Python Launcher for Windows

New in version 3.3.

The Python launcher for Windows is a utility which aids in locating and executing 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.

Unlike the PATH variable, the launcher will correctly select the most appropriate version of Python. It will prefer per-user installations over system-wide ones, and orders by language version rather than using the most recently installed version.

Getting started

From the command-line

Changed in version 3.6.

System-wide installations of Python 3.3 and later will put the launcher on your PATH. The launcher is compatible with all available versions of Python, so it does not matter which version is installed. To check that the launcher is available, execute the following command in Command Prompt:

py

You should find that the latest version of Python you have installed is started - it can be exited as normal, and any additional command-line arguments specified will be sent directly to Python.

If you have multiple versions of Python installed (e.g., 2.7 and 3.6) you will have noticed that Python 3.6 was started - to launch Python 2.7, try the command:

py -2.7

If you want the latest version of Python 2.x you have installed, try the command:

py -2

You should find the latest version of Python 2.x starts. If you see the following error, you do not have the launcher installed:

‘py’ is not recognized as an internal or external command,operable program or batch file.

Per-user installations of Python do not add the launcher to PATH unless the option was selected on installation.¹

How do I work with multiple versions of Python installed in parallel

On Linux, Mac OS X, and other POSIX systems, use the versioned Python commands in combination with the -m switch to run the appropriate copy of pip:

>python2   -m pip install SomePackage  # default Python 2
>python2.7 -m pip install SomePackage  # specifically Python 2.7
>python3   -m pip install SomePackage  # default Python 3
>python3.4 -m pip install SomePackage  # specifically Python 3.4

Appropriately versioned pip commands may also be available.

On Windows, use the py Python launcher in combination with the -m switch:

>py -2   -m pip install SomePackage  # default Python 2
>py -2.7 -m pip install SomePackage  # specifically Python 2.7
>py -3   -m pip install SomePackage  # default Python 3
>py -3.4 -m pip install SomePackage  # specifically Python 3.4

²

[官解]Windows上Python2和3如何兼容

想学习Python3，但是暂时又离不开Python2。在Windows上如何让它们共存呢？

目前国内网站经常会让大家把其中一个python.exe改个名字（嗯，我也这样讲过，在此纠正一下），这样区分开两个可执行文件的名字，但是这样做有一个重大的隐患，就是修改了名字的那个python对应的pip将无法使用。

官方的解法是什么？

事实上这个问题几年以前Python社区就给出了官方解决方案，只不过国内一直没有注意到罢了。我们在安装Python3（>=3.3）时，Python的安装包实际上在系统中安装了一个启动器py.exe，默认放置在文件夹C:\Windows\下面。这个启动器允许我们指定使用Python2还是Python3来运行代码（当然前提是你已经成功安装了Python2和Python3）。

如果你有一个Python文件叫 hello.py，那么你可以这样用Python2运行它

py -2 hello.py

类似的，如果你想用Python3运行它，就这样

py -3 hello.py

去掉参数 -2/-3

每次运行都要加入参数-2/-3还是比较麻烦，所以py.exe这个启动器允许你在代码中加入说明，表明这个文件应该是由python2解释运行，还是由python3解释运行。说明的方法是在代码文件的最开始加入一行

#! python2"

或者

`#! python3`

分别表示该代码文件使用Python2或者Python3解释运行。这样，运行的时候你的命令就可以简化为py hello.py

使用pip

当Python2和Python3同时存在于windows上时，它们对应的pip都叫pip.exe，所以不能够直接使用 pip install 命令来安装软件包。

而是要使用启动器py.exe来指定pip的版本。命令如下：

py -2 -m pip install XXXX-2

还是表示使用 Python2，-m pip 表示运行 pip 模块，也就是运行pip命令了。如果是为Python3安装软件，那么命令类似的变成

py -3 -m pip install XXXX

#! python2和# coding: utf-8哪个写在前面？

对于Python2用户还有另外一个困惑，Python2要在代码文件顶部增加一行说明，才能够在代码中使用中文。如果指明使用的Python版本也需要在文件顶部增加一行，那哪一行应该放在第一行呢？

#! python2 需要放在第一行，编码说明可以放在第二行。所以文件开头应该类似于:

#! python2
# coding: utf-8

有了这些技巧，Python2和Python3就可以愉快地在一起玩耍了～ ³