Mac安装NLTK报Uninstalling six-1.4.1错误

在Mac上安装NLTK时报以下错误：

Uninstalling six-1.4.1:

Exception:

Traceback (most recent call last):

  File "/Library/Python/2.7/site-packages/pip-9.0.1-py2.7.egg/pip/basecommand.py", line 215, in main

    status = self.run(options, args)

  File "/Library/Python/2.7/site-packages/pip-9.0.1-py2.7.egg/pip/commands/install.py", line 342, in run

    prefix=options.prefix_path,

  File "/Library/Python/2.7/site-packages/pip-9.0.1-py2.7.egg/pip/req/req_set.py", line 778, in install

    requirement.uninstall(auto_confirm=True)

  File "/Library/Python/2.7/site-packages/pip-9.0.1-py2.7.egg/pip/req/req_install.py", line 754, in uninstall

    paths_to_remove.remove(auto_confirm)

  File "/Library/Python/2.7/site-packages/pip-9.0.1-py2.7.egg/pip/req/req_uninstall.py", line 115, in remove

    renames(path, new_path)

  File "/Library/Python/2.7/site-packages/pip-9.0.1-py2.7.egg/pip/utils/__init__.py", line 267, in renames

    shutil.move(old, new)

  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/shutil.py", line 302, in move

    copy2(src, real_dst)

  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/shutil.py", line 131, in copy2

    copystat(src, dst)

  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/shutil.py", line 103, in copystat

    os.chflags(dst, st.st_flags)

OSError: [Errno 1] Operation not permitted: '/tmp/pip-jGajnO-uninstall/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/six-1.4.1-py2.7.egg-info'

google到six-1.4.1是系统内置的packages，因 系统集成保护 你是没有权限去修改/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/six-1.4.1-py2.7.egg-info目录的。因此在安装的时候需要选择忽略six的安装：

sudo pip install -U nltk --ignore-installed six