12. Virtual Environments and Packages
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 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:
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.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'] >>>
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 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 Installing Python Modules 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-184.108.40.206.tar.gz (136kB) Installing collected packages: novas Running setup.py install for novas Successfully installed novas-220.127.116.11
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: 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 environment:
(tutorial-env) -> pip list novas (18.104.22.168) 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==22.214.171.124 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==126.96.36.199 (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-188.8.131.52 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.