I have a high-performant function written in Julia, how can I use it from Python?

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Assuming your Python and Julia are installed you need to take the following steps.

  1. Run Julia and install PyCall

    using Pkg
pkg"add PyCall"

  2. Put your code into a Julia package

    using Pkg
Pkg.generate("MyPackage")

    In the folder src you will find MyPackage.jl, edit it to look like this:

    module MyPackage
f(x,y) = 2x.+y
export f
end

  3. Install pyjulia

    python -m pip install julia

    (On Linux systems you might want to use python3 instead of python command)

    For this step note that while an external Python can be used with Julia. However, for a convenience it might be worth
    to consider using a Python that got installed together with Julia as PyCall.
    In that case for installation use a command such this one:

    %HOMEPATH%\.julia\conda\3\python -m pip install julia

    or on Linux

    ~/.julia/conda/3/python -m pip install julia

    Note that if you have JULIA_DEPOT_PATH variable defined you can replace %HOMEPATH%\.julia or ~/.julia/ with its value.

  4. Run the appropiate Python and tell it to configure the Python-Julia integration:

    import julia
julia.install()

  5. Now you are ready to call your Julia code:

    >>> from julia import Pkg
>>> Pkg.activate(".\\MyPackage") #use the correct path
    Activating environment at `MyPackage\Project.toml`
>>> from julia import MyPackage
>>> MyPackage.f([1,2],5)
    [7,9]

It is worth noting that the proposed approach in this answer has several advantages over a standalone Julia file which would be possible, although is less recommended. The advantages include:

  1. Packages get precompiled (so they are faster in subsequent runs) and can be loaded as a package in Python.
  2. Packages come with their own virtual environment via 1Project.toml` which makes production deployments much comfortable.
  3. A Julia package can be statically compiled into Julia’s system image which can slash itsloading time — see https://github.com/JuliaLang/PackageCompiler.jl .

EDIT

In February 2022 juliacall was announced, as of December 2022 juliacall might be an easier option for some users – have a look at: How to load a custom Julia package in Python using Python’s juliacall

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