Catch multiple exceptions in one line (except block)

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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:

1. If you, also, need to catch other exceptions than those in the pre-defined tuple, you will need to define another except block.

2. If you just cannot tolerate a global variable, define it in main() and pass it around where needed...