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Since WindowsError became an alias of OSError, the error number shown in the stringification of an OSError err can either be a windows error code (err.winerror) or a posix style error number (err.errno), with no way to tell which.
For instance in
 >>> os.read(999, 0)
 Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 OSError: [Errno 9] Bad file descriptor
err.errno == EBADF == 9 and err.winerror == None, but in
 >>> _winapi.ReadFile(999, 0)
 Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 OSError: [Error 6] The handle is invalid
err.errno == EBADF == 9 and err.winerror == ERROR_INVALID_HANDLE == 6. (winerror gets priority over errno if it exists.)
With the attached patch the second will be shown as
 >>> _winapi.ReadFile(999, 0)
 Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 OSError: [WinError 6] The handle is invalid
 ^^^
Since this issue only applies to 3.3 and the patch is trivial, it would be nice to get this in before 3.3 is released.

I originally kept it that way to minimize disruption with 3.2, but I agree it's a welcome change. As for acceptance in 3.3, Georg will have to decide.

Uh, currently it's "Errno" or "Error", so there is a way to tell which.
I'll leave to Antoine to decide what to do here; it's not like changing it a big deal.
"WinError" looks kind of ugly to me though.

Well, "Error" and "Errno" are rather hard to distinguish, and the difference isn't obvious either, so Richard's patch is a good improvement IMHO.

> Well, "Error" and "Errno" are rather hard to distinguish
Embarrassingly, I did not even notice the was a difference.

New changeset 2e587b9bae35 by Richard Oudkerk in branch 'default':
Issue #15784: Modify OSError.__str__() to better distinguish between
http://hg.python.org/cpython/rev/2e587b9bae35 

The changeset is 2e587b9bae35.
Georg, could you copy it to your release branch please.

Done in 4e941113e4fa.

New changeset 4e941113e4fa by Richard Oudkerk in branch 'default':
Issue #15784: Modify OSError.__str__() to better distinguish between
http://hg.python.org/cpython/rev/4e941113e4fa