBeingMeanException:
# say please
Is there any way that I can do something like this (since the action to take in both exceptions is to say please):
try:
# do something that may fail
except IDontLikeYouException, YouAreBeingMeanException:
# say please
Now this really won't work, as it matches the syntax for:
try:
# do something that may fail
except Exception, e:
# say please
So, my effort to catch the two distinct exceptions doesn't exactly come through.
Is there a way to do this?
From Python Documentation:
An except clause may name multiple exceptions as a parenthesized tuple, for example
except (IDontLikeYouException, YouAreBeingMeanException) as e:
pass
Or, for Python 2 only:
except (IDontLikeYouException, YouAreBeingMeanException), e:
pass
Separating the exception from the variable with a comma will still work in Python 2.6 and 2.7, but is now deprecated and does not work in Python 3; now you should be using as.
How do I catch multiple exceptions in one line (except block)
Do this:
try:
may_raise_specific_errors():
except (SpecificErrorOne, SpecificErrorTwo) as error:
handle(error) # might log or have some other default behavior...
The parentheses are required due to older syntax that used the commas to assign the error object to a name. The as keyword is used for the assignment. You can use any name for the error object, I prefer error personally.
Best Practice
To do this in a manner currently and forward compatible with Python, you need to separate the Exceptions with commas and wrap them with parentheses to differentiate from earlier syntax that assigned the exception instance to a variable name by following the Exception type to be caught with a comma.
Here's an example of simple usage:
import sys
try:
mainstuff()
except (KeyboardInterrupt, EOFError): # the parens are necessary
sys.exit(0)
I'm specifying only these exceptions to avoid hiding bugs, which if I encounter I expect the full stack trace from.
This is documented here: https://docs.python.org/tutorial/errors.html
You can assign the exception to a variable, (e is common, but you might prefer a more verbose variable if you have long exception handling or your IDE only highlights selections larger than that, as mine does.) The instance has an args attribute. Here is an example:
import sys
try:
mainstuff()
except (KeyboardInterrupt, EOFError) as err:
print(err)
print(err.args)
sys.exit(0)
Note that in Python 3, the err object falls out of scope when the except block is concluded.
Deprecated
You may see code that assigns the error with a comma. This usage, the only form available in Python 2.5 and earlier, is deprecated, and if you wish your code to be forward compatible in Python 3, you should update the syntax to use the new form:
import sys
try:
mainstuff()
except (KeyboardInterrupt, EOFError), err: # don't do this in Python 2.6+
print err
print err.args
sys.exit(0)
If you see the comma name assignment in your codebase, and you're using Python 2.5 or higher, switch to the new way of doing it so your code remains compatible when you upgrade.
The suppress context manager
The accepted answer is really 4 lines of code, minimum:
try:
do_something()
except (IDontLikeYouException, YouAreBeingMeanException) as e:
pass
The try, except, pass lines can be handled in a single line with the suppress context manager, available in Python 3.4:
from contextlib import suppress
with suppress(IDontLikeYouException, YouAreBeingMeanException):
do_something()
So when you want to pass on certain exceptions, use suppress.
From Python documentation -> 8.3 Handling Exceptions:
A try statement may have more than one except clause, to specify
handlers for different exceptions. At most one handler will be
executed. Handlers only handle exceptions that occur in the
corresponding try clause, not in other handlers of the same try
statement. An except clause may name multiple exceptions as a
parenthesized tuple, for example:
except (RuntimeError, TypeError, NameError):
pass
Note that the parentheses around this tuple are required, because
except ValueError, e: was the syntax used for what is normally
written as except ValueError as e: in modern Python (described
below). The old syntax is still supported for backwards compatibility.
This means except RuntimeError, TypeError is not equivalent to
except (RuntimeError, TypeError): but to except RuntimeError as
TypeError: which is not what you want.
If you frequently use a large number of exceptions, you can pre-define a tuple, so you don't have to re-type them many times.
#This example code is a technique I use in a library that connects with websites to gather data
ConnectErrs = (URLError, SSLError, SocketTimeoutError, BadStatusLine, ConnectionResetError)
def connect(url, data):
#do connection and return some data
return(received_data)
def some_function(var_a, var_b, ...):
try: o = connect(url, data)
except ConnectErrs as e:
#do the recovery stuff
blah #do normal stuff you would do if no exception occurred
NOTES:
If you, also, need to catch other exceptions than those in the
pre-defined tuple, you will need to define another except block.
