How to overlay a Python package from unstable

Help
1

I’m familiar with overlaying a typical package from another channel—

final: previous: {
  cowsay = unstable.cowsay;
}

—but how would you do that for a Python package?

final: previous: {
  pythonPackagesExtensions = previous.pythonPackagesExtensions ++ [
    (finalPythonPackages: previousPythonPackages: {
      requests = unstable.???;
    })
  ];
}

Is there a way to reference the generic Python package before it’s been evaluated for a specific Python version?