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I installed latest release (https://www.python.org/downloads/release/python-351/) on OSX and get this error:
$ python3
...
ImportError: This package should not be accessible on Python 3. Either you are trying to run from the python-future src folder or your installation of python-future is corrupted.
Also tried installing via brew and got same error.

The full error log:
$ python3
Failed to import the site module
Traceback (most recent call last):
 File "/usr/local/lib/python2.7/site-packages/site.py", line 75, in <module>
 __boot()
 File "/usr/local/lib/python2.7/site-packages/site.py", line 23, in __boot
 loader.load_module('site')
 File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site.py", line 563, in <module>
 main()
 File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site.py", line 549, in main
 known_paths = addusersitepackages(known_paths)
 File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site.py", line 281, in addusersitepackages
 user_site = getusersitepackages()
 File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site.py", line 257, in getusersitepackages
 user_base = getuserbase() # this will also set USER_BASE
 File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site.py", line 247, in getuserbase
 USER_BASE = get_config_var('userbase')
 File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/sysconfig.py", line 582, in get_config_var
 return get_config_vars().get(name)
 File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/sysconfig.py", line 561, in get_config_vars
 import _osx_support
 File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/_osx_support.py", line 4, in <module>
 import re
 File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/re.py", line 335, in <module>
 import copyreg
 File "/usr/local/lib/python2.7/site-packages/copyreg/__init__.py", line 7, in <module>
 raise ImportError('This package should not be accessible on Python 3. '
ImportError: This package should not be accessible on Python 3. Either you are trying to run from the python-future src folder or your installation of python-future is corrupted.
What needs to be changed to avoid importing Python 2 modules?

Apparently "The PYTHONPATH variable is used by all versions of Python 2 and Python 3, so you should not permanently configure this variable unless it only includes code that is compatible with all of your installed Python versions."
That seems crazy...

It looks like your Python 3 is somehow using your Python 2 site directory. What does your PYTHONPATH contain?
Try running this:
python3 -E -S -c "import sys; print(sys.path)"
and see what it says.

> That seems crazy...
What's crazy about it? There are very very few legitimate situations where you need to permanently set any PYTHON* variable, particularly PYTHONPATH and PYTHONHOME. Adding site-packages to sys.path via PYTHONPATH is completely unnecessary anyway since site.py will do that for you in a much safer manner.
This looks like a local configuration issue; closing.

To add to Zach's comment, there have been extended discussions in the past about whether there should be a separate set of environment variables for Python 3 vs Python 2 interpreters; see, for instance, Issue2375 and http://comments.gmane.org/gmane.comp.python.devel/109797. In the latter, Guido pronounced on the matter: no separate values for PYTHONPATH since PYTHONPATH is not intended to be used on permanent basis. Either install distribution packages normally (to site-packages) or use a virtual environment.