Choose languages for your environment

You can define many different languages in your configuration files. This page describes how to use some of the more common ones.


Your environment will have Python (and specified dependencies) installed when you use one of the following configuration files:

  • requirements.txt

  • environment.yml


By default, the environment will have Python 3.7.

Changed in version 0.8: Upgraded default Python from 3.6 to 3.7.

Specifying a version of Python

To specify a specific version of Python, you have two options:

  • Use environment.yml. Conda environments let you define the Python version in environment.yml. To do so, add python=X.X to your dependencies section, like so:

    name: python 2.7
      - python=2.7
      - numpy
  • Use runtime.txt with requirements.txt. If you are using requirements.txt instead of environment.yml, you can specify the Python runtime version in a separate file called runtime.txt. This file contains a single line of the following form:


    For example:


The R Language

To ensure that R is installed, you must specify a version of R in a runtime.txt file. This takes the following form:


The date corresponds to the state of the MRAN repository at this day. Make sure that you choose a day with the desired version of your packages. For example, to use the MRAN repository on January 1st, 2018, add this line to runtime.txt:


Note that to install specific packages with the R environment, you should use the install.R configuration file.


To build an environment with Julia, include a configuration file called Project.toml. The format of this file is documented at the Julia Pkg.jl documentation. To specify a specific version of Julia to install, put a Julia version in the [compat] section of the Project.toml file, as described here:

Languages not covered here

If a language is not “officially” supported by a build pack, it can often be installed with a postBuild script. This will run arbitrary bash commands, and can be used to download / install a language.

Using multiple languages at once

It may also be possible to combine multiple languages in a single environment. The details on how to accomplish this with all possible combinations are outside the scope of this guide. However we recommend that you take a look at the Multi-Language Demo repository for some inspiration.