Windows 上同时使用Python2.x 与 3.x

Python在2.x与3.x上存在较大差异，在同时安装这两个版本时，可以通过

py -2

在命令行模式下可以启动2.7版本Python

py -3

同样可以启动python 3.x

官方描述：

The Python launcher for Windows is a utility which aids in the location and execution of different Python versions. It allows scripts (or the command-line) to indicate a preference for a specific Python version, and will locate and execute that version.