12. Virtual Environments and Packages

12. 虚拟环境和包

12.1. Introduction

12.1. 概述

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.
Python应用程序通常会使用不在标准库内的软件包和模块。应用程序有时需要特定版本的库，因为应用程序可能需要修复特定的错误，或者可以使用库的过时版本的接口编写应用程序。

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.
这意味着一个Python安装可能无法满足每个应用程序的要求。如果应用程序A需要特定模块的1.0版本但应用程序B需要2.0版本，则需求存在冲突，安装版本1.0或2.0将导致某一个应用程序无法运行。

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.
这个问题的解决方案是创建一个 virtual environment，一个目录树，其中安装有特定Python版本，以及许多其他包。

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.
然后，不同的应用将可以使用不同的虚拟环境。要解决先前需求相冲突的例子，应用程序 A 可以拥有自己的安装了 1.0 版本的虚拟环境，而应用程序 B 则拥有安装了 2.0 版本的另一个虚拟环境。如果应用程序 B 要求将某个库升级到 3.0 版本，也不会影响应用程序 A 的环境。

12.2. Creating Virtual Environments

12.2. 创建虚拟环境

The module used to create and manage virtual environments is called "venv". "venv" will usually install the most recent version of Python that you have available. If you have multiple versions of Python on your system, you can select a specific Python version by running "python3" or whichever version you want.
用于创建和管理虚拟环境的模块称为 "venv"。"venv" 通常会安装你可用的最新版本的 Python。如果您的系统上有多个版本的 Python，您可以通过运行"python3" 或您想要的任何版本来选择特定的Python版本。

To create a virtual environment, decide upon a directory where you want to place it, and run the "venv" module as a script with the directory path:
要创建虚拟环境，请确定要放置它的目录，并将 "venv" 模块作为脚本运行目录路径:

   python3 -m venv tutorial-env

This will create the "tutorial-env" directory if it doesn't exist, and also create directories inside it containing a copy of the Python interpreter, the standard library, and various supporting files.
如果它不存在，这将创建 "tutorial-env" 目录，并在其中创建包含Python解释器，标准库和各种支持文件的副本的目录。

A common directory location for a virtual environment is ".venv". This name keeps the directory typically hidden in your shell and thus out of the way while giving it a name that explains why the directory exists. It also prevents clashing with ".env" environment variable definition files that some tooling supports.
虚拟环境的常用目录位置是 ".venv"。这个名称通常会令该目录在你的终端中保持隐藏，从而避免需要对所在目录进行额外解释的一般名称。它还能防止与某些工具所支持的 ".env" 环境变量定义文件发生冲突。

Once you've created a virtual environment, you may activate it.
创建虚拟环境后，您可以激活它。

On Windows, run:
在Windows上，运行:

   tutorial-env\Scripts\activate.bat

On Unix or MacOS, run:
在Unix或MacOS上，运行:

   source tutorial-env/bin/activate

(This script is written for the bash shell. If you use the csh* or fish* shells, there are alternate "activate.csh" and "activate.fish" scripts you should use instead.)
（这个脚本是为bash shell编写的。如果你使用 csh* 或 fish* shell，你应该改用 "activate.csh" 或 "activate.fish" 脚本。）

Activating the virtual environment will change your shell's prompt to show what virtual environment you're using, and modify the environment so that running "python" will get you that particular version and installation of Python. For example:
Activating the virtual environment will change your shell's prompt toshow what virtual environment you're using, and modify the environmentso that running "python" will get you that particular version andinstallation of Python. For example:

   $ source ~/envs/tutorial-env/bin/activate 
   (tutorial-env) $ python 
   Python 3.5.1 (default, May  6 2016, 10:59:36) 
     ... 
   >>> import sys 
   >>> sys.path 
   ['', '/usr/local/lib/python35.zip', ..., 
   '~/envs/tutorial-env/lib/python3.5/site-packages'] 
   >>>

12.3. Managing Packages with pip

12.3. 使用pip管理包

You can install, upgrade, and remove packages using a program called pip*. By default "pip" will install packages from the Python Package Index, . You can browse the Python Package Index by going to it in your web browser, or you can use "pip"'s limited search feature:
你可以使用一个名为 pip* 的程序来安装、升级和移除软件包。默认情况下"pip" 将从 Python Package Index 安装软件包。你可以在浏览器中访问 Python Package Index 或是使用 "pip" 受限的搜索功能:

   (tutorial-env) $ pip search astronomy 
   skyfield               - Elegant astronomy for Python 
   gary                   - Galactic astronomy and gravitational dynamics. 
   novas                  - The United States Naval Observatory NOVAS astronomy library 
   astroobs               - Provides astronomy ephemeris to plan telescope observations 
   PyAstronomy            - A collection of astronomy related tools for Python. 
   ...

