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P.S. - I was able to solve this issue, however since I didn't find any answer responding to this aspect anywhere, thought it might help the community.

I was trying to run some python codes from my dotnet core project by setting up a local python 3.7 virtual environment created using venv. I setup the compile-time constants in project properties accordingly and followed the steps mentioned here to use my virtual environment. I was trying to import numpy which was already installed in the venv, but every time I was getting a missing basic python library error (like codec etc).
On further inspection I found that the virtual environment directory does not have these "basic" python libraries. These libraries are present in the parent python 3.7 directory (the python which was used to create the venv itself). And since the path to the parent library is alerady present in the PythonPath, they are referred from there when you run python in CMD.
But as mentioned in your documentation, instead of appending new values to PYTHONPATH, you are changing it completely and pointing it towards just the venv directory which does not have all the python files required.
I was finally able to run python and import the modules in venv after appending the venv path to original PYTHONPATH, instead of assigning it directly.
To summarise, the PythonEngine.PythonPath should have the </path/to/Lib/>, </path/to/Lib/SitePackages/> of the virtual environment python directory and also of the parent python used to create the virtual env. (Another approach could be to check the paths in sys.path in the python virtual environment and ensure those values are present here too.)
What commands did I run to trigger this issue?

    string pathToVirtualEnv = /path/to/venv/;

    Environment.SetEnvironmentVariable("PATH", pathToVirtualEnv, EnvironmentVariableTarget.Process);
    Environment.SetEnvironmentVariable("PYTHONHOME", pathToVirtualEnv, EnvironmentVariableTarget.Process);
    Environment.SetEnvironmentVariable("PYTHONPATH", $"{pathToVirtualEnv}\\Lib\\site-packages;{pathToVirtualEnv}\\Lib", EnvironmentVariableTarget.Process);
            
            
    PythonEngine.PythonHome = pathToVirtualEnv;
    PythonEngine.PythonPath = Environment.GetEnvironmentVariable("PYTHONPATH", EnvironmentVariableTarget.Process);

One of the errors which I faced

ImportError : No module named '_ctypes'

This change to the above code finally worked for me. Note - my initial PYTHONPATH points to python path corresponding the parent python I used to create the virtual environment (i.e. Python 3.7)

PythonEngine.PythonPath = PythonEngine.PythonPath + ";"+Environment.GetEnvironmentVariable("PYTHONPATH", EnvironmentVariableTarget.Process);

Reacted by Vlad, Joyce Cahoon, Bùi Lê Duy Nhất (Nick), jdunne525, myPythonCode and msrouchou

Reacted by Tu Tran

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vks2 commented on Jan 12, 2021
vks2
on Jan 12, 2021
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you saved my day. thx
Reacted by Joyce Cahoon, Shivji Bhagat, Nightrollix and kesshei
lostmsu commented on Feb 21, 2021
lostmsu
on Feb 21, 2021
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Starting with Python.NET 3.0 you will also need to set Runtime.PythonDLL to the python dynamic library, otherwise you will get TypeInitializationException.
Reacted by Shivji Bhagat
lostmsu
closed this as completed on Feb 21, 2021
shv07 commented on Apr 20, 2021
shv07
on Apr 20, 2021
Author
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Starting with Python.NET 3.0 you will also need to set Runtime.PythonDLL to the python dynamic library, otherwise you will get TypeInitializationException.

thnx for the heads up!

screig commented on Nov 8, 2022

screig

on Nov 8, 2022

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How I got it working

*Python 3.8
*VS2022
*Target framework 4.7.2
*Platform Target x64 (note you must change this, you can't leave it on ANYCPU)
*Pythonnet 3.0.1 (via nuget)

    private static void setup_py_venv_003()
    {
        Runtime.PythonDLL = @"C:\Python38\python38.dll";
        var pathToVirtualEnv = Path.GetFullPath(Path.Combine(Directory.GetCurrentDirectory(), @"..\..\..\venv"));
        Console.WriteLine(pathToVirtualEnv);


