[Python-checkins] r45925 - in python/trunk: Lib/tempfile.py Lib/test/test_os.py Misc/NEWS Modules/posixmodule.c

[M.-A. Lemburg]

Martin v. Löwis wrote:
> M.-A. Lemburg wrote:
>> That's not what I meant: in errno, many WSA windows error codes
>> are aliased to their corresponding Unix error codes to
>> enhance portability across platforms.
> 
> Can you please explain how such aliasing could work? errno can
> only have one value: either 183, or 17, but not both.

Right. The idea used in errno is to let the Windows
error code override the value possibly defined in the
C lib (that's why the order of the inscode() in errno
matters). This is based on:

http://msdn.microsoft.com/library/en-us/winsock/winsock/error_codes_errno_h_errno_and_wsagetlasterror_2.asp?frame=true

>> I don't know where ERROR_ALREADY_EXISTS is defined and
>> how it relates to the WSA codes, but since EEXIST doesn't
>> have a corresponding, it's probably a good idea to alias
>> EEXIST to ERROR_ALREADY_EXISTS and use that in tempfile.py
>> for both Windows and Unix instead of adding another except
>> clause.
> 
> ERROR_ALREADY_EXISTS is defined in winerror.h, and it is possible
> result of the GetLastError system call.
> 
> I don't understand the fragment "but since EEXIST doesn't have a
> corresponding": A corresponding what?

Sorry, it should read "corresponding WSA alias", ie. there's
no WSAEEXIST, probably because Windows sockets don't support
domain sockets.

> You mean, I should define EEXIST to have the value of 183, instead
> of having the value 17?  That's certainly not a good idea. First,
> EEXIST should map to either ERROR_FILE_EXISTS (80) or
> ERROR_ALREADY_EXISTS (183), depending on context. Furthermore,
> the C library will still put its own value (17) into errno.

Ah, ok. I didn't know that they use different error values.
That makes things complicated.

>>> In what sense would the backwards compatibility be improved ?!
>> I suppose that the code used to raise an OSError with
>> EEXIST error code before the change to use Win32 APIs
>> on Windows.
> 
> Correct. It now raises WindowsError instead.
> 
>> With the alias, existing code looking for EEXIST on Windows
>> will continue to work without change.
> 
> I still cannot see how this could work, at least not in a way
> that wouldn't break something else.

Indeed.

Perhaps we should have hybrid error code objects that compare
equal to more than one integer code, but otherwise behave like
integers ?!

E.g.

# Error code objects
class ErrorCode(int):
    values = ()
    def __new__(cls, basevalue, *aliasvalues):
        return int.__new__(cls, basevalue)
    def __init__(self, basevalue, *aliasvalues):
        self.values = (basevalue,) + aliasvalues
    def __cmp__(self, other):
        if other in self.values:
            return 0
        if other < self.values:
            return 1
        else:
            return -1

EEXISTS = ErrorCode(17, 183, 80)


-- 
Marc-Andre Lemburg
eGenix.com

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