Project specific environments in Julia

One of the great things about the Julia programming language is that it is very easy to manage the dependencies of your project. In short, two files Project.toml and Manifest.toml uniquely specify what packages and in what versions are required by your scripts. You can find out how it works in detail in the Julia manual.

What I describe in this post was tested under Julia 1.4.1.

If not specified otherwise, Julia is started in a default (global) environment. Here is a quick example. Start a new Julia session and press ] to switch to package manager mode. You will see the following scrren

~$ julia
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.4.1 (2020-04-14)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

(@v1.4) pkg>

The (@v1.4) prefix tells you that you are in a default environment.

You can switch to the project specific environment using two options:

the activate command from Julia package manager, or
by passing --project command line argument when starting Julia.

For instance to activate the project environment in the current working directory use . as a path specifier and write either:

(@v1.4) pkg> activate .
 Activating new environment at `~/Project.toml`

or start Julia with the following command julia --project=..

Also as a special case you can write julia --project=@. in which case Julia will be looking for files specifying the project files in the current directory or its parents, as described here.

All this functionality is very powerful and greatly simplifies reproducibility of my work. In particular on a single machine one can have many projects, and each of them can have different dependencies.

My daily workflow with project environments

I like the Project.toml/Manifest.toml combo very much and use it for every project I work on. There is one problem though — you have to remember to activate the project environment when you start Julia.

If you use Jupyter Notebooks, the default Julia kernel that is installed by IJulia.jl automatically uses --project=@. flag when starting Julia, see here. So in this case everything works nice.

However, my typical working setup is to use a classical text editor and a terminal where Julia (a.k.a Julia REPL) is run. A natural question is how can one easily get a similar functionality in this case.

What I do is the following. In my ~/.julia/config/startup.jl file I have added the following lines:

using Pkg
if isfile("Project.toml") && isfile("Manifest.toml")
    Pkg.activate(".")
end

Now, every time I start my Julia REPL and the current working directory contains Project.toml and Manifest.toml files they will get activated.

Note that I use . — I typically do not want Julia to look for the files in the parent directories.

In rare cases when I want to disable this functionality I run julia with --startup-file=no flag to disable loading ~/.julia/config/startup.jl file.

If you have never used startup.jl file you can read a description how it works in the Julia manual.