        Console.WriteLine(Runtime.PythonDLL);
        Console.WriteLine(PythonEngine.Platform);
        Console.WriteLine(PythonEngine.MinSupportedVersion);
        Console.WriteLine(PythonEngine.MaxSupportedVersion);
        Console.WriteLine(PythonEngine.BuildInfo);
        Console.WriteLine(PythonEngine.PythonPath);

        string additional = $"{pathToVirtualEnv};{pathToVirtualEnv}\\Lib\\site-packages;{pathToVirtualEnv}\\Lib";
        PythonEngine.PythonPath = PythonEngine.PythonPath + ";" + additional;
        Console.WriteLine(PythonEngine.PythonPath);

        PythonEngine.Initialize();
        PythonEngine.BeginAllowThreads();
    }

So I am adding three parts on to the end of the existing python path

Root {pathToVirtualEnv}
Site Packages {pathToVirtualEnv}\Lib\site-packages
Lib folder {pathToVirtualEnv}\Lib

Output

C:\Users\sean\_CODE\_SMALL\PythonCore\venv
----------
C:\Python38\python38.dll
win32
4.7
4.11.2147483647.2147483647
tags/v3.8.0:fa919fd, Oct 14 2019, 19:37:50
C:\Python38\python38.zip;C:\Python38\Lib\;C:\Python38\DLLs\;C:\Users\sean\_CODE\_SMALL\PythonCore\ConsoleCshrp\bin\Debug
----------
C:\Python38\python38.zip;C:\Python38\Lib\;C:\Python38\DLLs\;C:\Users\sean\_CODE\_SMALL\PythonCore\ConsoleCshrp\bin\Debug;C:\Users\sean\_CODE\_SMALL\PythonCore\venv;C:\Users\sean\_CODE\_SMALL\PythonCore\venv\Lib\site-packages;C:\Users\sean\_CODE\_SMALL\PythonCore\venv\Lib
----------

Then it goes on to run these successfully

 private static void run_some_tests()
    {
        using (Py.GIL())
        {
            dynamic np = Py.Import("numpy");
            Console.WriteLine(np.cos(np.pi * 2));

            dynamic sin = np.sin;
            Console.WriteLine(sin(5));

            double c = (double)(np.cos(5) + sin(5));
            Console.WriteLine(c);

            dynamic a = np.array(new List<float> { 1, 2, 3 });
            Console.WriteLine(a.dtype);

            dynamic b = np.array(new List<float> { 6, 5, 4 }, dtype: np.int32);
            Console.WriteLine(b.dtype);

            Console.WriteLine(a * b);
            Console.ReadKey();
        }
    }

A bit silly, but note in your venv you obviously need to have installed numpy into your venv first.

Reacted by msrouchou and Dan Kuida

cwt2021 commented on Jan 13, 2023
cwt2021
on Jan 13, 2023 · Hidden as off-topic
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calebd-anderson commented on Feb 10, 2023

calebd-anderson

on Feb 10, 2023

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I found, to use a venv, what finally worked for me, was a modified version of the documentation:

Runtime.PythonDLL = @"C:\Users\<username>\AppData\Local\Programs\Python\Python310\python310.dll";
var pathToVirtualEnv = @"path\to\env";

// be sure not to overwrite your existing "PATH" environmental variable.
var path = Environment.GetEnvironmentVariable("PATH").TrimEnd(';');
path = string.IsNullOrEmpty(path) ? pathToVirtualEnv : path + ";" + pathToVirtualEnv;
Environment.SetEnvironmentVariable("PATH", path, EnvironmentVariableTarget.Process);
Environment.SetEnvironmentVariable("PATH", pathToVirtualEnv, EnvironmentVariableTarget.Process);
// Environment.SetEnvironmentVariable("PYTHONHOME", pathToVirtualEnv, EnvironmentVariableTarget.Process);
Environment.SetEnvironmentVariable("PYTHONPATH", $"{pathToVirtualEnv}\\Lib\\site-packages;{pathToVirtualEnv}\\Lib", EnvironmentVariableTarget.Process);

PythonEngine.Initialize();

PythonEngine.PythonHome = pathToVirtualEnv;
PythonEngine.PythonPath = Environment.GetEnvironmentVariable("PYTHONPATH", EnvironmentVariableTarget.Process);

Notice

Comment out: // Environment.SetEnvironmentVariable("PYTHONHOME", pathToVirtualEnv, EnvironmentVariableTarget.Process);
Then call: PythonEngine.Initialize(); before PythonEngine

Not sure why yet.

