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 虚拟环境 (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.
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
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.
Once you’ve created a virtual environment, you need to activate it.
在 Windows，运行：
tutorial-env/Scripts/activate
On Unix or MacOS, run:
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.)
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'] >>>
Once you’ve activated a virtual environment, you can install, upgrade, and remove packages using a program called
pip
。默认情况下
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
has a number of subcommands: “search”, “install”, “uninstall”, “freeze”, etc. (Consult the
安装 Python 模块
guide for complete documentation for
pip
)。
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
followed by one or more package names will remove the packages from the virtual environment.
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
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
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
has many more options. Consult the
安装 Python 模块
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
分发 Python 模块
guide.
