python虚拟环境

virtual-environment.jpeg

一 什么是虚拟环境

Python applications will often use packages and modules that don’t come as part of the standard library. Applications will sometimes need a specific version of a library, because the application may require that a particular bug has been fixed or the application may be written using an obsolete version of the library’s interface.This means it may not be possible for one Python installation to meet the requirements of every application. If application A needs version 1.0 of a particular module but application B needs version 2.0, then the requirements are in conflict and installing either version 1.0 or 2.0 will leave one application unable to run.The solution for this problem is to create a virtual environment, a self-contained directory tree that contains a Python installation for a particular version of Python, plus a number of additional packages.Different applications can then use different virtual environments. To resolve the earlier example of conflicting requirements, application A can have its own virtual environment with version 1.0 installed while application B has another virtual environment with version 2.0. If application B requires a library be upgraded to version 3.0, this will not affect application A’s environment.

引用自Virtual Environments and Packages

Python 虚拟环境与docker容器的概念有点类似，也和操作系统的沙箱有点类似。用户可以创建独立的虚拟环境，然后在虚拟环境安装指定的外部包，每个虚拟环境之间不会相互影响。

所以，独立的虚拟环境可以方便制作不同的application runtime，也是用来制作测试环境。

二 python虚拟环境使用场景

• 同一个服务器上，不同的程序依赖同一个包不同的版本
• 创建测试环境，安装一些特殊的包
• 创建测试环境，测试同一个包不同的版本
• 防止影响os原来安装的包，创建独立的环境运行程序

三 如何创建 Python 虚拟环境

python3 -m venv tutorial-env
# start
source tutorial-env/bin/activate
# stop
deactivate

加入找不到venv这个module，可以通过apt-get或者yum等包管理工具安装

e.g. ubuntu
sudo apt-get install python3-venv

四 如何运行 python 虚拟环境的程序

方法一：hardcode python bin，并执行文件

vi example.py

#!/home/tom/py3-venv/bin/python
....

./myscript.py

方法二：编写一个脚本，通过脚本制定python bin

#!/bin/bash

/home/tom/py3-venv/bin/python myscript.py