一旦你激活了一个虚拟环境，可以使用一个叫做 pip 程序来安装，升级以及删除包。默认情况下 pip 将会从 Python Package Index，https://pypi.python.org/pypi， 中安装包。你可以通过 Web 浏览器浏览它们，或者你也能使用 pip 有限的搜索功能:
(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 有许多子命令：“搜索”，“安装”，“卸载”，“freeze”（译者注：这个词语暂时没有合适的词语来翻译），等等。（请参考 installing-index 指南获取 pip 更多完整的文档。）
你可以安装一个包最新的版本，通过指定包的名称:
-> 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
你也能安装一个指定版本的包，通过给出包名后面紧跟着 == 和版本号:
-> 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
如果你重新运行命令（pip install requests==2.6.0），pip 会注意到要求的版本已经安装，不会去做任何事情。你也可以提供一个不同的版本号来安装，或者运行 pip install --upgrade 来升级包到最新版本:
-> 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 后跟一个或者多个包名将会从虚拟环境中移除这些包。
pip show 将会显示一个指定的包的信息:
(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 将会列出所有安装在虚拟环境中的包:
(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 将会生成一个类似需要安装的包的列表，但是输出采用了 pip install 期望的格式。常见的做法就是把它们放在一个 requirements.txt 文件:
(tutorial-env) -> pip freeze > requirements.txt
(tutorial-env) -> cat requirements.txt
novas==3.1.1.3
numpy==1.9.2
requests==2.7.0
requirements.txt 能够被提交到版本控制中并且作为一个应用程序的一部分。用户们可以使用 install -r 安装所有必须的包:
-> 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 还有更多的选项。请参考 installing-index 指南获取关于 pip 完整的文档。当你编写一个包并且在 Python Package Index 中也出现的话，请参考 distributing-index 指南。