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 (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. Creating Virtual Environments¶
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
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:
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.
On Windows, run:
On Unix or MacOS, run:
(This script is written for the bash shell. If you use the
csh or fish shells, there are alternate
activate.fish scripts you should use
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. Managing Packages with pip¶
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
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 Installing Python Modules guide for
complete documentation for
You can install the latest version of a package by specifying a package’s name:
-> pip install novas Collecting novas Downloading novas-22.214.171.124.tar.gz (136kB) Installing collected packages: novas Running setup.py install for novas Successfully installed novas-126.96.36.199
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
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: email@example.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
(tutorial-env) -> pip list novas (188.8.131.52) 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
(tutorial-env) -> pip freeze > requirements.txt (tutorial-env) -> cat requirements.txt novas==184.108.40.206 numpy==1.9.2 requests==2.7.0
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
-> pip install -r requirements.txt Collecting novas==220.127.116.11 (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-18.104.22.168 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.