When running Python unit tests with tox, you might see the following errors:
C:\Program Files\dotnet\sdk\2.0.0\NuGet.targets(466,5): error : Value cannot be null. C:\Program Files\dotnet\sdk\2.0.0\NuGet.targets(466,5): error : Parameter name: path1
System.ArgumentNullException: Value cannot be null. Parameter name: path1 at System.IO.Path.Combine(String path1, String path2) at Microsoft.DotNet.Configurer.CliFallbackFolderPathCalculator.get_DotnetUserProfileFolderPath() at Microsoft.DotNet.Configurer.FirstTimeUseNoticeSentinel..ctor(CliFallbackFolderPathCalculator cliFallbackFolderPathCalculator) at Microsoft.DotNet.Cli.Program.ProcessArgs(String[] args, ITelemetry telemetryClient) at Microsoft.DotNet.Cli.Program.Main(String[] args)
That is happening because all environment variables are not passed to tox by default.
Open tox.ini and add the passenv = APPDATA ProgramFiles USERPROFILE line: