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    Manage Dependencies with Python Virtual Environments


    Virtual Environments ensure that dependencies from one Python application don’t overwrite the dependencies of another application. In this lesson, you will learn how to create a virtual environment, switch between virtual environments, and manage dependencies within a virtual environment.



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    Virtual environments allow us to keep dependencies for different projects in separate places. To use it, we need to install it, so we'll type pip install virtual env, and I've already got it installed, so there's nothing for it to do.

    To get started using it, I'll create a directory for my project, change into that directory, and then create the virtual environment with the virtual env command, and a name for my virtual environment.

    At the same time, I can also tell it which Python I want to use, whether it's Python 2 or Python 3. Since my system defaults to Python 2, I want to use Python 3, so I'll provide the path to Python 3.

    If we take a look at the directory here, there's a new directory called py3, and if we look inside that directory, there's all of the Python requirements to run that virtual environment.

    Look at this. I can type python--version, and it shows I'm running Python 2.7. To activate this virtual environment, I'll type source, the name we gave it, the bin directory, and the activate command.

    Notice that it changed my prompt there to show that I'm now working from the py3 virtual environment. If I run the same command, python--version, I'm now running Python 3.6.1.

    I can use Python like I normally would, including installing packages using pip. I can pip install Request to install Request Library. That gets installed, and now watch this.

    I can type pip freeze, and pipe the output of that to a file called requirements.txt. If we take a look at that, it's listed all of the libraries that I have installed in this virtual environment, as well as their version.

    What that means is I can include this requirements.txt as part of my repo for this. Whenever someone else checks out this repo, they can type pip install-r, requirements.txt, and pip will install the exact same requirements that this virtual environment specifies so that we know that everyone's running on the same dependency versions.

    When I'm all done, I can just type deactivate. Notice that my cursor changed back to remove me from the virtual environment, and I can type python--version, and I'm back on Python 2.7.

    One thing you'll want to do, though, is be sure that you exclude this py3, or whatever name you gave your virtual environment, from your source control system so that the virtual environment itself isn't included in your repository chest or requirements text specifying the library versions.