(this script is written for the bash shell if you use the csh or fish shellsthere are alternate activate csh and activate fish scripts you should use instead activating the virtual environment will change your shell' prompt to show what virtual environment you're usingand modify the environment so that running python will get you that particular version and installation of python for examplesource ~/envs/tutorial-env/bin/activate (tutorial-envpython python (defaultmay : : import sys sys path ['''/usr/local/lib/python zip''~/envs/tutorial-env/lib/python /site-packages' managing packages with pip you can installupgradeand remove packages using program called pip by default pip will install packages from the python package index<by going to it in your web browseror you can use pip' limited search feature(tutorial-envpip 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 collection of astronomy related tools for python pip has number of subcommands"search""install""uninstall""freeze"etc (consult the installingindex guide for complete documentation for pip you can install the latest version of package by specifying package' name(tutorial-envpip install novas collecting novas downloading novas tar gz ( kbinstalling collected packagesnovas running setup py install for novas successfully installed novas you can also install specific version of package by giving the package name followed by =and the version number(tutorial-envpip install requests=collecting requests=using cached requests--py py -none-any whl installing collected packagesrequests successfully installed requestsif you re-run this commandpip will notice that the requested version is already installed and do nothing you can supply different version number to get that versionor you can run pip install --upgrade to upgrade the package to the latest version virtual environments and packages