Sometimes you want both Python 2 and Python 3 (e.g., if you have a mixture of notebooks that use each
version of the language). This repository demonstrates how to handle these cases with repo2docker. You can
specify a Python 2.7 environment with the runtime.txt file. In this case, repo2docker will install Python 2
alongside Python 3 (though all commands will default to Python 2). In this case, you can install Python 3 dependencies
with requirements3.txt, while a file called requirements.txt alone will install to the Python 2 environment.