NumPy's fftn in C# with pythonnet

Question

I'm using pythonnet to integrate Python code into my C# program. Specifically, I'm attempting to apply a Fast Fourier Transform (FFT) on a complex array. While the code works flawlessly in Python, I encounter strange errors when attempting to use it in C#. The error message suggests that the input must be a real number and not complex, which seems contradictory.

Here's the C# code snippet:

using System.Globalization;
using System.Net.Mime;
using System.Numerics;
using Python.Runtime;

static void Initialize()
{
    string pythonDll = @"C:\Users\Mohsen\AppData\Local\Programs\Python\Python39\python39.dll";
    Environment.SetEnvironmentVariable("PYTHONNET_PYDLL", pythonDll);
    PythonEngine.Initialize();    
}


Initialize();
Complex[] complexes = [
    new Complex(1.0, 2.0),
    new Complex(3.0, 4.0),
    new Complex(5.0, 6.0),
    new Complex(5.0, 6.0)

];
List<Complex> comlist = new List<Complex>(complexes);


using (Py.GIL())
{
    dynamic lst = new PyList();
    lst.append(2);
    lst.append(2);
    
    dynamic tuple = PyTuple.AsTuple(lst);
    
    dynamic np = Py.Import("numpy");
    dynamic rawData = np.array(comlist);
    rawData.reshape(tuple);
    dynamic k = np.reshape(a: rawData.T, newshape:tuple, order:'F');

    k = np.fft.fftshift(k);
    k = np.fft.fftn(k);

    k = np.fft.fftshift(k);
    k = np.rot90(k);
    k = np.flip(k, axis : 0);
    
    
    k = np.flip(np.rot90(np.fft.fftshift(np.fft.fftn(np.fft.fftshift(k)))));
    Console.WriteLine(k.shape);
}

And here's the exception encountered:

Unhandled exception. Python.Runtime.PythonException: must be real number, not Complex
  File "C:\Users\Mohsen\AppData\Local\Programs\Python\Python39\lib\site-packages\numpy\fft\_pocketfft.py", line 74, in _raw_fft
    r = pfi.execute(a, is_real, is_forward, fct)
  File "C:\Users\Mohsen\AppData\Local\Programs\Python\Python39\lib\site-packages\numpy\fft\_pocketfft.py", line 185, in fft
    output = _raw_fft(a, n, axis, False, True, inv_norm)
  File "<__array_function__ internals>", line 5, in fft
  File "C:\Users\Mohsen\AppData\Local\Programs\Python\Python39\lib\site-packages\numpy\fft\_pocketfft.py", line 652, in _raw_fftnd
    a = function(a, n=s[ii], axis=axes[ii], norm=norm)
  File "C:\Users\Mohsen\AppData\Local\Programs\Python\Python39\lib\site-packages\numpy\fft\_pocketfft.py", line 755, in fftn
    return _raw_fftnd(a, s, axes, fft, norm)
  File "<__array_function__ internals>", line 5, in fftn
   at Python.Runtime.PythonException.ThrowLastAsClrException()
   at Python.Runtime.PyObject.Invoke(PyTuple args, PyDict kw)
   at Python.Runtime.PyObject.InvokeMethod(String name, PyTuple args, PyDict kw)
   at Python.Runtime.PyObject.TryInvokeMember(InvokeMemberBinder binder, Object[] args, Object& result)
   at CallSite.Target(Closure, CallSite, Object, Object)
   at System.Dynamic.UpdateDelegates.UpdateAndExecute2[T0,T1,TRet](CallSite site, T0 arg0, T1 arg1)
   at Program.<Main>$(String[] args) in C:\Users\Mohsen\RiderProjects\ConsoleApp2\ConsoleApp2\Program.cs:line 39

Despite thorough checks, the code works smoothly in Python without any errors.

Answer 1

NumPy doesn't know .NET Complex type. You need to either use Python complex type or have NumPy convert a 1D array of floats.