Develop Reference

Python: How to catch a list of exceptions? [duplicate]

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.

Re-raise and re-catch error immediately in python

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.

Does `try... except Exception as e` catch every possible exception?

In Python 2, are all exceptions that can be raised required to inherit from Exception?
That is, is the following sufficient to catch any possible exception:
try:
code()
except Exception as e:
pass
or do I need something even more general like
try:
code()
except:
pass

With the first variant you'll catch "all built-in, non-system-exiting exceptions" (https://docs.python.org/2/library/exceptions.html), and should catch user defined exceptions ("all user-defined exceptions should also be derived from this class").
For example, the first variant will not catch user-pressed Control-C (KeyboardInterrupt), but the second will.

In python why ever use except:

In python is it true that except Exception as ex or except BaseException as ex is the the same as except: but you get a reference to the exception?
From what I understand BaseException is the newer default catch-all.
Aside from that why would you ever want just an except: clause?

The difference between the three is:
bare except catches everything, including system-exiting things like KeyboardInterrupt;
except Exception[ as ex] will catch any subclass of Exception, which should be all your user-defined exceptions and everything built-in that is non-system-exiting; and
except BaseException[ as ex] will, like bare except, catch absolutely everything.
Generally, I would recommend using 2. (ideally as a fallback, after you have caught specific/"expected" errors), as this allows those system-exiting exceptions to percolate up to the top level. As you say, the as ex part for 2. and 3. lets you inspect the error while handling it.
There is a useful article on "the evils of except" here.

There are several differences, apart from Pokémon exception handling* being a bad idea.
Neither except Exception: nor except BaseException: will catch old-style class exceptions (Python 2 only):
>>> class Foo(): pass
...
>>> try:
... raise Foo()
... except Exception:
... print 'Caught'
...
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
__main__.Foo: <__main__.Foo instance at 0x10ef566c8>
>>> try:
... raise Foo()
... except BaseException:
... print 'Caught'
...
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
__main__.Foo: <__main__.Foo instance at 0x10ef56680>
>>> try:
... raise Foo()
... except:
... print 'Caught'
...
Caught
because the old-style object is not derived from BaseException or Exception. This is a good reason to never use custom exceptions that do not derive from Exception, in any case.
Next, there are three exceptions that derive from BaseException, but not from Exception; in most cases you don't want to catch those. SystemExit, KeyboardInterrupt and GeneratorExit are not exceptions you would want to catch in the normal course of exception handling. If you use except BaseException: you do catch these, except Exception will not.
* Pokémon exception handling because you gotta catch em all.

If you truly do not care what the reason or message of the failure was, you can use a bare except:. Sometimes this is useful if you are trying to access some functionality which may or may not be present or working, and if it fails you plan to degrade gracefully to some other code path. In that case, what the error type or string was does not affect what you're going to do.

It's not quite the case, no.
If you have a look at the Python documentation on built-in exceptions (specifically this bit) you see what exceptions inherit from where. If you use raw except: it will catch every exception thrown which even includes KeyboardInterrupt which you almost certainly don't want to catch; the same will happen if you catch BaseException with except BaseException as exp: since all exceptions inherit from it.
If you want to catch all program runtime exceptions it's proper to use except Exception as exp: since it won't catch the type of exceptions that you want to end the program (like KeyboardInterrupt).
Now, people will tell you it's a bad idea to catch all exceptions in this way, and generally they're right; but if for instance you have a program processing a large batch of data you may rightfully want it to not exit in case of an exception. So long as you handle the exception properly (ie, log it and make sure the user sees an exception has occurred) but never just pass; if your program produces errors you're unaware of, it will do strange things indeed!

Aside from that why would you ever want just an except: clause?
Short answer: You don't want that.
Longer answer: Using a bare except: takes away the ability to distinguish between exceptions, and even getting a hand on the exception object is a bit harder. So you normally always use the form except ExceptionType as e:.

How to catch a default error as something?

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.