pyenv lets you easily switch between multiple versions of Python. It’s simple, unobtrusive, and follows the UNIX tradition of single-purpose tools that do one thing well.
This project was forked from rbenv and ruby-build, and modified for Python.
$ pyenv doctor
Cloning /Users/Leo/.pyenv/plugins/pyenv-doctor/bin/.....
Installing python-pyenv-doctor...
BUILD FAILED (OS X 10.12.3 using python-build 20160602)
Last 10 log lines:
checking for memory.h... yes
checking for strings.h... yes
checking for inttypes.h... yes
checking for stdint.h... yes
checking for unistd.h... yes
checking openssl/ssl.h usability... no
checking openssl/ssl.h presence... no
checking for openssl/ssl.h... no
configure: error: OpenSSL development header is not installed.
make: *** No targets specified and no makefile found. Stop.
Problem(s) detected while checking system.
通过检测，可以发现本地环境可能存在的问题，例如，从以上输出可以看出，本地的OpenSSL development header还没有安装。根据提示的问题，逐一进行修复，直到检测不再出现问题为止。
$ pyenv -h
Usage: pyenv <command> [<args>]
Some useful pyenv commands are:
commands List all available pyenv commands
local Set or show the local application-specific Python version
global Set or show the global Python version
shell Set or show the shell-specific Python version
install Install a Python version using python-build
uninstall Uninstall a specific Python version
rehash Rehash pyenv shims (run this after installing executables)
version Show the current Python version and its origin
versions List all Python versions available to pyenv
which Display the full path to an executable
whence List all Python versions that contain the given executable
See `pyenv help <command>' for information on a specific command.
For full documentation, see: https://github.com/yyuu/pyenv#readme
如果想修改系统全局的Python环境，可以采用pyenv global PYTHON_VERSION命令。该命令执行后会在$(pyenv root)目录（默认为~/.pyenv）中创建一个名为version的文件（如果该文件已存在，则修改该文件的内容），里面记录着系统全局的Python版本号。
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$ pyenv global 2.7.13
$ cat ~/.pyenv/version
2.7.13
$ pyenv version
2.7.13 (set by /Users/Leo/.pyenv/version)$ pyenv global 3.6.0
$ cat ~/.pyenv/version
3.6.0
$ pyenv version
3.6.0 (set by /Users/Leo/.pyenv/version)
通常情况下，对于特定的项目，我们可能需要切换不同的Python环境，这个时候就可以通过pyenv local PYTHON_VERSION命令来修改当前目录的Python环境。命令执行后，会在当前目录中生成一个.python-version文件（如果该文件已存在，则修改该文件的内容），里面记录着当前目录使用的Python版本号。
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$ cat ~/.pyenv/version
2.7.13
$ pyenv local 3.6.0
$ cat .python-version
3.6.0
$ cat ~/.pyenv/version
2.7.13
$ pyenv version
3.6.0 (set by /Users/Leo/MyProjects/.python-version)$ pip -V
pip 9.0.1 from /Users/Leo/.pyenv/versions/3.6.0/lib/python3.6/site-packages (python 3.6)
$ pyenv virtualenvs
2.7.13/envs/py27_XDiff (created from /Users/Leo/.pyenv/versions/2.7.13)* 3.6.0/envs/py36_XDiff (created from /Users/Leo/.pyenv/versions/3.6.0) py27_XDiff (created from /Users/Leo/.pyenv/versions/2.7.13) py36_XDiff (created from /Users/Leo/.pyenv/versions/3.6.0)