"pip" has a number of subcommands: "search", "install", "uninstall", "freeze", etc. (Consult the Installing Python Modules guide for complete documentation for "pip".)
"pip" 有许多子命令：“search”、“install”、“uninstall”、“freeze”等等。（请参阅安装 Python 模块指南以了解 "pip" 的完整文档。）

You can install the latest version of a package by specifying a package's name:
您可以通过指定包的名称来安装最新版本的包：

   (tutorial-env) $ python -m pip install novas 
   Collecting novas 
     Downloading novas-3.1.1.3.tar.gz (136kB) 
   Installing collected packages: novas 
     Running setup.py install for novas 
   Successfully installed novas-3.1.1.3

You can also install a specific version of a package by giving the package name followed by "==" and the version number:
您还可以通过提供包名称后跟 "==" 和版本号来安装特定版本的包：

   (tutorial-env) $ python -m pip install requests==2.6.0 
   Collecting requests==2.6.0 
     Using cached requests-2.6.0-py2.py3-none-any.whl 
   Installing collected packages: requests 
   Successfully installed requests-2.6.0

If you re-run this command, "pip" will notice that the requested version is already installed and do nothing. You can supply a different version number to get that version, or you can run "pip install --upgrade" to upgrade the package to the latest version:
如果你重新运行这个命令，"pip" 会注意到已经安装了所请求的版本并且什么都不做。您可以提供不同的版本号来获取该版本，或者您可以运行 "pip install--upgrade" 将软件包升级到最新版本：

   (tutorial-env) $ python -m pip install --upgrade requests 
   Collecting requests 
   Installing collected packages: requests 
     Found existing installation: requests 2.6.0 
       Uninstalling requests-2.6.0: 
         Successfully uninstalled requests-2.6.0 
   Successfully installed requests-2.7.0

"pip uninstall" followed by one or more package names will remove the packages from the virtual environment.
"pip uninstall" 后跟一个或多个包名称将从虚拟环境中删除包。

"pip show" will display information about a particular package:
"pip show" 将显示有关特定包的信息：

   (tutorial-env) $ pip show requests 
   --- 
   Metadata-Version: 2.0 
   Name: requests 
   Version: 2.7.0 
   Summary: Python HTTP for Humans. 
   Home-page: http://python-requests.org 
   Author: Kenneth Reitz 
   Author-email: me@kennethreitz.com 
   License: Apache 2.0 
   Location: /Users/akuchling/envs/tutorial-env/lib/python3.4/site-packages 
   Requires:

"pip list" will display all of the packages installed in the virtual environment:
"pip list" 将显示虚拟环境中安装的所有软件包：

   (tutorial-env) $ pip list 
   novas (3.1.1.3) 
   numpy (1.9.2) 
   pip (7.0.3) 
   requests (2.7.0) 
   setuptools (16.0)

"pip freeze" will produce a similar list of the installed packages, but the output uses the format that "pip install" expects. A common convention is to put this list in a "requirements.txt" file:
pip freeze` 将生成一个类似的已安装包列表，但输出使用 "pip install"期望的格式。一个常见的约定是将此列表放在 "requirements.txt" 文件中：

   (tutorial-env) $ pip freeze > requirements.txt 
   (tutorial-env) $ cat requirements.txt 
   novas==3.1.1.3 
   numpy==1.9.2 
   requests==2.7.0

The "requirements.txt" can then be committed to version control and shipped as part of an application. Users can then install all the necessary packages with "install -r":
然后可以将 "requirements.txt" 提交给版本控制并作为应用程序的一部分提供。然后用户可以使用 "install -r" 安装所有必需的包：

   (tutorial-env) $ python -m pip install -r requirements.txt 
   Collecting novas==3.1.1.3 (from -r requirements.txt (line 1)) 
     ... 
   Collecting numpy==1.9.2 (from -r requirements.txt (line 2)) 
     ... 
   Collecting requests==2.7.0 (from -r requirements.txt (line 3)) 
     ... 
   Installing collected packages: novas, numpy, requests 
     Running setup.py install for novas 
   Successfully installed novas-3.1.1.3 numpy-1.9.2 requests-2.7.0

"pip" has many more options. Consult the Installing Python Modules guide for complete documentation for "pip". When you've written a package and want to make it available on the Python Package Index, consult the Distributing Python Modules guide.
"pip" 有更多选择。有关 "pip" 的完整文档，请参阅安装 Python 模块指南。当您编写一个包并希望在 Python 包索引中使它可用时，请参考分发 Python模块指南。

12. Virtual Environments and Packages 虚拟环境和包