I just installed Python 3.0a4 from source on an Ubuntu system and noticed that it doesn't seem to set the distutils.sysconfig config var: "SO":
```
vagrant@ubuntu:~/src/Python-3.4.0a4$ python3.4
Python 3.4.0a4 (default, Nov 11 2013, 17:11:59)
[GCC 4.6.3] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from distutils import sysconfig
>>> sysconfig.get_config_var("SO")
>>> sysconfig.get_config_var("SO") is None
True
```
This worked fine for me in Python 3.3:
```
vagrant@ubuntu:~/src/Python-3.4.0a4$ python3.3
Python 3.3.2 (default, May 16 2013, 18:32:41)
[GCC 4.6.3] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from distutils import sysconfig
>>> sysconfig.get_config_var("SO")
'.so'
```

Indeed, this happens for me too in default head.

Never mind, this is an intentional change:
- Issue #16754: Fix the incorrect shared library extension on linux. Introduce
  two makefile macros SHLIB_SUFFIX and EXT_SUFFIX. SO now has the value of
  SHLIB_SUFFIX again (as in 2.x and 3.1). The SO macro is removed in 3.4.
Although this does introduce some backward compatibility issues.  Perhaps sysconfig.get_config_var('SO') should be deprecated in 3.4 and removed in 3.5.

Thanks Barry, for tracking down that this is intentional.
I wonder how one gets this value in Python code now? For example, the reason I stumbled upon this in the first place is that there is some code in PyCrypto (https://github.com/dlitz/pycrypto/blob/master/lib/Crypto/SelfTest/PublicKey/test_RSA.py#L464) that uses `get_config_var("SO")` thusly:
```
    except ImportError:
        from distutils.sysconfig import get_config_var
        import inspect
        _fm_path = os.path.normpath(os.path.dirname(os.path.abspath(
            inspect.getfile(inspect.currentframe())))
            +"/../../PublicKey/_fastmath"+get_config_var("SO"))
        if os.path.exists(_fm_path):
            raise ImportError("While the _fastmath module exists, importing "+
                "it failed. This may point to the gmp or mpir shared library "+
                "not being in the path. _fastmath was found at "+_fm_path)
```
What would be the way to express this now in Python >= 3.4?

Here's a patch, sans NEWS and any docs.

Note that obviously the DeprecationWarning is not raised if you do
sysconfig.get_config_vars()['SO']
but it still gets mapped to EXT_SUFFIX in that case.

On Nov 11, 2013, at 06:27 PM, Marc Abramowitz wrote:
>What would be the way to express this now in Python >= 3.4?
For now, use sysconfig.get_config_var('EXT_SUFFIX') though if no one objects
to my patch, I'll restore 'SO' for 3.4.  We'll add a DeprecationWarning and
get rid of it in 3.5.

New changeset fedc2b8fbb6e by Barry Warsaw in branch 'default':
- Issue #19555: Restore sysconfig.get_config_var('SO'), with a
http://hg.python.org/cpython/rev/fedc2b8fbb6e

Test is failing on Windows:
http://buildbot.python.org/all/builders/x86%20Windows%20Server%202003%20%5BSB%5D%203.x/builds/1758/steps/test/logs/stdio
======================================================================
ERROR: test_SO_in_vars (test.test_sysconfig.TestSysConfig)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "E:\Data\buildslave\cpython\3.x.snakebite-win2k3r2sp2-x86\build\lib\test\test_sysconfig.py", line 388, in test_SO_in_vars
    self.assertIsNotNone(vars['SO'])
KeyError: 'SO'
======================================================================
FAIL: test_SO_value (test.test_sysconfig.TestSysConfig)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "E:\Data\buildslave\cpython\3.x.snakebite-win2k3r2sp2-x86\build\lib\test\test_sysconfig.py", line 382, in test_SO_value
    sysconfig.get_config_var('EXT_SUFFIX'))
AssertionError: None != '.pyd'

New changeset 331b7a8bb830 by Barry Warsaw in branch 'default':
A fix for issue 19555 on Windows.
http://hg.python.org/cpython/rev/331b7a8bb830

New changeset 8a130fd92255 by Barry Warsaw in branch 'default':
Issue 19555 for distutils, plus a little clean up (pyflakes, line lengths).
http://hg.python.org/cpython/rev/8a130fd92255

New changeset ac2ee9fc353a by R David Murray in branch 'default':
whatsnew: deprecation of sysconfig SO key (#19555).
http://hg.python.org/cpython/rev/ac2ee9fc353a

nosy: + vstinner
messages: + msg203735

resolution: fixed -> (no value)

messages: + msg202648

nosy: - tarek, eric.araujo