12. 虚拟环境和包

12.1. 概述

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。

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 (often shortened to “virtualenv”), 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 virtualenv with version 2.0. If application B requires a library be upgraded to version 3.0, this will not affect application A’s environment.

12.2. 创建虚拟环境

The script used to create and manage virtual environments is called pyvenv. pyvenv will usually install the most recent version of Python that you have available; the script is also installed with a version number, so if you have multiple versions of Python on your system you can select a specific Python version by running pyvenv-3.4 or whichever version you want.

To create a virtualenv, decide upon a directory where you want to place it and run pyvenv with the directory path:

pyvenv tutorial-env

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。

Once you’ve created a virtual environment, you need to activate it.

在Windows上，运行:

tutorial-env/Scripts/activate

在Unix或MacOS上，运行:

source tutorial-env/bin/activate

（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 virtualenv will change your shell’s prompt to show what virtualenv you’re using, and modify the environment so that running python will get you that particular version and installation of Python. For example:

-> source ~/envs/tutorial-env/bin/activate
(tutorial-env) -> python
Python 3.4.3+ (3.4:c7b9645a6f35+, May 22 2015, 09:31:25)
  ...
>>> import sys
>>> sys.path
['', '/usr/local/lib/python34.zip', ...,
'~/envs/tutorial-env/lib/python3.4/site-packages']
>>>

12.3. 使用pip管理包

Once you’ve activated a virtual environment, you can install, upgrade, and remove packages using a program called pip. By default pip will install packages from the Python Package Index, <https://pypi.python.org/pypi>. You can browse the Python Package Index by going to it in your web browser, or you can use pip’s limited search feature:

(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 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:

-> 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:

-> 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:

-> 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 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:

(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:

(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:

(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:

-> 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 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。