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Calling Python, C, and FORTRAN from Julia#

The following sections describe Julia packages and native function calls that can be used to call Python, C, and FORTRAN libraries.

Calling Python#

We can use the PyCall.jl package to call Python code from Julia.

using PyCall
# The following makes it so that print statements in python will appear in this notebook
# This is not necessary when using PyCall in a terminal based Julia instance
pyimport("sys")."stdout" = PyTextIO(stdout)
pyimport("sys")."stderr" = PyTextIO(stderr);

We can execute arbitrary Python code with the special Julia strings py"..." and py"""...""".

import math
class Point:
    def __init__(self, x,y):
        self.x = x
        self.y = y
    def distance(self, p):
        return math.sqrt((self.x - p.x)**2 + (self.y - p.y)**2)

p = py"Point(1.0, 2.0)"
PyObject <__main__.Point object at 0x7fa3d66bd340>

We can even use Julia's string interpolation to give values to the Python code:

x = rand()
q = py"Point($(x), $rand())"
PyObject <__main__.Point object at 0x7fa3d66bdb80>

Attributes are directly accessible through the standard dot syntax:

@show p.x
@show p.distance(q);
p.x = 1.0
p.distance(q) = 1.7581695820873517

But say we have a module in Python that we want to call from Julia. We can do that too (otherwise this wouldn't be much use would it?). The pyimport function returns an object that gives us access to that modules functions:

np = pyimport("numpy")
A = rand(3,3)
b = rand(3)
x = np.linalg.solve(A, b)
@show maximum(abs.(A * x - b));
maximum(abs.(A * x - b)) = 1.1102230246251565e-16

In the previous slide A and b are created by Julia while x is created by Python, but we are using them interchangeably. We can do this because PyCall handles most type conversions automatically.

for x in [5.0, 2, ["a", "b"], Dict("a"=>rand(), "b"=>rand()), A]
    @show typeof(x)
typeof(x) = Float64
<class 'float'>
typeof(x) = Int64
<class 'int'>
typeof(x) = Vector{String}
<class 'list'>
typeof(x) = Dict{String, Float64}
<class 'dict'>
typeof(x) = Matrix{Float64}
<class 'numpy.ndarray'>

Note that the matrix is converted to a numpy array if numpy is installed.

The same is true going from Python to Julia.

objs = [{'a':1,'b':2}, [1, 'a', 3.0], 2.0+3j]
for k in range(len(objs)):
Dict{Any, Any}
<class 'dict'>
<class 'list'>
<class 'complex'>

We do need to be a little careful with some of Julia's less common types especially if we give it to python and bring it back:

a = Int32(5)
@show typeof(a)
@show typeof(py"$a");
typeof(a) = Int32
typeof(py"$a") = Int64

In these cases, we may want to handle the conversion ourselves. One option is getting the raw PyObject back by using the py"..."o syntax and then calling an appropriate convert function:

@show typeof(a)
@show typeof(py"$a"o)
@show typeof(convert(Int32, py"$a"o));
typeof(a) = Int32
typeof(py"$a"o) = PyObject
typeof(convert(Int32, py"$a"o)) = Int32

Another way of handling (or preventing) type conversions is to use the pycall function.

pycall(np.random.normal, PyObject, size=3)
PyObject array([ 1.27173788, -0.55905635, -1.81371862])

Here we specified to leave the object as a raw PyObject (i.e. no type conversion at all)

We can also give it a Julia type to convert to

pycall(np.random.normal, Vector{ComplexF32}, size=3)
3-element Vector{ComplexF32}:
 0.82824904f0 + 0.0f0im
 -1.8152742f0 + 0.0f0im
  0.6555549f0 + 0.0f0im

Here we forced the type conversion to complex numbers with 32-bit precision for the real and imaginary parts.

But what if we need to call a Python function that requires a callback? Not a problem. PyCall will automatically convert Julia functions to Python callable objects!

si = pyimport("scipy.integrate")
tk = 0.0:1e-2:10.0
function my_ode(t::Float64, y::Vector{Float64})::Vector{Float64}
    dy = zeros(length(y))
    dy[1] = 5.0*y[1] - 5.0*y[1]*y[2]
    dy[2] = y[1]*y[2] - y[2]
    return dy
soln = si.solve_ivp(my_ode, (0.0, 10.0), [5.0, 1.0], t_eval=tk);
using Plots
plot(soln["t"], soln["y"]')


plot(soln["y"][1,:], soln["y"][2,:])


For more details, see the PyCall github repo.

Calling C/FORTRAN Libraries#

Here we will discuss how to call a C library function from within Julia.

Calling a FORTRAN library function is the same except that FORTRAN compilers "mangle" the function names. This means that they are not precisely the same names as in the source code and you need to know what compiler was used to compile the FORTRAN library so you can determine the mangling scheme.

Note that the library we are calling must be compiled as a shared library.

As an example we will use the "silly" library that was written just for this.

Here are the functions available in the silly library:

void fill_zeros(double *to_fill, int size);
void fill_value(double *to_fill, int size, double value);
void fill_cb(double *to_fill, int size, double (*func)(int));

To call one of these functions, we will use the builtin Julia function ccall:

N = 4
my_vector = Vector{Float64}(undef, N)
@show my_vector
ccall((:fill_zeros,"fake-lib/libsilly"), # function and library
    Cvoid, # return type
    (Ref{Float64}, Cint), # argument types
    my_vector, N # arguments
@show my_vector
    (Ref{Float64}, Cint, Cdouble),
    my_vector, N, pi
@show my_vector;
my_vector = [2.257468188e-314, 0.0, 2.257517705e-314, 2.257468188e-314]
my_vector = [0.0, 0.0, 0.0, 0.0]
my_vector = [3.141592653589793, 3.141592653589793, 3.141592653589793, 3.141592653589793]

What if we want to use a function that requires a callback (so one of its arguments is a function pointer)? We can create a pointer to a Julia function with the @cfunction macro.

function my_filler(index::Int)::Float64
    return index / 10.0
cfunc = @cfunction(my_filler, Float64, (Int,))
Ptr{Nothing} @0x000000017ee10ec0

Now we call the C function with ccall as before. The type of the function pointer is Ptr{Cvoid}.

ccall((:fill_cb, "fake-lib/libsilly"),
    (Ref{Float64}, Cint, Ptr{Cvoid}),
    my_vector, N, cfunc)
@show my_vector;
my_vector = [0.0, 0.1, 0.2, 0.3]

For more details, see the Calling C and FORTRAN Code section of the Julia documentation. (If the link does not work, just google "julia call c library".)

A more complex example is provided by Ipopt.jl. You may also wish to look at the Ipopt library C API. The easiest way to do this is actually to just look at the header file at src/Interfaces/IpStdCInterface.h which is viewable at the Ipopt github repo.

Other Interfaces#

Interested in calling a function/library written in something other than Python, C or FORTRAN? Checkout the Julia Interop group on GitHub. Interfaces already exist for C++, MATLAB, Mathematica and R to name a few.