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 -2 *.py #运行python2程序
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:]