The virtuals part 2: using virtualenv to create isolated python environments

In this video we will learn how to use virtualenv to create isolated Python environments, and how to use it with pip.
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Video content:
0:00 : An "isolated"/"virtual" python environment enables installing/uninstalling packages "locally", for the isolated environment only, thus unaffecting other such environments. For example, one can create two isolated environments for two projects, installing for one environment version X of some package, and for the other version Y of the same package. It is recommended to create an isolated environment for every python project.
0:35 : virtualenv |path|. Creates a virtual environment in the given path. Doesn't overwrite any files that may be in the given path. The default python version of the computer is used in the new environment. The following packages are installed by default in the new environment: pip, wheel, setuptools, pkg_resources.
1:17 : virtualenv --python=|python-version| |path|. Creates a virtual environment in the given path that uses the given python version. For example, in the university's computers one can use python2 or python3.
1:34 :
cd |path|
source bin/activate.csh
Activates the virtual environment in the given |path|, if you're using tcsh. If you use a different shell, you'll probably need to activate the environment like so:
cd |path|
source bin/activate
2:37 : To show that the environment is active you will see [|environment-name|] before the prompt.
2:43 : When an environment is active, installing/uninstalling packages using pip is done for the active environment only!
4:05 : pip freeze. Prints a list of every package installed in the active environment, including the exact version number of each.
4:26 : pip freeze greater-than-sign |path|. Saves to |path| a list of every package installed in the active environment, including the exact version number of each.
4:32 : pip install -r |path|. If a file with package names is located at |path| (for example, a file generated by pip freeze greater-than-sign |path|) pip will install all packages listed.
5:00 : deactivate. Deactivates ("exits") the current environment.
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Relevant links:
virtualenv tutorial: http://wiki.cs.huji.ac.il/wiki/Python#virtualenv
The complete "Virtuals" playlist: https://www.youtube.com/playlist?list=PLuHmgt1HXB7CjVnJwwwJIk6LkatqLMfmL