If you just cannot tolerate a global variable, define it in main()
and pass it around where needed...
One of the way to do this is..
try:
You do your operations here;
......................
except(Exception1[, Exception2[,...ExceptionN]]]):
If there is any exception from the given exception list,
then execute this block.
......................
else:
If there is no exception then execute this block.
and another way is to create method which performs task executed by except block and call it through all of the except block that you write..
try:
You do your operations here;
......................
except Exception1:
functionname(parameterList)
except Exception2:
functionname(parameterList)
except Exception3:
functionname(parameterList)
else:
If there is no exception then execute this block.
def functionname( parameters ):
//your task..
return [expression]
I know that second one is not the best way to do this, but i'm just showing number of ways to do this thing.
As of Python 3.11 you can take advantage of the except* clause that is used to handle multiple exceptions.
PEP-654 introduced a new standard exception type called ExceptionGroup that corresponds to a group of exceptions that are being propagated together. The ExceptionGroup can be handled using a new except* syntax. The * symbol indicates that multiple exceptions can be handled by each except* clause.
For example, you can handle multiple exceptions
try:
raise ExceptionGroup('Example ExceptionGroup', (
TypeError('Example TypeError'),
ValueError('Example ValueError'),
KeyError('Example KeyError'),
AttributeError('Example AttributeError')
))
except* TypeError:
...
except* ValueError as e:
...
except* (KeyError, AttributeError) as e:
...
For more details see PEP-654.
Related
As in the following documentation for SSH:
http://docs.paramiko.org/en/latest/api/ssh_exception.html
How can I catch the following exception:
except paramiko.ssh_exception.BadAuthenticationType(explanation, types)
# Do your work here...
It is easy to catch normal exceptions (that doesn't have parameters) as follows:
except paramiko.ssh_exception.AuthenticationException as e:
# Do your work here...
The parameters are used to create the exception; it has already been created by the time you can catch it. All exceptions are caught the same way; the only issue is how you access whatever attributes those parameters may have been used to set.
except paramiko.ssh_exception.BadAuthenticationType as exc:
...
I have this rather intricate try except block:
try:
self.sorting = sys.argv[1]
try:
test_sorting_var = int(self.sorting)
if test_sorting_var < 1:
print "Sorting column number not valid."
raise ValueError
else:
self.sorting = test_sorting_var
except ValueError:
print "There's a problem with the sorting value provided"
print "Either the column doesn't exists or the column number is invalid"
print "Please try a different sorting value or omit it."
sys.exit(1)
except:
if self.sorting not in self.output_table.column_headers:
print "Sorting column name not valid."
raise ValueError
except:
pass
Basically I'm checking:
If there's a sys.argv[1]
If so, try it as int, and see if it's less than 1
If int fails, test it as string
In both 2+3, if the tests don't succeed, I'm raising a ValueError that should be caught in the except ValueError block and it does as expected:
Sorting column number not valid.
There's a problem with the sorting value provided
Either the column doesn't exists or the column number is invalid
Please try a different sorting value or omit it.
BUT! The sys.exit(1) is not invoked and the program just continues.
How can I fix it and even make it more readable?
In the last two lines you catch any exception:
except:
pass
This includes the exception SystemExit, which is raised by sys.exit.
To fix this, only catch exceptions deriving from Exception, which
SystemExit does not:
except Exception:
pass
In general, it's (almost) never a good idea to do a bare except, always catch Exception, or if possible, something more specific.
The builtint sys.exit() raises a SystemExit-Exception. As you are catching any type of exception when you don't define the Exception to catch (except: without an Exception Type) the SystemExit gets also caught. Ultimately the function will run until the last line where you wrote pass.
Best thing to do is to always catch specific Exceptions and never ever catch all Exceptions with an except:.
Furthermore you should put the check if self.sorting is not in self.output_table.column_headers outside the try catch where you check for a valid self.sorting.
From the documentation for sys.exit:
Exit from Python. This is implemented by raising the SystemExit exception, so cleanup actions specified by finally clauses of try statements are honored, and it is possible to intercept the exit attempt at an outer level.
This means that the outer try except loop is catching the SystemExit exception and causing it to pass. You can add this exception to the outer block and call it again.
