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I cannot find correct repro steps for this, but:
/Library/Frameworks/Python.framework/Versions/2.7/bin/python" -B -s -c "import sys;print('%d.%d' % tuple(sys.version_info)[:2])
Traceback (most recent call last):
 File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site.py", line 558, in <module>
 main()
 File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site.py", line 540, in main
 known_paths = addusersitepackages(known_paths)
 File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site.py", line 264, in addusersitepackages
 user_site = getusersitepackages()
 File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site.py", line 239, in getusersitepackages
 user_base = getuserbase() # this will also set USER_BASE
 File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site.py", line 229, in getuserbase
 USER_BASE = get_config_var('userbase')
 File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/sysconfig.py", line 518, in get_config_var
 return get_config_vars().get(name)
 File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/sysconfig.py", line 421, in get_config_vars
 _init_posix(_CONFIG_VARS)
 File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/sysconfig.py", line 300, in _init_posix
 raise IOError(msg)
IOError: $MACOSX_DEPLOYMENT_TARGET mismatch: now "10.3" but "10.5" during configure
Python was built on a Snow Leopard machine with MACOSX_DEPLOYMENT_TARGET=10.5 environment variable. But on the user's 10.6 machine, no such environment variable is necessarily set.
Why is this check required? Shouldn't it be restricted to building modules using distutils, and not happen during an innocuous "import site" that happens in interpreter startup?