Windows中Python2与Python3并存，随意切换版本 ：

首先需要知道，python默认执行版本由系统环境变量中Python2和Python3的先后顺序决定；

这儿给大家展示在不同情况下利用不同的方法调用python；

#1：Python Launcher调用参数

> py --help

Python Launcher for Windows Version 3.7.150.1013

usage:
py [launcher-args][python-args] script [script-args]

Launcher arguments:
-2     : Launch the latest Python 2.x version
-3     : Launch the latest Python 3.x version
-X.Y   : Launch the specified Python version
     The above all default to 64 bit if a matching 64 bit python is present.
-X.Y-32: Launch the specified 32bit Python version
-X-32  : Launch the latest 32bit Python X version
-X.Y-64: Launch the specified 64bit Python version
-X-64  : Launch the latest 64bit Python X version
-0  --list       : List the available pythons
-0p --list-paths : List with paths

py -2 #调用Python2
py -3 #调用Python3

img

py -3 *.py#运行python3程序
下面是一个python2程序的例子，当使用py -3时，运行失败；

img

#2：修改执行程序.exe名字

..\Python37\python.exe可以修改为python2.exe

..\Python27\python.exe可以修改为python3.exe

这样直接调用python2与python3就不会混乱了；

#3：程序中申明

在python程序首行添加#! python3指定由python3解释运行；

在python程序首行添加#! python2指定由python2解释运行；

#3：pip运行

py -2 -m pip install numpy  #pip2 install numpy

py -3 -m pip install numpy #pip3 install numpy

#4：Python Launcher for Windows

python launcher是python3.3新增的一个功能，它对所有有的python版本都是兼容的；Python launcher for Windows可以帮助程序或命令选择最合适的python版本作为解释器，而不是最近安装的python版本；安装3.3及更新版本的python时，python launcher默认加入到path中。

详细信息见Python Launcher for Windows

py是python launcher下的一个命令；

##打开最近安装的python版本

> py 
Python 3.7.0 (v3.7.0:1bf9cc5093, Jun 27 2018, 04:59:51) [MSC v.1914 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.

##查看py详细参数

>py -h

Python Launcher for Windows Version 3.7.150.1013

usage:
py [launcher-args][python-args] script [script-args]

Launcher arguments:

-2     : Launch the latest Python 2.x version
-3     : Launch the latest Python 3.x version
-X.Y   : Launch the specified Python version
     The above all default to 64 bit if a matching 64 bit python is present.
-X.Y-32: Launch the specified 32bit Python version
-X-32  : Launch the latest 32bit Python X version
-X.Y-64: Launch the specified 64bit Python version
-X-64  : Launch the latest 64bit Python X version
-0  --list       : List the available pythons
-0p --list-paths : List with paths

The following help text is from Python:

usage: C:\Program Files\Python37\python.exe [option] ... [-c cmd | -m mod | file | -][arg] ...
Options and arguments (and corresponding environment variables):
-b     : issue warnings about str(bytes_instance), str(bytearray_instance)
         and comparing bytes/bytearray with str. (-bb: issue errors)
-B     : don't write .pyc files on import; also PYTHONDONTWRITEBYTECODE=x
-c cmd : program passed in as string (terminates option list)
-d     : debug output from parser; also PYTHONDEBUG=x
-E     : ignore PYTHON* environment variables (such as PYTHONPATH)
-h     : print this help message and exit (also --help)
-i     : inspect interactively after running script; forces a prompt even
         if stdin does not appear to be a terminal; also PYTHONINSPECT=x
-I     : isolate Python from the user's environment (implies -E and -s)
-m mod : run library module as a script (terminates option list)
-O     : remove assert and __debug__-dependent statements; add .opt-1 before
         .pyc extension; also PYTHONOPTIMIZE=x
-OO    : do -O changes and also discard docstrings; add .opt-2 before
         .pyc extension
-q     : don't print version and copyright messages on interactive startup
-s     : don't add user site directory to sys.path; also PYTHONNOUSERSITE
-S     : don't imply 'import site' on initialization
-u     : force the stdout and stderr streams to be unbuffered;
         this option has no effect on stdin; also PYTHONUNBUFFERED=x
-v     : verbose (trace import statements); also PYTHONVERBOSE=x
         can be supplied multiple times to increase verbosity
-V     : print the Python version number and exit (also --version)
         when given twice, print more information about the build
-W arg : warning control; arg is action:message:category:module:lineno
         also PYTHONWARNINGS=arg
-x     : skip first line of source, allowing use of non-Unix forms of #!cmd
-X opt : set implementation-specific option
--check-hash-based-pycs always|default|never:
    control how Python invalidates hash-based .pyc files
file   : program read from script file

         : program read from stdin (default; interactive mode if a tty)
arg ...: arguments passed to program in sys.argv[1:]