Reacted by Shamil Urazbakhtin

Reacted by mgamache

mgamache commented on Dec 18, 2023

mgamache

on Dec 18, 2023

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I found, to use a venv, what finally worked for me, was a modified version of the documentation:

Runtime.PythonDLL = @"C:\Users\<username>\AppData\Local\Programs\Python\Python310\python310.dll";
var pathToVirtualEnv = @"path\to\env";

// be sure not to overwrite your existing "PATH" environmental variable.
var path = Environment.GetEnvironmentVariable("PATH").TrimEnd(';');
path = string.IsNullOrEmpty(path) ? pathToVirtualEnv : path + ";" + pathToVirtualEnv;
Environment.SetEnvironmentVariable("PATH", path, EnvironmentVariableTarget.Process);
Environment.SetEnvironmentVariable("PATH", pathToVirtualEnv, EnvironmentVariableTarget.Process);
// Environment.SetEnvironmentVariable("PYTHONHOME", pathToVirtualEnv, EnvironmentVariableTarget.Process);
Environment.SetEnvironmentVariable("PYTHONPATH", $"{pathToVirtualEnv}\\Lib\\site-packages;{pathToVirtualEnv}\\Lib", EnvironmentVariableTarget.Process);

PythonEngine.Initialize();

PythonEngine.PythonHome = pathToVirtualEnv;
PythonEngine.PythonPath = Environment.GetEnvironmentVariable("PYTHONPATH", EnvironmentVariableTarget.Process);

Notice

Comment out: // Environment.SetEnvironmentVariable("PYTHONHOME", pathToVirtualEnv, EnvironmentVariableTarget.Process);
Then call: PythonEngine.Initialize(); before PythonEngine

Not sure why yet.

Just moving the PythonEngine.Initialize(); before PythonEngine worked for me, thanks.

bukowa commented on Dec 20, 2023
bukowa
on Dec 20, 2023
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If anyone has some nice examples (this repo lacks them) would be great to see them./
danp-hh commented on Feb 14, 2024
danp-hh
on Feb 14, 2024
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On Windows the provided example from https://github.com/pythonnet/pythonnet/wiki/Using-Python.NET-with-Virtual-Environments worked just fine (no need to call PythonEngine.Initialize(); before setting PythonHome and PythonPath). However, I can not get this working on Linux (Ubuntu 20.04). If someone has any advices, please add a comment below. Thanks.
Reacted by heartacker
tracktownsoftware commented on Feb 28, 2024
tracktownsoftware
on Feb 28, 2024
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@helldanno Instead of ";" and "\" on Windows, I think Linux uses ":" and "/".

Maybe change code to use System.IO.Path.DirectorySeparatorChar instead of "\" backslash

For environment variable delimiter find it with below. There might be a better way.
string environmentVarDelimiter = (Environment.OSVersion.Platform == PlatformID.Win32NT) ? ";" : ":";
pythonnet
locked and limited conversation to collaborators on Feb 29, 2024
filmor
converted this issue into a discussion #2334 on Feb 29, 2024

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Setting Virtual Environment while Embedding Python in C# #1348

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vks2 commented on Jan 12, 2021

lostmsu commented on Feb 21, 2021

shv07 commented on Apr 20, 2021

screig commented on Nov 8, 2022

cwt2021 commented on Jan 13, 2023

calebd-anderson commented on Feb 10, 2023

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mgamache commented on Dec 18, 2023

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bukowa commented on Dec 20, 2023

danp-hh commented on Feb 14, 2024

tracktownsoftware commented on Feb 28, 2024

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