Printing passed bytes from Exception

This seems to be the code for the poplib.error_proto.

class error_proto(Exception): pass

It just passes the bytes from the POP response in the exception. What I would like to do is catch any exception, take those bytes, use .decode('ascii') on them, and print them as a string. I've written my own test setup like so:

class B(Exception): pass

def bex(): raise B(b'Problem')

try:
    bex()
except B as err:
    print(err.decode('ascii'))

I've tried replacing the last line with:

    b = bytes(err)
    print(b.decode('ascii'))

But to no avail. Is this possible and if so, how would I implement this?

UPDATE: Though, as falsetru points out, the documentation says results are returned as strings, this is not the case:

>>> p = poplib.POP3('mail.site.com')
>>> p.user('skillian@site.com')
b'+OK '
>>> p.pass_('badpassword')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python33\lib\poplib.py", line 201, in pass_
    return self._shortcmd('PASS %s' % pswd)
  File "C:\Python33\lib\poplib.py", line 164, in _shortcmd
    return self._getresp()
  File "C:\Python33\lib\poplib.py", line 140, in _getresp
    raise error_proto(resp)
poplib.error_proto: b'-ERR authorization failed  Check your server settings.'
>>>

According to poplib.error_proto documentation:

Exception raised on any errors from this module (errors from socket module are not caught). The reason for the exception is passed to the constructor as a string.

So, you don't need to decode it.

---

UPDATE It seems like the documentation does not match the actual implementation.

You can access the arguments passed to the exception constructor using args attribute.

p = poplib.POP3('mail.site.com')
try:
    p.user('skillian@site.com')
    p.pass_('badpassword')
except poplib.error_proto as e:
    print(e.args[0].decode('ascii')) # `'ascii'` is not necessary.