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Okay I see your point about backwards compatibility now. Indeed, I have written code myself before Python 3.3 that used to inspect err.args[0], and it would suffer a similar problem if I had not updated it to use the new Python 3.3 exception subclasses.
Below I have written a comparison of two options that have been proposed. Both have disadvantages and would potentially require changes in user code.
A third middle-ground option might be to deprecate OSError.errno as well, and replace it with OSError.posix_errno or something. But I would still prefer the option to stop setting SSLError.errno, even though it breaks backwards compatibility. I think we can at least agree that "errno" and "args" should be deprecated for SSLError instances.
## Option 1: Add OSError.errno_scope ##
To avoid the bug in new Python versions we would still have to modify code handling a general OSError (EnvironmentError), for example:
# Buggy handler compatible with Python 3.2
try:
 return ssl_sock.read()
except EnvironmentError as e:
 if e.errno != errno.EINTR:
 raise
# Fixed handler, supporting new Python versions only
try:
 return ssl_sock.read()
except OSError as e:
 if e.errno_scope != POSIX or e.errno != errno.EINTR:
 raise
Advantages:
* Existing code still works if you don’t care about this bug
* No code changes required in SSL error handlers
## Option 2: Stop setting SSLError.errno ##
Existing SSLError handlers written for Python < 3.3 would stop catching exceptions, because "errno" defaults to None. This would probably cause an unhandled exception, and could be fixed as follows:
# Broken handler written for Python 3.2
try:
 return ssl_sock.read()
except SSLError as e:
 if e.errno != SSL_ERROR_WANT_READ:
 raise
 # Handle non-blocking read
# Fixed handler, supporting Python 3.3+ only
try:
 return ssl_sock.read()
except SSLWantReadError as e:
 # Handle non-blocking read
Advantages:
* Affected code should be easy to find due to breakage
* Only requires the user to change code that directly handles SSL errors
* No changes for code already written for Python 3.3
* Code changes only when relying on the undocumented errno behaviour, which is already asking for trouble