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Python and Julia in harmony

Python and Julia play very well together. You can see in the examples above that I am using Matplotlib for plotting, both in Python and in Julia. Julia has a built-in package manager. You can use it to install and remove Julia packages, but also to install or remove Python packages. That works like this:

Using Python packages from Julia

There are three options:

Option one:

Use wrapper libraries that exist for a few, very popular Python packages, e.g. PyPlot.jl for Matplotlib or SymPy.jl for SymPy. You can install them like any other Julia package, e.g.

]     # enter package manager mode
add SymPy
<DEL> # leave the package manager

and on the Julia prompt:

using SymPy

Option two:

Use PyCall to use Python packages for Julia. This works for all Python packages, but is a little bit less comfortable than option one. Example:

using PyCall
np = pyimport("numpy")

Now you can use NumPy from Julia:

julia> np.zeros(3)
3-element Vector{Float64}:
 0.0
 0.0
 0.0

If the package is not yet installed, you can use the notation:

as = pyimport_conda("assimulo", "assimulo")

If the command using PyCall should fail, you can execute:

cd bin
./build_pycall

to re-build it.

Option three:

Using the package PythonCall. You cannot use it together with PyCall, it is the newer successor of PyCall, and it is symmetric, you can use it to call Julia from Python or Python from Julia.

We need to create a new project to try it out:

cd repos # or any other folder that you use for your projects
mkdir PythonDemo
cd PythonDemo
julia --project="." # this creates a new, empty project

Then enter at the Julia prompt: Example of using Python from Julia:

re = pyimport("re")   # import the re module
words = re.findall("[a-zA-Z]+", "PythonCall.jl is very useful!")

Output:

Python: ['PythonCall', 'jl', 'is', 'very', 'useful']

Type:

sentence = Py(" ").join(words)

Output:

Python: 'PythonCall jl is very useful'

And finally, convert this Python object to a Julia string:

pyconvert(String, sentence)  # convert the Python string to a Julia string

For adding Python packages that you want to use with PythonCall use CondaPkg as explained in the next section.

Sometimes needed: Install CondaPkg

] # by pressing the closing square bracket you enter the package manager mode of Julia
add CondaPkg # add the Python package manger

Now press the \<DEL\> key to leave the package manager. In the Julia REPL, type:

using CondaPkg
]                 # enter the package manager mode
help              # will show you all available commands; try for example
conda add ipython # this will add ipython

Press \<DEL\> to leave the package manager mode. In the Julia REPL, type:

CondaPkg.withenv() do
    run(`ipython`)
end

You should now get an interactive Python prompt and can program in Python.

Further reading