Python one exception multiple handlers

Question

I want to do this:

try:
    raise A()
except A:
    print 'A'
except (A, B):
    print 'A,B'

Which I hoped would print both A and A,B.

That doesn't work (only the first except is executed). It makes sense for the first except to swallow the error, in case you want to catch a subclass before it's parent.

But is there another elegant way to get this to work?

I could of course do the following, but that is seemingly redundant code duplication, especially if more than just A and B are involved:

try:
    raise A()
except A:
    print 'A'
    print 'A,B'
except B:
    print 'A,B'

(Related to Multiple exception handlers for the same Exception but not a duplicate. The usage is different and I want to know how best to handle it with minimal code duplication.)

Why do you want to print A, B when the exception is only related to A? — Padraic Cunningham, Apr 26 '15 at 10:03
@PadraicCunningham: With `except (A, B)` the exception could also be related to `B`. — Mark, Apr 26 '15 at 10:23
why not just `catch (exceptions...) as e` and print e.whatever — Padraic Cunningham, Apr 26 '15 at 10:24
Both the `print` and the `raise A()` are meant as examples; it could be other code. — Mark, Apr 26 '15 at 10:25
@Mark, you can subclass and do whatever you want in the print e.whatever part. — Padraic Cunningham, Apr 26 '15 at 10:58

vaultah · Accepted Answer · 2015-04-27T08:04:20.723

1

It's rather common to catch possible exceptions and get exception type from the instance later:

try:
    raise A()
except (A, B) as e:
    if isinstance(e, A):
        print('A')
    print('A', 'B')

Another option is to inherit one class from another e.g.

class B(Exception):
    def do(self):
        print('A', 'B')

class A(B, Exception):
    def do(self):
        print('A')
        super().do()

Then

try:
    raise B()
except (A, B) as e:
    e.do()

will print A B, and

try:
    raise A()
except (A, B) as e:
    e.do()

will print A and A B.

edited Apr 27 '15 at 08:04

answered Apr 26 '15 at 09:45

vaultah

44,105
12
114
143

I'm not a big fan of this style but it seems this is the closest way possible – Mark Apr 26 '15 at 10:27
@Mark see the update, I added another possible option – vaultah Apr 26 '15 at 10:44
Could be useful, but I think often the error handling code isn't related enough to the error object to make it a method. Still, occasionally useful. – Mark Apr 26 '15 at 12:06

score -1 · Answer 2 · answered Apr 26 '15 at 10:08

-1

You can use nested try-except-blocks:

try:
    try:
        raise A()
    except A:
        print 'A'
        raise
except (A, B):
    print 'A,B'

answered Apr 26 '15 at 10:08

Daniel

42,087
4
55
81

Python one exception multiple handlers

2 Answers2