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>>> print tempfile.gettempdir()
c:\users\dreamkxd\appdata\local\temp
>>>
And the real path is
C:\Users\dreamkxd\AppData\Local\Temp.

The actual implementation calls os.path.normcase from tempfile._get_default_tempdir. In this scenario here, that resolves to the ntpath module & triggers a lowercase conversion.
On the other hand, the posixpath module is simply an identity function.
Now, it *is* possible to force a Windows file system to run in a case-sensitive configuration. Since ntpath.py forces lower case, this could actually cause breakage in that situation, however rare.
In my opinion, changing the ntpath.normcase function to retain case probably yields more correct behavior.

There's not much sense in having normcase() if it never does anything :)
But in this case, I don't really see why tempfile needs to use normcase().

+1 for fixing.

The normcase call isn't exactly a no-op in ntpath.py as it also swaps / for \\. Removing the .lower() seems to simply make it a watered down version of normpath.
There are a couple of options I guess. We could simply keep the altsep, or update _get_default_tempdir to use normpath? 
I'd be happy to do it once everyone agrees =)

+1
 _get_default_tempdir to use normpath

Here's a tiny patch that just changes normcase->normpath. This fixes the casing issue at the 'gettempdir' level, though it doesn't address the 'normcase' function itself.
Note that *both* macpath.py and ntpath.py use <s>.lower, which obviously won't fly on case-sensitive mounts. If someone more experienced than I has a suggestion on how to handle that -- or whether its worthwhile to even touch -- I'll gladly implement.

It's possible to getting actual file name under Windows.
http://stackoverflow.com/questions/74451/getting-actual-file-name-with-proper-casing-on-windows 

It doesn't seem to me that the right approach on either platform is to simply downcase. Maybe just deprecate the call altogether as its not doing anything normpath isn't if the <s>.lower call is removed?

This issue is much nastier than it seems when dealing with character sets that contain multi-byte characters in Python 2.7 on Windows. For example, on a Japanese system:
1. The ANSI code page is cp932 and Python 2.7 will return the TEMP environment variable as a str with this encoding.
2. If a user has a username containing the character \u5c71, this will be encoded as \x8eR, so the TEMP environment variable will include this character.
3. When normcase is called, \x8eR is converted to \x8er.
4. Because \x8eR is a multi-byte character, \x8er is an unrelated character; case insensitivity for "R" doesn't apply.
5. The result is that the correct temp directory is skipped. In fact, on most systems, IOError is raised because none of the other candidate directories are valid either.

Yeah clearly the wrong behavior on Winders.
I think that moving to 'normpath' instead of 'normcase' is likely the right thing to do. Patch is attached, so if someone with commit powers could review real quick I'll address whatever needs to be addressed.
IMHO, the 'normcase' calls are of questionable benefit since the posix call is simply an identity and as this bug details, it isn't necessarily the right behavior on Win.

> I think that moving to 'normpath' instead of 'normcase'
Official manual says 'abspath' include 'normpath' functionality.
I think it is need JUST remove '_os.path.normcase' calling.
But, I do not understand the circumstances which use 'normcase'.

normpath doesn't really buy anything here as abspath already has the same effect (plus more). Patch attach removes normcase|path leaving only abspath.

If no-one objects, I'll commit in a day or two.

Could you perhaps add a test?

Added, including a slightly surprising change needed to test_zipimport_support (which arguably should have been there from the start for robustness).

The test fails under Linux here:
======================================================================
FAIL: test_case_sensitive (test.test_tempfile.TestGetTempDir)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/home/antoine/cpython/default/Lib/test/test_tempfile.py", line 490, in test_case_sensitive
 self.assertEqual(tempfile.gettempdir(), case_sensitive_tempdir)
AssertionError: '/tmp' != 'C:\\Temp'
- /tmp
+ C:\Temp

New patch, tested on Windows & Linux

New changeset d5a9a1ba47ee by Tim Golden in branch 'default':
Issue14255 Don't flatten case of tempdir
http://hg.python.org/cpython/rev/d5a9a1ba47ee 

Fixed. Thanks for the report.