Set PYTHON_HOME and PYTHON_VERSION environment variables. If using Visual Studio, this has to be done before launching Visual Studio.

Alternatively, create LAUNCH.cs file in Codecs.Tests project, and add the following code there:

partial class TestsRuntimeConfig
{
    static TestsRuntimeConfig()
    {
        Environment.SetEnvironmentVariable("PYTHON_HOME", @"PATH/TO/PYHOME");
        Environment.SetEnvironmentVariable("PYTHON_VERSION", "3.5");
    }
}

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