Switch python version in MAC-OS is a confused problem, so i use pyenv to switch python version, it works very well:

vinllen@ ~$ python --version  
Python 2.7.11  
vinllen@ ~$ pyenv global <tab>  
--help  2.7.10  3.4.0   system
vinllen@ ~$ pyenv global 3.4.0  
--help  2.7.10  3.4.0   system
vinllen@ ~$ python --version  
Python 3.4.0  

  Problem occurs when i using cmake to compile a software called SimpleElastix, the error shows that my python version is unsuitable:

 CMake Error at /usr/local/Cellar/cmake/3.3.1/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:148 (message):
   Could NOT find PythonLibs: Found unsuitable version "2.7.6", but required
   is exact version "2.7.11" (found /usr/lib/libpython2.7.dylib)
 Call Stack (most recent call first):
   /usr/local/Cellar/cmake/3.3.1/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:386 (_FPHSA_FAILURE_MESSAGE)
   /usr/local/Cellar/cmake/3.3.1/share/cmake/Modules/FindPythonLibs.cmake:205 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
   /Users/vinllen/opt/elastix/SimpleElastix/CMake/sitkLanguageOptions.cmake:38 (find_package)
   SuperBuild.cmake:244 (include)
   CMakeLists.txt:26 (include)

After i switching python version into 3.4.0 by pyenv, the required version of python also switch to "3.4":

 CMake Error at /usr/local/Cellar/cmake/3.3.1/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:148 (message):
   Could NOT find PythonLibs: Found unsuitable version "2.7.6", but required
   is exact version "3.4" (found /usr/lib/libpython2.7.dylib)
 Call Stack (most recent call first):
   /usr/local/Cellar/cmake/3.3.1/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:386 (_FPHSA_FAILURE_MESSAGE)
   /usr/local/Cellar/cmake/3.3.1/share/cmake/Modules/FindPythonLibs.cmake:205 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
   /Users/vinllen/opt/elastix/SimpleElastix/CMake/sitkLanguageOptions.cmake:38 (find_package)
   SuperBuild.cmake:244 (include)
   CMakeLists.txt:26 (include)

  It's very strange the required version dynamic change when my python version changed.
  At first, i guess maybe it's my problem of cmake-3,3, so i download a new cmake-3.5 with GUI. Now, i can pass the compile and build, and then installed by python, but when i import the package of SimpleITK, errors happened again:

>>> import SimpleITK
Fatal Python error: PyThreadState_Get: no current thread  
Abort trap: 6  

  So i seach this problem in stackoverflow and find this solution, i got the reason: cmake-3.5 with GUI compile SimpleElastix by python-app which is a pre-installed version by OS-X, but the excute python in my command line is installed by homebrew or pyenv or macport or anything else. After failed using otool -L and install_name_tool to switch dynmaic link library, i follow this post to switch python by myself:

1. Delete old python version by OS-X:
   sudo mv /System/Library/Frameworks/Python.framework/Versions/2.7 /System/Library/Frameworks/Python.framework/Versions/2.7_bak
2. Move the new python:
   sudo mv /usr/local/Cellar/python/2.7.11/Frameworks/Python.framework/Versions/2.7 /System/Library/Frameworks/Python.framework/Versions/2.7

3. Setting group to wheel if needed:
   sudo chown -R root:wheel /System/Library/Frameworks/Python.framework/Versions/2.7
4. Updating the current link if needed:
   sudo rm /System/Library/Frameworks/Python.framework/Versions/Current sudo ln -s /System/Library/Frameworks/Python.framework/Versions/2.7 /System/Library/Frameworks/Python.framework/Versions/Current

Now, rebuild the SimpleElastix.

Reference:
https://wolfpaulus.com/journal/mac/installing_python_osx/
http://stackoverflow.com/questions/15678153/homebrew-python-on-mac-os-x-10-8-fatal-python-error-pythreadstate-get-no-cu