When you start developing software, it is of utmost importance to have an isolated programming environment in which you can control precisely the packages installed. This will allow you, for example, to use experimental libraries without overwriting software that other programs use in your computer. Isolated environments allow you, for example, to update a package only within that specific environment, without altering the dependencies in every other development you are doing.

Python provides a very convenient tool called Virtual Environment that allows you to do exactly what was described. The instructions are slightly different depending on the operating system that you use, but they are easy to adapt from one to the other. To install Virtual Environment you can do it with pip, the package manager of Python. Remember that sometimes you may have more than one version of pip in your computer, in the same way, you may have more than one version of Python. In the command line you run:

pip3 install virtualenv

If you are on Linux, you may need to add sudo to the command: sudo pip3 install virtualenv. This is the last installation that you do system-wide; from now on, everything else will happen within a Virtual Environment. We create a folder to hold all the needed files and initialize the environment.

mkdir myproject
cd myproject
virtualenv venv -p python3

The first two lines create the folder and mode into it. In the last line, we create the virtual environment within a folder called venv and using a specific version of Python, python3. On Linux you would do:

source venv/bin/activate

while on Windows it would be:

venv\Scripts\activate

If everything went well, you will see that a (venv) appears in at the beginning of the command line. Now you are working inside the Virtual Environment called venv and all the packages you install are going to be stored within it. Let's install a package to see how it works.

pip install Flask==0.9

The command will install a very specific version of Flask, which is not the most recent one. One of the useful aspects of virtual environments is that they allow you to keep track of all the packages that you have installed, including their versions:

pip freeze

You can output the results to a file that you can use later on for automatically installing all the requirements:

pip freeze > requirements.txt

If you open the file requirements.txt you will notice that it contains a list with all the packages from venv. To see the full potential of Virtual Environment, let's create a second one. First, we need to deactivate the one we are working on now by running:

deactivate

And now we repeat the step above to create a new environment, but with a different name:

virtualenv test -p python3

And we activate it:

source test/bin/activate

or for Windows:

venv\Scripts\activate

If we run again pip freeze you will notice that your environment is empty. We can install all the packages contained in the requirements.txt file by simply running:

pip install -r requirements.txt

If you check again with pip freeze you will notice that you have exactly the same packages than in the venv environment. You can upgrade Flask, for example:

pip install --upgrade Flask

And if you run again pip freeze you will notice that the version of Flask has changed. Repeat the steps mentioned above in order to deactivate test and activate venv. You will see that the version of Flask stayed at 0.9 and was not upgraded.