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If you want to keep things simple and are sure that, for all environments, you will be using the same dependencies, you can use just one file for your requirements named requirements.txt, generated by definition, as shown in the following:

(env)$ pip freeze > requirements.txt

To install the modules in a new virtual environment, simply use the following command:

(env)$ pip install -r requirements.txt

If you need to install a Python library from another version control system, or on a local path, then you can learn more about pip from the official documentation at https://pip.pypa.io/en/stable/user_guide/.

Another approach to managing Python dependencies that is getting more and more popular is Pipenv. You can get it and learn about it at https://github.com/pypa/pipenv.