I think I would do something like this:
import sys
def MyClass(object):
def method(self, argv, ...):
# ...
if len(argv) < 2:
raise RuntimeError("Usage: {} <sorting column>".format(argv[0]))
sorting = argv[1]
try:
self.sorting = int(sorting)
except ValueError:
try:
self.sorting = self.output_table.column_headers.index(sorting)
except ValueError:
raise ValueError("Invalid sorting column '{}'.".format(sorting))
# ...
try:
# ...
obj.method(sys.argv, ...)
except Exception as e:
sys.exit(e.message)
It's okay to ask for forgiveness instead of permission when it makes things easier (for example to parse a number), but if you need to make sure if sys.argv has enough elements just check it, it will make the program flow clearer.
Avoid using sys.exit within regular code, try to use it only in the outermost levels. For the most part, it is generally better to let exceptions bubble up and catch them at top level or let them crash the program if necessary.
Do make use of exception parameters to store error information, you can decide at a later point whether to print the error, log it, show it in a popup, ...
Instead of using sys.argv directly from within a class, you can pass it as an argument to the method/constructor, it will make the code easier to test and more flexible towards the future.
I have some exception handling code in python where two exceptions can be raised, the first one being a "superset" of the second one.
I.e. the following code summarizes what I need to do (and works fine)
try:
normal_execution_path()
except FirstError:
handle_first_error()
handle_second_error()
except SecondError:
handle_second_error()
But it requires me to abstract everything into independent functions for the code to remain clean and readable. I was hopping for some simpler syntax like:
try:
normal_execution_path()
except FirstError:
handle_first_error()
raise SecondError
except SecondError:
handle_second_error()
But this does not seem to work (SecondError does not get re-catched if it is raised inside this block). Is there anything doable in that direction though ?
If you wish to manually throw the second error to be handled, you can use nested try-catch blocks like these:
try:
normal_execution_path()
except FirstError:
try:
handle_first_error()
raise SecondError
except SecondError:
handle_second_error()
except SecondError:
handle_second_error()
Perhaps it is worth reviewing the code architecture. But for your particular case:
Create a generic class that handles this type of error. To inherit from it for the first and second error cases. Create a handler for this type of error. In the handler, check the first or second special case and process it with a waterfall.
class SupersetException(Exception):
pass
class FirstError(SupersetException):
pass
class SecondError(SupersetException):
pass
def normal_execution_path():
raise SecondError
def handle_superset_ex(state):
# Our waterfall
# We determine from whom the moment to start processing the exception.
if type(state) is FirstError:
handle_first_error()
# If not the first, the handler above will be skipped
handle_second_error()
try:
normal_execution_path()
except SupersetException as state:
handle_superset_ex(state)
Then just develop the idea.
I have a code for a function which is called inside another function.(Result of refactoring).
So in the called function I have a huge block of try-catch statements as.
def Called():
try:
#All statements for the function in the try block.
except A:
# Exception handler.
except B:
# Exception handler.
except A:
# Exception handler.
The problem I have is that I need to catch two exceptions of the same type (At different locations in the Called function). Which then are handled by the Calling function.
One way would be to define two try-except blocks within the Called function. But I am not understanding how the Calling function can handle two exceptions of the same type differently.
This won't work as advertised; only the first except A clause will ever get executed. What you need is either some logic inside the clause to further inspect the exception, or (if the code inside the try block permits) several try-except blocks.
Example of the former approach:
try:
something_that_might_fail()
except A as e:
if e.is_harmless():
pass
elif e.is_something_we_can_handle():
handle_it()
else:
raise # re-raise in the hope it gets handled further up the stack
I think this will work
def Called():
try:
#All statements for the function in the try block.
except A:
try:
do_someting()
except B:
try:
do_somthing_else()
except:
except A:
# Exception handler.
Then using try catch in python you can catch errors and assign them to a variable with the as keyword
try
do something..
except IOError as e:
do something with e..
However then trying to do the same thing without knowing the type of error python complains about the syntax.
try
do something..
except as e:
do something with e..
Is there any way to catch a default error and assign it to a variable?
Yes there is. All exceptions derive from the Exception class.
So you can do:
try:
doSomething()
except Exception as e:
doSomethingWithException(e)
It is a kind of catch-all line.
You can also use sys.exc_info(). This allows you to handle exceptions on Python 2.x and Python 3.x with the same code.
The conventional manner is:
try:
do_whatever()
except Exception as e:
handle_it()
Although it's ill-advised to catch such broad excepts - it's preferable to catch specific exceptions you know you can handle and let anything else propogate.
It's worth noting that KeyboardInterrupt and SystemExit inherit from BaseException and not Exception, so these wouldn't be caught were you expecting to cater for those, but that shouldn't be a problem as they should be handled at the top level anyway.